Nedcim Posted June 10, 2020 Author Posted June 10, 2020 19 hours ago, J.C.MacSwell said: What instant do you believe equilibrium exists for the bat and ball? (I don't think it's a single instant when or as you think it is) The ball changes direction. There must be an instant when the force from the bat is equal but opposite the force of the ball, for that time there is no net force in the horizontal direction. Both the ball and bat will deform, negating the net force of gravity. External force of zero gives equilibrium. 10 hours ago, swansont said: Way to miss the point. If they are not an action-reaction pair, Newton’s 3rd law does not apply. They don’t have to be equal in magnitude, opposite in direction, or the same type of force. How can the forces be third law pair but not be equal magnitude or opposite in direction? Quote The ball hits the bat, the bat hits the ball. The forces are not on the same object. They are an action-reaction pair. The contact force and gravity, the two forces acting on the ball, are not an action-reaction pair. That gives a contradiction. The same acting forces on two examples but with different results. There is must be more consideration other than two forces acting on the same object. Quote Still no. The force an be unknown or not accounted for. You can be in a rotating frame of reference. Those circumstances will not change the action-reaction relationship of other forces that are present Unknown or not accounted force? That's never going to be the case with an object that has a net external force or torque. A suitable reaction will give equilibrium. That is the basis of statics. Quote CAN be. Not MUST be, or CAN ONLY be. And perhaps consider that a BBC website might be a less trustworthy source than someone with a PhD in physics If it "can be" that implies that there must an action/reaction pair for equilibrium or else Newton's third law would not apply. In any case, the BBC website has a good reputation in terms of fact based reporting: Quote Factual Reporting: HIGH https://mediabiasfactcheck.com/bbc Are you suggesting that the website is somehow not being honest in its application of Newton's third law?
Ghideon Posted June 10, 2020 Posted June 10, 2020 21 minutes ago, Nedcim said: That gives a contradiction. The same acting forces on two examples but with different results. There is must be more consideration other than two forces acting on the same object. Can you provide a diagram showing what you believe to be a contradiction?
swansont Posted June 10, 2020 Posted June 10, 2020 1 hour ago, Nedcim said: The ball changes direction. There must be an instant when the force from the bat is equal but opposite the force of the ball, for that time there is no net force in the horizontal direction. Both the ball and bat will deform, negating the net force of gravity. External force of zero gives equilibrium. “No net force” is a concept that can only apply to one object. It has no bearing on action-reaction These forces are horizontal, and can’t “negate” gravity. Quote How can the forces be third law pair but not be equal magnitude or opposite in direction? They can’t. I said they weren’t a third law pair. So asking me how they can be is...odd. Quote That gives a contradiction. The same acting forces on two examples but with different results. Are they the same? The force the ball exerts on the bat is being exerted on the ball? How does that happen? Quote There is must be more consideration other than two forces acting on the same object There are. They’ve been discussed about a bazillion times. Are you not paying attention? Quote Unknown or not accounted force? That's never going to be the case with an object that has a net external force or torque. A suitable reaction will give equilibrium. That is the basis of statics Why are you assuming statics? Quote If it "can be" that implies that there must an action/reaction pair for equilibrium or else Newton's third law would not apply. can ≠ must Quote In any case, the BBC website has a good reputation in terms of fact based reporting: physics is not reporting. Quote Are you suggesting that the website is somehow not being honest in its application of Newton's third law? No. I said nothing like that. I don’t think a lack of expertise is dishonesty. I don’t think you’re being dishonest by misunderstanding this. Why would you go there?
Nedcim Posted June 11, 2020 Author Posted June 11, 2020 23 hours ago, Ghideon said: Can you provide a diagram showing what you believe to be a contradiction? There is no consideration of the forces acting on the ball and bat only their resulting opposed normal forces. While the desk and book considers the forces that result in the opposed normal forces. 22 hours ago, swansont said: “No net force” is a concept that can only apply to one object. It has no bearing on action-reaction That's not true at all. Again, refer back to the derrick example. It's comprised of many objects yet the derrick itself has no net force because of its suitable reactions. Quote These forces are horizontal, and can’t “negate” gravity. The deformation forces will definitely negate gravity. Take a piece of clay and press it against a wall, it not resist gravity until sufficient deformation. Quote They can’t. I said they weren’t a third law pair. So asking me how they can be is...odd Then what what was you referring to with "they" if not action/reaction forces: Quote They don't have to be equal in magnitude, opposite in direction, or the same type if force. Quote Are they the same? The force the ball exerts on the bat is being exerted on the ball? How does that happen? Yes, they are the same, ultimately both examples consist of equal but oppositely directed normal forces. The force of the desk exerts on the book is being exerted on the book. Quote There are. They’ve been discussed about a bazillion times. Are you not paying attention? Then I missed them. What are those considerations? Quote Why are you assuming statics? Because the results of statics is fundamental to the issue you mentioned. Why are you ignoring the results of statics? Quote can ≠ must I never implied that. Only that there must be an action/reaction or there's no application of Newton's third law. Quote physics is not reporting. It most definitely is reporting. The BBC corporation gave a written account about Newton's third law. Quote No. I said nothing like that. I don’t think a lack of expertise is dishonesty. I don’t think you’re being dishonest by misunderstanding this. Why would you go there? You said: "BBC website might be less trustworthy source than someone with a PhD in physics." Meanwhile, the BBC science staff is comprised of writers who have PhDs in physics.
swansont Posted June 11, 2020 Posted June 11, 2020 20 minutes ago, Nedcim said: That's not true at all. Again, refer back to the derrick example. It's comprised of many objects yet the derrick itself has no net force because of its suitable reactions. Either the derrick is an object or it isn’t. You can’t have it both ways in the same sentence. Only forces (and torques) acting on the derrick are shown. No reaction forces are depicted in that diagram. 20 minutes ago, Nedcim said: The deformation forces will definitely negate gravity. Take a piece of clay and press it against a wall, it not resist gravity until sufficient deformation. Orthogonal forces cannot negate each other. If you don’t understand that, you need to open a new thread to clear up your misconception Quote Then what what was you referring to with "they" if not action/reaction forces: There are two forces. You use “they” when there’s more than one of something. They - the two forces - are not an action-reaction pair. It’s not a situation where Newton’s 3rd law tells you anything. 20 minutes ago, Nedcim said: Yes, they are the same, ultimately both examples consist of equal but oppositely directed normal forces. The force of the desk exerts on the book is being exerted on the book. Opposite directIon, and acting on different objects, means they are not the same. Not identical. Quote Then I missed them. What are those considerations? Go back and read the thread Quote Because the results of statics is fundamental to the issue you mentioned. Why are you ignoring the results of statics? Where did Newton say that his laws only applies to statics? How do you use the 2nd law, if you are limited to statics Quote I never implied that. Only that there must be an action/reaction or there's no application of Newton's third law. You did more than imply it, you literally said it. And where you saiid itx you mistake zero net force to mean action-reaction. They aren’t the same thing. Quote It most definitely is reporting. The BBC corporation gave a written account about Newton's third law. So all the times I was teaching physics I was actually reporting? Quote You said: "BBC website might be less trustworthy source than someone with a PhD in physics." Meanwhile, the BBC science staff is comprised of writers who have PhDs in physics. 1. Prove it. 2. You haven’t understood my objection, which really is to your (lack of) understanding of what the site said.
Ghideon Posted June 11, 2020 Posted June 11, 2020 42 minutes ago, Nedcim said: There is no consideration of the forces acting on the ball and bat only their resulting opposed normal forces. While the desk and book considers the forces that result in the opposed normal forces. There is no contradiction. Draw a diagram, mark the action reaction pairs try to tell where the contradiction is. If you prefer to compare the ball and the bat with the book and the table then tilt the ball and the bat situation to vertical; drop the ball from above and hit it with the bat from straight below. The tilted case will resemble the book and table situation; gravity acting to pull the ball down and the ball acting to pull the earth up. But the action/reaction pairs will be the same. Action/reaction pairs may change in magnitude (as I said back in my first post in this thread) but not which two forces that are action/reaction pairs.
swansont Posted June 12, 2020 Posted June 12, 2020 12 hours ago, Nedcim said: There is no consideration of the forces acting on the ball and bat only their resulting opposed normal forces. While the desk and book considers the forces that result in the opposed normal forces. This points out the two different situations we've encountered. One is Newton's second law: F = ma. The acceleration of an object (which may* be zero) depends on the net force acting on it. So you identify and add all the forces acting on it together. Usually highlighted by a free-body diagram (FBD). The other is Newton's third law. This identifies pairs of forces - each pair consisting of one exerted by the object, the other exerted on the object. (The is no FBD, because a FBD only includes forces exerted on the object). These pairs are equal in magnitude and opposite in direction, as well as being the same type of force. Neither case requires that the net force be zero. Resolving a FBD will tell you if there is a net force. Identifying action-reaction pairs will not (though if there is only one pair you can deduce that there is) *may be zero does not mean must be zero. Zero is a possibility, but not a requirement.
Nedcim Posted June 12, 2020 Author Posted June 12, 2020 On 6/11/2020 at 6:23 PM, swansont said: Either the derrick is an object or it isn’t. You can’t have it both ways in the same sentence. No, an object can be composed of other objects. Stack up two books and now two objects have combined to form one object with no net force acting them. Quote Only forces (and torques) acting on the derrick are shown. No reaction forces are depicted in that diagram. No reaction forces? It explictly notes that the normal force of the rock counters gravity. What is countering the normal from the rock? The normal force from the base of the derrick. They must form an action/reaction pair. Quote Orthogonal forces cannot negate each other. If you don’t understand that, you need to open a new thread to clear up your misconception The book and desk are in equilibrium, solely by opposed normal forces. Quote There are two forces. You use “they” when there’s more than one of something. They - the two forces - are not an action-reaction pair. It’s not a situation where Newton’s 3rd law tells you anything. Your mistake. You mentioned that Newton's third law does not apply. Then noted that they don't have to be equal etc. Those conditions are always true for forces that are not action/reaction pair. Quote Opposite directIon, and acting on different objects, means they are not the same. Not identical. The normal forces are identical but oppositely directed. If they were not identical there would be a net force. Quote Go back and read the thread Your claim. Quote Where did Newton say that his laws only applies to statics? How do you use the 2nd law, if you are limited to statics Newton's laws are fundamental to statics. You use Newton's 2nd law when acceleration is equal to 0. Quote You did more than imply it, you literally said it. And where you saiid itx you mistake zero net force to mean action-reaction. They aren’t the same thing. I never implied that "can" has the same meaning as "must". That was a strawman argument by you. I said Newton's third law can be applied to equilibrium as did the webpage Hence, equilibrium must be composed of action/reaction pair. Quote So all the times I was teaching physics I was actually reporting? Sure, by the definition of reporting. Quote 1. Prove it. I will, but I doubt you'll prove your phD: Quote https://www.bbc.co.uk/programmes/p011v42h/p011v3t She graduated from Churchill College, Cambridge, with degrees Master of Arts and Master of Science in Natural Sciences (Physics) and a PhD[10] in experimental explosives physics, particularly Research Department Explosive (RDX).https://en.m.wikipedia.org/wiki/Helen_Czerski Quote 2. You haven’t understood my objection, which really is to your (lack of) understanding of what the site said You dismissed the website's claim in favor of your own on the basis they might not be qualified. On 6/11/2020 at 6:53 PM, Ghideon said: There is no contradiction. Draw a diagram, mark the action reaction pairs try to tell where the contradiction is. If you prefer to compare the ball and the bat with the book and the table then tilt the ball and the bat situation to vertical; drop the ball from above and hit it with the bat from straight below. The tilted case will resemble the book and table situation; gravity acting to pull the ball down and the ball acting to pull the earth up. But the action/reaction pairs will be the same. Action/reaction pairs may change in magnitude (as I said back in my first post in this thread) but not which two forces that are action/reaction pairs. The tilted case is not the same as the book and desk. The force of gravity is greater than the normal force, so there is a net force acting on the ball. 13 hours ago, swansont said: This points out the two different situations we've encountered. One is Newton's second law: F = ma. The acceleration of an object (which may* be zero) depends on the net force acting on it. So you identify and add all the forces acting on it together. Usually highlighted by a free-body diagram (FBD). The other is Newton's third law. This identifies pairs of forces - each pair consisting of one exerted by the object, the other exerted on the object. (The is no FBD, because a FBD only includes forces exerted on the object). These pairs are equal in magnitude and opposite in direction, as well as being the same type of force. Neither case requires that the net force be zero. Resolving a FBD will tell you if there is a net force. Identifying action-reaction pairs will not (though if there is only one pair you can deduce that there is) *may be zero does not mean must be zero. Zero is a possibility, but not a requirement. Ok, that's all fine. It's still doesn't give consideration to the forces that result in the oppositely directed normal forces that was the justification to include the a third body (Earth) as with the book and desk. The ball changes direction. The ball loses all of its energy and momentum to the bat. Then the bat loses some of its energy and momentum to the ball. I don't see how you can describe that situation with a single FBD. How would it not consist of three intermediate steps where the acting forces change with time?
swansont Posted June 13, 2020 Posted June 13, 2020 16 hours ago, Nedcim said: No, an object can be composed of other objects. Stack up two books and now two objects have combined to form one object with no net force acting them. This is a been a problem with your posts - you either don't read what was said, or comprehension isn't there. So you respond to something other than what was claimed Like here. I did not make a general claim about objects, I made a specific claim that the derrick is an object. That in no way can be (validly) extrapolated to mean that other systems can’t be comprised of multiple objects. So bringing up the fact that you can stack two books doesn't have any bearing on this. Quote No reaction forces? It explictly notes that the normal force of the rock counters gravity. What is countering the normal from the rock? The normal force from the base of the derrick. They must form an action/reaction pair. The can't form an action/reaction pair, since they don't satisfy the criteria for action/reaction pairs. Since you refuse to go look this up (and somehow you are arguing about Newton's third law without knowing this), I will once again list them. Make note, since I won't do it again. The criteria are: 1. Equal in magnitude 2. Opposite in direction 3. Act on different objects (they act on each other) 4. Same kind of force ALL criteria must be met. Normal force and gravity fail to meet 3 and 4, thus, any student of physics hoping to demonstrate understanding of the concept must realize that these are not an action/reaction pair. And, as I said, there are no reaction forces present, since a reaction force is one exerted BY the object. All of the forces in the diagram are exerted ON the object. Quote The book and desk are in equilibrium, solely by opposed normal forces. The example in question was the bat and the ball. Quote Your mistake. You mentioned that Newton's third law does not apply. Then noted that they don't have to be equal etc. Those conditions are always true for forces that are not action/reaction pair. It doesn't apply to that pair of forces, since they are not an action/reaction pair. Quote The normal forces are identical but oppositely directed. If they were not identical there would be a net force. If they were identical they would be in the same direction, and identifying an action/reaction pair does not tell you whether or not there is a net force. Quote Your claim. See above. I can't believe you are this effing lazy. Quote Newton's laws are fundamental to statics. You use Newton's 2nd law when acceleration is equal to 0. Newton's laws are fundamental to dynamics, too. Lots of problems are solved where F ≠ 0. You use the second law for them too. But it's pretty obvious you haven't studied more advanced physics if you have the misunderstandings you do. Quote I never implied that "can" has the same meaning as "must". That was a strawman argument by you. I said Newton's third law can be applied to equilibrium as did the webpage Hence, equilibrium must be composed of action/reaction pair. Not a strawman as you actually did it. Equilibrium is a conclusion from the second law. Action/reaction is the third law. They are different concepts. Quote I will, but I doubt you'll prove your phD: Did Dr. Czerski write that article? I didn't find her name on the first page, or on page 5 It really hasn't come into question when I talk about work, which is a tad more advanced than lecture 2 of physics 101. Quote You dismissed the website's claim in favor of your own on the basis they might not be qualified. I realized that the problem is your interpretation of the website, and the issue is less of one of qualification as in clearly explaining the concepts. You can't ask a website for clarification. I didn't dismiss their claim. I inquired why YOU dismissing mine in favor of theirs. AFTER I pointed out that they did not actually back up your claim. (a can vs. must issue you have asserted didn't happen, and yet, there it is) Quote Ok, that's all fine. It's still doesn't give consideration to the forces that result in the oppositely directed normal forces that was the justification to include the a third body (Earth) as with the book and desk. If it's all fine then how about applying that information? The book and desk example mentioned gravity. Gravity in this kind of problem is exerted by the earth or some other (typically very large) mass. Thus, it can't be ignored. But, once again looking at the above list of criteria for action/reaction, we see that these force fail to fulfill all of the criteria. Quote The ball changes direction. The ball loses all of its energy and momentum to the bat. Then the bat loses some of its energy and momentum to the ball. I don't see how you can describe that situation with a single FBD. How would it not consist of three intermediate steps where the acting forces change with time? In that kind of problem you are worried about the moment of impact, though it would depend on the specific question. But in all the time the ball is in contact with the bat, the single FBD would apply, since those are the forces present. They may change magnitude, but that doesn't negate a FBD that is just identifying the forces Energy and momentum are not things that appear in the FBD. Forces only. 1
Ghideon Posted June 13, 2020 Posted June 13, 2020 15 hours ago, Nedcim said: The tilted case is not the same as the book and desk. The force of gravity is greater than the normal force, so there is a net force acting on the ball. I did not claim them to be the same. I claim that there is no contradiction. I tried to highlight that by using a thought experiment to modify the ball and the bat example to show the similarities with the book and the table. Sorry that it did not work out. Again: What is the contradiction? The cases with the bat and the ball and the table and the book have similarities and they ave differences. The behaviour is explained by the same principles and physical laws.
Nedcim Posted June 13, 2020 Author Posted June 13, 2020 8 hours ago, swansont said: This is a been a problem with your posts - you either don't read what was said, or comprehension isn't there. So you respond to something other than what was claimed Like here. I did not make a general claim about objects, I made a specific claim that the derrick is an object. That in no way can be (validly) extrapolated to mean that other systems can’t be comprised of multiple objects. So bringing up the fact that you can stack two books doesn't have any bearing on this. The derrick is composed of many objects. I can't help if you choose to ignore simplest case of two books stacked together because it goes against your claim. Quote The can't form an action/reaction pair, since they don't satisfy the criteria for action/reaction pairs. Since you refuse to go look this up (and somehow you are arguing about Newton's third law without knowing this), I will once again list them. Make note, since I won't do it again. The criteria are: 1. Equal in magnitude 2. Opposite in direction 3. Act on different objects (they act on each other) 4. Same kind of force ALL criteria must be met. Normal force and gravity fail to meet 3 and 4, thus, any student of physics hoping to demonstrate understanding of the concept must realize that these are not an action/reaction pair. And, as I said, there are no reaction forces present, since a reaction force is one exerted BY the object. All of the forces in the diagram are exerted ON the object. You failed to acknowledge that some force must be countering the normal force from the rock. Those two forces satisfy the given criteria. Quote The example in question was the bat and the ball Sure, ignore the simple case because it refutes your claim. Quote It doesn't apply to that pair of forces, since they are not an action/reaction pair Only in the strictest case. Refer to vollyball diagram. Quote If they were identical they would be in the same direction, and identifying an action/reaction pair does not tell you whether or not there is a net force. Same direction for identical forces? That's not true: https://www.chegg.com/homework-help/questions-and-answers/three-cases-involving-identical-forces-acting-identical-sticks-shown-figure-case-compare-m-q35472072 Quote Not a strawman as you actually did it. Equilibrium is a conclusion from the second law. Action/reaction is the third law. They are different concepts No, I never said that "can" has the same definition as "must." You tried to make that insinuation. Of course, Newton's 2nd and 3rd laws are different concepts. Again, another strawman argument from you. Equilibrium is dependent on all of Newton's laws of motion. Quote Did Dr. Czerski write that article? I didn't find her name on the first page, or on page 5 You obviously didn't make much of an effort. Quote I didn't dismiss their claim. I inquired why YOU dismissing mine in favor of theirs. AFTER I pointed out that they did not actually back up your claim. You are holding on to a unique case, which is ignored in most applications. You have ignored or made excuses for all cases I cited and their applications. Quote it's all fine then how about applying that information? The book and desk example mentioned gravity. Gravity in this kind of problem is exerted by the earth or some other (typically very large) mass. Thus, it can't be ignored. But, once again looking at the above list of criteria for action/reaction, we see that these force fail to fulfill all of the criteria. Gravity is not ignored. The book and desk example gave a normal force which is a result of gravity. The interaction of the third body is what is usually ignored. Quote In that kind of problem you are worried about the moment of impact, though it would depend on the specific question. But in all the time the ball is in contact with the bat, the single FBD would apply, since those are the forces present. They may change magnitude, but that doesn't negate a FBD that is just identifying the forces Energy and momentum are not things that appear in the FBD. Forces only The moment of impact is critical but it does not relate enough information. Compare that with a rocket blasting off. In that case a single FBD is wholly adequate. I searched for a relevant FBD for a bat hitting a ball. Though not a true FBD, they make use of 4 diagrams to fully explain the situation: http://physicsgroup4.tripod.com/id23.htm 4 hours ago, Ghideon said: I did not claim them to be the same. I claim that there is no contradiction. I tried to highlight that by using a thought experiment to modify the ball and the bat example to show the similarities with the book and the table. Sorry that it did not work out. Again: What is the contradiction? The cases with the bat and the ball and the table and the book have similarities and they ave differences. The behaviour is explained by the same principles and physical laws. The tilted case example, the ball has the same forces acting on it as the book, so it fails by the same convention.
swansont Posted June 13, 2020 Posted June 13, 2020 Quote The derrick is composed of many objects. Yes, and objects are comprised of many, many atoms. It's irrelevant to this particular conversation. The object is rigid, and can be easily modeled. The fact that it's an extended object means the possibility of torques, but the acceleration of it will be the result of the net force. Quote I can't help if you choose to ignore simplest case of two books stacked together because it goes against your claim. I ignored because it wasn't the example I was discussing. It's a red herring you introduced into the conversation. You seem to introduce new examples when you want to avoid admitting you're wrong about a different example. The act is getting tired. Quote You failed to acknowledge that some force must be countering the normal force from the rock. Those two forces satisfy the given criteria. I don't acknowledge it because it's irrelevant. That pair of forces only satisfy two of the criteria, so they are not an action reaction pair. The other criteria become irrelevant at that point, since all four must be satisfied. Once you find one that isn't, you know the answer (well, not you, specifically, but the colloquial you) 42 minutes ago, Nedcim said: Sure, ignore the simple case because it refutes your claim. Again, I ignore it because it's not what we were discussing. I'm ignoring billions of other example we were not discussing, too. Quit changing the subject. Quote Only in the strictest case. Refer to vollyball diagram .<sigh> OK, change the subject AGAIN. What about the volleyball diagram? It clearly explains what the action/reaction pairs are. It doesn't pair a normal force with the gravitational force. Quote Same direction for identical forces? That's not true: https://www.chegg.com/homework-help/questions-and-answers/three-cases-involving-identical-forces-acting-identical-sticks-shown-figure-case-compare-m-q35472072 So you found a badly written example. Great. You've shown that somebody else out there doesn't know what identical means, or has forgotten that forces are a vector. Quote Of course, Newton's 2nd and 3rd laws are different concepts. Again, another strawman argument from you. If it's a strawman, why do you keep confusing the two? Quote Equilibrium is dependent on all of Newton's laws of motion. No. Quote You obviously didn't make much of an effort. It's your claim, and you are increasingly not worth the effort. Obstinance and ignorance are not a good combination. Quote You are holding on to a unique case, which is ignored in most applications. You have ignored or made excuses for all cases I cited and their applications. Unique case? What's unique and what's ignored? Quote Gravity is not ignored. The book and desk example gave a normal force which is a result of gravity. The interaction of the third body is what is usually ignored. Yes, gravity. I didn't say it was ignored, I said it can't be ignored. I can't tell if you're being deliberately obtuse when you do this. Quote The moment of impact is critical but it does not relate enough information. Compare that with a rocket blasting off. In that case a single FBD is wholly adequate. Enough information for what? You didn't pose a physics problem here that needed to be solved. It was just identifying action/reaction, and the identity of those forces doesn't change. Quote I searched for a relevant FBD for a bat hitting a ball. Though not a true FBD, they make use of 4 diagrams to fully explain the situation: http://physicsgroup4.tripod.com/id23.htm They use 4 diagrams because they are explaining more than the impact. Only one of the drawings depicts the impact and it shows the action/reaction forces. As I have been saying: ball hits bat, bat hits ball. One can say it depicts two FBDs, and notice how there's only one force on each object. Nothing is in equilibrium, because equilibrium is not required to identify action/reaction force pairs.
Ghideon Posted June 14, 2020 Posted June 14, 2020 (edited) 7 hours ago, swansont said: The other criteria become irrelevant at that point, since all four must be satisfied. Once you find one that isn't, you know the answer I'll borrow that and try to add another way to address the initial question. Not 100% sure this will hold but worth a try. Postulate*: Downward force of gravity (Fg) on the book exerted by Earth and Upward force (Fu) on the book exerted by the table are a Newton third law pair. That means for the pair of forces Fg and Fu criteria 1-4 must hold: 1. Fg and Fu equal in magnitude 2. Fg and Fu Opposite in direction 3. Fg and Fu act on different objects (they act on each other) 4. Fg and Fu Same kind of force First we check criteria no 1. Note that it says equal in magnitude. It does not say equal in magnitude sometimes. Let’s increase the force from the table on the book by some method. Examples: we can accelerate the table upwards or we can put a second book on top of the first. The force Fu upwards from the table on the book will increase by both methods. (The fact that the force Fu changes can be measured if necessary.) Now this means that according to (1), equal in magnitude, the downward force of gravity on the book exerted by Earth must also increase. But that is not true. The gravitational force Fg, the force on the book exerted by earth does not change, it is unaffected by any of the changes in the second force Fu. So criteria 1 does not always hold for the postulated pair of forces, a small change breaks the symmetry** . There is no need to analyse the other criteria as @swansont said, the first one failed. Result: Postulate is false. Downward force of gravity on the book exerted by Earth and Upward force on the book exerted by the table are not a Newton third law pair. This gives another way to express the answer to the question: Question "The upward normal force from the table and the downward force due to gravity are not an action/reaction pair of Newton's third law, why?" Answer: "Because it is possible to change one of the forces in the pair while the other force remains unaffected." If we would perform the above line of reasoning with a correct version the postulate it will pass all four criteria and be considered true. (Postulate would be: Downward force (Fb) on the table exerted by the book and Upward force (Fu) on the book exerted by the table are a Newton third law pair. I don't think a walk-through is necessary.) Note to @Nedcim, I fail to see the contradiction you claim. Maybe the comparisons of cases are confusing; different cases with different pairs of forces may fail or pass @swansont's four criteria for different reasons. Maybe comparing each case directly, separately, with newtons laws or the list of criteria will help more than trying to compare the various examples with each other? Or are the various entangled examples an expression for some underlaying question or point you wish to make? If so, can you express that question or claim more clearly to add progress to the discussion? *) intentionally incorrect **)criteria 1 always holds for the other force that really is in the correctly expressed action/reaction pair: the downward force on the table exerted by the book. But that was not the question and not necessary for an analysis of the intentionally incorrect postulate. Edited June 14, 2020 by Ghideon grammar & spelling
Nedcim Posted June 14, 2020 Author Posted June 14, 2020 19 hours ago, swansont said: OK, change the subject AGAIN. What about the volleyball diagram? It clearly explains what the action/reaction pairs are. It doesn't pair a normal force with the gravitational force. The subject is identical. The only change is that it ignores that interaction from Earth which is trvial for the vast majority of applications. Instead it focuses on the objects in contact, and simply labels that interaction as a third law pair. You won't acknowledge that a force must be countering the normal force from the rock from derrick example. Quote So you found a badly written example. Great. You've shown that somebody else out there doesn't know what identical means, or has forgotten that forces are a vector. More excuses. Quote If it's a strawman, why do you keep confusing the two? I haven't. You made the decision to resort strawman to avoid the consequences of accepting BBC article which went against your claim. Quote Unique case? What's unique and what's ignored? What is unique is that the desk/book interaction is not an action/reaction pair because the strict consideration of the Earth interaction. However, generally that is the exception not the rule. Here is the same example but different results: Quote This law is important in analyzing problems of static equilibrium, where all forces are balanced, but it also applies to bodies in uniform or accelerated motion. The forces it describes are real ones, not mere bookkeeping devices. For example, a book resting on a table applies a downward force equal to its weight on the table. According to the third law, the table applies an equal and opposite force to the book. This force occurs because the weight of the book causes the table to deform slightly so that it pushes back on the book like a coiled spring. https://www.britannica.com/science/Newtons-laws-of-motion More examples: Quote Diagram 1: The building is not stable enough to offer any resistance to the resultant force. Structural failure results since the building is no longer in a state of equilibrium. Diagram 2: The building is now fitted with a diagonal brace or strut. The diagonal brace acts as a reaction force against the resultant force. The building is now in a state of equilibrium.https://emedia.rmit.edu.au/dlsweb/Toolbox/buildright/content/bcgbc4010a/02_force_systems/05_newtons_third_law/page_001.htm Newton's Third Law of Motion can be stated as an object that is subjected to a force will respond with an equal but opposite force. A simple example explains why one does not fall through the floor on which they are standing. https://teachers.yale.edu/curriculum/viewer/initiative_08.04.08_u Quote Yes, gravity. I didn't say it was ignored, I said it can't be ignored. Gravity obviously wasn't ignored.
swansont Posted June 14, 2020 Posted June 14, 2020 1 hour ago, Nedcim said: The subject is identical. The only change is that it ignores that interaction from Earth which is trvial for the vast majority of applications. Instead it focuses on the objects in contact, and simply labels that interaction as a third law pair. You won't acknowledge that a force must be countering the normal force from the rock from derrick example. Why is it that in the volleyball example, you apparently agree that the reaction force of the gravitation force is another gravitational force, and the reaction force to the normal force is another normal force, and yet insist the for the derrick, it's a normal force paired with a gravitational force? Can you explain your inconsistency?
Ghideon Posted June 15, 2020 Posted June 15, 2020 22 hours ago, Nedcim said: What is unique is that the desk/book interaction is not an action/reaction pair because the strict consideration of the Earth interaction. However, generally that is the exception not the rule. Here is the same example but different results: Quote This law is important in analyzing problems of static equilibrium, where all forces are balanced, but it also applies to bodies in uniform or accelerated motion. The forces it describes are real ones, not mere bookkeeping devices. For example, a book resting on a table applies a downward force equal to its weight on the table. According to the third law, the table applies an equal and opposite force to the book. This force occurs because the weight of the book causes the table to deform slightly so that it pushes back on the book like a coiled spring. https://www.britannica.com/science/Newtons-laws-of-motion The britannica example is the same as in the example in the first post. Britannica provides a description of the same setup, the same physical result according to Newton, as in this thread. Compare britannica with explanations provided in this thread, for instance the first reply: On 5/25/2020 at 7:43 AM, Ghideon said: The books force pushing down on the table Britannica says "a book resting on a table applies a downward force equal to its weight". Britannica uses different words and also adds a description of the magnitude of the forces in the action/reaction pair. They do not speak of gravity in the article, they left that action/reaction pair* out the description. They also mention accelerated motion, also included earlier in this thread. I can't find any difference in the results, can you explain what the difference is? *) Earth's gravitational pull on the book, the book' gravitational pull on earth.
Nedcim Posted June 15, 2020 Author Posted June 15, 2020 21 hours ago, swansont said: Why is it that in the volleyball example, you apparently agree that the reaction force of the gravitation force is another gravitational force, and the reaction force to the normal force is another normal force, and yet insist the for the derrick, it's a normal force paired with a gravitational force? Can you explain your inconsistency? All objects with mass exert an attractive force on one another by universal gravition. However, the force that the book exerts on Earth is negligible. I have never insisted that the normal force be paired with gravity that was done by you. What I've said is that the force of gravity on an object results in an external force. It's the external force that is countered by a nongravitational force. My inconsistency? I suppose you're going to ignore the BBC Corp., Encyclopedia Britannica, Yale University etc. They all published material that ignores the strict considerations and simply label the interaction as a third law pair. 57 minutes ago, Ghideon said: The britannica example is the same as in the example in the first post. Britannica provides a description of the same setup, the same physical result according to Newton, as in this thread. Compare britannica with explanations provided in this thread, for instance the first reply: Britannica says "a book resting on a table applies a downward force equal to its weight". Britannica uses different words and also adds a description of the magnitude of the forces in the action/reaction pair. They do not speak of gravity in the article, they left that action/reaction pair* out the description. They also mention accelerated motion, also included earlier in this thread. I can't find any difference in the results, can you explain what the difference is? *) Earth's gravitational pull on the book, the book' gravitational pull on earth. It's not the same. Encyclopedia Britannic labels the interaction between the table and book as having equal but opposite forces as a result of Newton's third law.
swansont Posted June 15, 2020 Posted June 15, 2020 10 minutes ago, Nedcim said: All objects with mass exert an attractive force on one another by universal gravition. However, the force that the book exerts on Earth is negligible. I have never insisted that the normal force be paired with gravity that was done by you. No, it was done by you: “You failed to acknowledge that some force must be countering the normal force from the rock. Those two forces satisfy the given criteria.” The force that counters the normal force is the gravitational force. That does not make them an action/reaction pair. Quote What I've said is that the force of gravity on an object results in an external force. It's the external force that is countered by a nongravitational force. What relevance does this have to action/reaction?
Ghideon Posted June 15, 2020 Posted June 15, 2020 (edited) 1 hour ago, Nedcim said: It's not the same. Encyclopedia Britannic labels the interaction between the table and book as having equal but opposite forces as a result of Newton's third law. Please clarify where britannica differs from the action/reaction pair, the equal and opposite forces as a result of Newton's third law, described here: On 5/25/2020 at 7:43 AM, Ghideon said: The books force pushing down on the table and the tables push on the book The intention is to describe the same physical setup and the same result according to Newton. The words are different, is this a language issue? (English is not my first language) Edit: Maybe you mean "different" because Britannica does not choose to explain all the details; that the two gravitational forces are also a separate pair? The fact that britannica does not include the word "gravity" or "gravitation" does not mean they do not exist in the book-and-table-example, it just means britannica don't cover that aspect. Here is another example if you prefer. As expected it agrees with britannica but adds the details covered in this topic https://www.khanacademy.org/science/physics/forces-newtons-laws/newtons-laws-of-motion/a/what-is-newtons-third-law Spoiler since it is a little quiz to test that the reader grasps the concept of Newtons third law. Spoiler The partner force for the downward force of gravity on the box exerted by Earth would be the upward force of gravity on Earth exerted by the box. The partner force for the downward force on the table exerted by the box would be the upward force on the box exerted by the table. Khan academy and Britannica explains the same setup with different words and different level of detail. Does that make one of the wrong? If so which one? How? I also don't see a response to https://www.scienceforums.net/topic/122178-newtons-third-law-pair/page/5/?tab=comments#comment-1144776 Edited June 15, 2020 by Ghideon New idea what the issue could be, added
Nedcim Posted June 17, 2020 Author Posted June 17, 2020 On 6/15/2020 at 5:11 PM, swansont said: No, it was done by you: “You failed to acknowledge that some force must be countering the normal force from the rock. Those two forces satisfy the given criteria.” You quoted me, yet my quote says nothing about a normal force countering gravity. In any case, the diagram from the physics book said: "The normal force of the rock counters gravity." Are you saying that the book is wrong? Quote What relevance does this have to action/reaction? The nongravitational forces result in an action/reaction pair. On 6/15/2020 at 5:28 PM, Ghideon said: Please clarify where britannica differs from the action/reaction pair, the equal and opposite forces as a result of Newton's third law, described here: The intention is to describe the same physical setup and the same result according to Newton. The words are different, is this a language issue? (English is not my first language) Edit: Maybe you mean "different" because Britannica does not choose to explain all the details; that the two gravitational forces are also a separate pair? The fact that britannica does not include the word "gravity" or "gravitation" does not mean they do not exist in the book-and-table-example, it just means britannica don't cover that aspect. Here is another example if you prefer. As expected it agrees with britannica but adds the details covered in this topic https://www.khanacademy.org/science/physics/forces-newtons-laws/newtons-laws-of-motion/a/what-is-newtons-third-law Spoiler since it is a little quiz to test that the reader grasps the concept of Newtons third law. Reveal hidden contents The partner force for the downward force of gravity on the box exerted by Earth would be the upward force of gravity on Earth exerted by the box. The partner force for the downward force on the table exerted by the box would be the upward force on the box exerted by the table. Khan academy and Britannica explains the same setup with different words and different level of detail. Does that make one of the wrong? If so which one? How? I also don't see a response to https://www.scienceforums.net/topic/122178-newtons-third-law-pair/page/5/?tab=comments#comment-1144776 Early in this thread, I mentioned about the desk pushes the book; book pushes the desk. I noted about the opposing normal forces from the desk and book. I talked about the results of gravity i.e. the normal forces etc. You, nor anyone else acknowledged that those were third law pairs. Instead the concern was about two forces acting on the same object not being consistent with Newton's third law.
Ghideon Posted June 17, 2020 Posted June 17, 2020 (emphasis mine) 3 hours ago, Nedcim said: Early in this thread, I mentioned about the desk pushes the book; book pushes the desk. I noted about the opposing normal forces from the desk and book. I talked about the results of gravity i.e. the normal forces etc. You, nor anyone else acknowledged that those were third law pairs. Instead the concern was about two forces acting on the same object not being consistent with Newton's third law. Which pair of forces are "those"? Please clarify. Two forces acting on the same object are never a third law pair. Two forces of equal size and opposite direction acting on two different objects are sometimes a third law pair. Me, and several others, have acknowledged the various third law pairs that are third law pairs in agreement with Newton's third law and in agreement with sources provided. Any pairs of forces that are not Newton's third law pairs have not and will not be acknowledged as Newton's third law pairs. I have posted some questions, intended to take the discussion forward and add insight in these topics, but I fail to see answers.
swansont Posted June 17, 2020 Posted June 17, 2020 10 hours ago, Nedcim said: You quoted me, yet my quote says nothing about a normal force countering gravity. In any case, the diagram from the physics book said: "The normal force of the rock counters gravity." Are you saying that the book is wrong? The book is not wrong; you are. The book isn't presenting a 3rd-law problem. It's presenting a 2nd-law problem. IOW, in the problem you're trying to tabulate the net force, not identify action/reaction pairs. (The two concepts you swear that you aren't mixing up. But you're mixing them up) The normal force counters gravity means they add to zero, and make no net contribution to a net force. (Any time you discuss net force, that's 2nd law, not 3rd law) As I had stated earlier, there are no reaction forces labeled in that diagram. A reaction force is always exerted BY the object. (see #3 on the list of criteria) Since both the normal force and gravity are exerted ON the object, they are not an action/reaction pair. Quote The nongravitational forces result in an action/reaction pair. A gravitational and non-gravitational force can NEVER be an action/reaction pair. It would violate criterion #4 Quote Early in this thread, I mentioned about the desk pushes the book; book pushes the desk. I noted about the opposing normal forces from the desk and book. I talked about the results of gravity i.e. the normal forces etc. You, nor anyone else acknowledged that those were third law pairs. Because they aren't third law pairs. The normal forces (one acting on the book, one exerted by the book) are a third law pair. The gravitational forces (one exerted on the book, one exerted by the book) are a third law pair. Quote Instead the concern was about two forces acting on the same object not being consistent with Newton's third law. Two forces acting on the same object are not a third law issue. It's not the concept to apply. Much like thermodynamics doesn't apply to the problem. It's not that thermodynamics is wrong in any way, it's that it's the wrong tool for the job.
Nedcim Posted June 17, 2020 Author Posted June 17, 2020 9 hours ago, swansont said: The book is not wrong; you are. The book isn't presenting a 3rd-law problem. It's presenting a 2nd-law problem. IOW, in the problem you're trying to tabulate the net force, not identify action/reaction pairs. (The two concepts you swear that you aren't mixing up. But you're mixing them up) You are entirely wrong. You have repeatedly failed to acknowledge that a normal force from the derrick must be countering the normal force from the rock. Quote The normal force counters gravity means they add to zero, and make no net contribution to a net force. (Any time you discuss net force, that's 2nd law, not 3rd law) As I had stated earlier, there are no reaction forces labeled in that diagram. A reaction force is always exerted BY the object. (see #3 on the list of criteria) Since both the normal force and gravity are exerted ON the object, they are not an action/reaction pair. A reaction is exerted by the object. Are you denying that the derrick is exerting a normal force on the rock? Quote A gravitational and non-gravitational force can NEVER be an action/reaction pair. It would violate criterion #4 More strawman from you. I said nongravitational forces result in action/reaction pair. Quote Because they aren't third law pairs. The normal forces (one acting on the book, one exerted by the book) are a third law pair. That's exactly what I said early in this thread: Quote I mentioned about the desk pushes the book; book pushes the desk. I noted about the opposing normal forces from the desk and book. I talked about the results of gravity i.e. the normal forces etc. You, nor anyone else acknowledged that the normal forces from the desk and book constituted a third law pair until I cited several sources which explictly made that claim. You contradicted that claim with your earlier quotes: Quote Equilibrium is irrelevant. All equilibrium (something at rest in its own frame) tells you is that there isn’t a net force on an object, or pair of objects, which says nothing about action-reaction. You failed to acknowledge that the opposed normal forces are an action/reaction pair, and static equilibrium in the example, is dependent on that result.
swansont Posted June 17, 2020 Posted June 17, 2020 2 hours ago, Nedcim said: You are entirely wrong. You have repeatedly failed to acknowledge that a normal force from the derrick must be countering the normal force from the rock. I said that no reaction forces are depicted in the diagram (no forces exerted by the derrick are shown), and you argued with me about it! The diagram shows two forces, and states that the normal force from the rock counters gravity. "Counters" is used to show the forces add to zero, and give a zero net force. It does not have any force arrows depicting forces exerted by the derrick. The normal force exerted by the derrick does not "counter" the normal force exerted by the rock - they are not acting on the same thing, so they cannot be combined to calculate a net force. But in presenting the example you said "If gravity acts on an object then there is a net external force on that object until equal but oppositely set of action/reaction offsets it:" That force that counters gravity is the normal force, and you identified them here as action/reaction forces. Quote A reaction is exerted by the object. Are you denying that the derrick is exerting a normal force on the rock? Not at all. I'm just saying that this force is not identified in the drawing. Quote More strawman from you. I said nongravitational forces result in action/reaction pair. What you said was "What I've said is that the force of gravity on an object results in an external force. It's the external force that is countered by a nongravitational force." Since we're discussing action/reaction pairs, and you had been using "counters" to refer to these pairs, what is one to conclude? Quote You, nor anyone else acknowledged that the normal forces from the desk and book constituted a third law pair until I cited several sources which explictly made that claim. Ghideon stated that in the second post in the thread (the first response). In my second post, I stated that Ghideon was correct. A couple of posts later, studiot agreed. A few posts later, I said "The reaction force to the normal force exerted by the table on the book would be the normal force exerted by the book on the table." So aside from us three (and whoever else; I stopped looking at that point), sure, nobody else acknowledged it... Quote You contradicted that claim with your earlier quotes: You failed to acknowledge that the opposed normal forces are an action/reaction pair, and static equilibrium in the example, is dependent on that result. Action/reaction is not dependent on equilibrium. I know this because you can have action/reaction forces when a system is not in equilibrium. I gave an example - the earth/moon system. They each exert a gravitational force on each other, equal in magnitude, opposite in direction. That's action/reaction. And yet, no equilibrium. Equilibrium, if it exists, is a matter of the second law. Not the third.
J.C.MacSwell Posted June 18, 2020 Posted June 18, 2020 3 hours ago, Nedcim said: You, nor anyone else acknowledged that the normal forces from the desk and book constituted a third law pair until I cited several sources which explictly made that claim. You contradicted that claim with your earlier quotes: You failed to acknowledge that the opposed normal forces are an action/reaction pair, and static equilibrium in the example, is dependent on that result. On 5/30/2020 at 11:22 PM, J.C.MacSwell said: A book, object 'A', on a table, object 'B', exert equal and opposite forces on each other. They are an action-reaction pair. Don't feel bad about me though...others have tried much harder to make points you prefer to ignore. (I quoted you directly when I posted that)
Recommended Posts