Robittybob1 Posted May 20, 2015 Posted May 20, 2015 Quote: "feel the force".. That force that they 'feel' is the centripetal force that they themselves are applying to the weight through the tension in the rope which gives the weight it's circular motion.... otherwise it would not go in a circle. Did you watch the video? Centripetal vs Centrifugal? ALL OF IT was explained very clearly and simply in that vid. Yes I watched it several times. The tension in the string is a two way force and the direction of the arrow showing the person sliding off is wrong. The "sliding off" bit is radial but the motion through the air once off is the addition of the radial rate and the tangential rate vectors. You try putting tension in the string without pulling it with two opposing forces.
DrP Posted May 20, 2015 Posted May 20, 2015 The video is not wrong - you are just misunderstanding it. I don't know how many more times we can say that it is NOT radial. Fine - keep choosing not to believe it then, even though it is there for you to see/read/experience. I think the misconception may be that the tangential motion, once the wheel/roundabout/pendulum/whatever, turns through 90 degrees then it appears radial. It is about your frame of reference. I give up for now. Over and out.
swansont Posted May 20, 2015 Posted May 20, 2015 Now it is quite obvious in the "moving mass" situation that since the inner part is not tied down the inward force is less than the outward force and that imbalance continues as the mass slides along the tube. There is no outward force on the mass. There is nothing outside of it that it can be pulling on. Stop assuming that you are right about this, and ways to explain it, if you think there is an outward force. You aren't. 3
Robittybob1 Posted May 20, 2015 Posted May 20, 2015 (edited) There is no outward force on the mass. There is nothing outside of it that it can be pulling on. Stop assuming that you are right about this, and ways to explain it, if you think there is an outward force. You aren't. I agree with you. The forces aren't from the outside, as you say, but from within, as I tried to explain The video is not wrong - you are just misunderstanding it. I don't know how many more times we can say that it is NOT radial. Fine - keep choosing not to believe it then, even though it is there for you to see/read/experience. I think the misconception may be that the tangential motion, once the wheel/roundabout/pendulum/whatever, turns through 90 degrees then it appears radial. It is about your frame of reference. OK if radial is not the right word - if I draw a radial line on the merry go round and put a coin on that line and spin it till the coin slides, I think it will follow that line to the outer edge. That would be a very easy experiment to do. Edited May 20, 2015 by Robittybob1 -2
imatfaal Posted May 20, 2015 Posted May 20, 2015 ...That would be a very easy experiment to do. So do it and stop speculating
DrP Posted May 20, 2015 Posted May 20, 2015 Rob: No it would NOT!!!! You have even posted a video YOURSELF (post #333) showing this to be false... it SPIRALS outward. It is moving tangentally and when the friction is overcome it slides off at a TANGENT. Because the whole system is rotating the coin then goes OUTWARD, YES, because it is following the tangent it was on just a second ago, but the circle has moved, so it APPEARS as if it is going outward.
pzkpfw Posted May 20, 2015 Posted May 20, 2015 (edited) ... if I draw a radial line on the merry go round and put a coin on that line and spin it till the coin slides, I think it will follow that line to the outer edge. ... Remember that experiment with the ball on the string, and how when the string was cut, the ball went off at a tangent? (Not outwards). That would be the equivalent of your merry go round, if there was suddenly zero friction between it and the coin. At the opposite end of the scale, if the friction was "enough", the coin wouldn't move at all (relative to the merry go round). In between, where the friction exists, but isn't enough to counter the momentum of the coin, the coin will appear to slide outwards (relative to the merry go round), as it tries to follow it's straight line course. The exact amount of friction will determine the path - but as noted above it will be a spiral, relative to the merry go round. At no time will there be a real outwards (centrifugal) force. (Edit: there might be a very specific balance of speed (perhaps needing continuous variable adjustment) of merry go round rotation, friction and coin weight where you'd get that spiral to be an apparent radial - but that (if even possible) would be a special case, not a proof of a general principle.) That would be a very easy experiment to do. Well, yeah, these experiments are easy. That's why modern science came to the conclusion it did about centrifugal vs. centripetal force. (Argument by authority is of course bogus, but do you really think this simple experiment would have the outcome you expect, and all of science has missed the conclusion you expect? Nobody has noticed? Really?) Edited May 20, 2015 by pzkpfw 3
swansont Posted May 20, 2015 Posted May 20, 2015 I agree with you. The forces aren't from the outside, as you say, but from within, as I tried to explain And yet you spoke of an outward force. There is none. As soon as you cite that, your explanation fails. OK if radial is not the right word - if I draw a radial line on the merry go round and put a coin on that line and spin it till the coin slides, I think it will follow that line to the outer edge. That would be a very easy experiment to do. That's the rotating frame of reference I mentioned. It's invalid for the analysis of forces. Newton's laws (as written) cannot be applied in a rotating frame. So even if this worked, it is not evidence of an outward force. I have provided the analysis. I think your experiment will fail, though, because when the friction fails, the coin will slide in a transverse direction. But by all means do it. 1
Mike Smith Cosmos Posted May 20, 2015 Author Posted May 20, 2015 (edited) Bringing in yet another scenario is probably not helpful. If you are at the center, the issue is different. You feel an outward force, because you have to exert the inward force on the mass that's moving in a circle.Yes , well I can see that and agree that. That is surely the reactive force that shows up, all over the place. ( no doubt , this is the initiative is with the centripetal force inward, the balance with the inertia ( mass )( moving even accelerating mass ) is the reactive centrifugal force ( wether you term this fictitious, cooking the books or whatever, Similarly with pushing a large concrete block along a concrete pavement. The push is real , the friction is fictitious , or reactive. The friction can not rear up and move a block about , and will disappear when you stop pushing. So it is with the whirling ' whatever ' --------- But ----- But there appears occasions when the centrifugal is acting as an initiative , not the reaction . This when the mass is in the mode of straight line motion ( inertia ) , and curves Barr the way .then the Barr surely reacts to being shoved in the way friction resists to being Overcome. In this case the initiative is in the moving mass ( possibly even accelerating ) , therefore a force , away from centre , and the fictiseous is in the reactive mode towards the centre. Some not all of the examples mentioned above . For example I initiate a straight line velocity, acceleration , in my cloth and lead pellet Lizard. I do this by my long cardboard tube . It's not long before my straight line trajectory and force is met with an obstruction by way of the walls of the tube that are attempting to follow a curve as my arms are restricting , to some extent the movement of the tube into a circle. A reactive force ( centripetal ) reacts with the lizard , and produces a resistive force ( equivalent of friction ) . But the forward inertia wins , and out pops the lizard. Then everything changes , as there is no longer restriction, reaction , and the motion is as previously described as tangential plus , some gained velocity in another direction . Now whether the names and directions of the forces is leading to ambiguity , I wonder ? Mike Edited May 20, 2015 by Mike Smith Cosmos
swansont Posted May 20, 2015 Posted May 20, 2015 Yes , well I can see that and agree that. That is surely the reactive force that shows up, all over the place. ( no doubt , this is the initiative is with the centripetal force inward, the balance with the inertia ( mass )( moving even accelerating mass ) is the reactive centrifugal force ( wether you term this fictitious, cooking the books or whatever, If you are the one moving in a circular path, the fictitious centrifugal force is when you analyze things in the rotating frame you are in. The reactive centrifugal force is the force you exert in reaction to the centripetal force on you. These are different things. Similarly with pushing a large concrete block along a concrete pavement. The push is real , the friction is fictitious , or reactive. The friction can not rear up and move a block about , and will disappear when you stop pushing. The frictional force is real, and the reaction force to the frictional force is also a frictional force. The ground exerts a frictional force on the block, so the block exerts a frictional force on the ground. --------- But ----- But there appears occasions when the centrifugal is acting as an initiative , not the reaction . This when the mass is in the mode of straight line motion ( inertia ) , and curves Barr the way .then the Barr surely reacts to being shoved in the way friction resists to being Overcome. In this case the initiative is in the moving mass ( possibly even accelerating ) , therefore a force , away from centre , and the fictiseous is in the reactive mode towards the centre. Some not all of the examples mentioned above . For example I initiate a straight line velocity, acceleration , in my cloth and lead pellet Lizard. I do this by my long cardboard tube . It's not long before my straight line trajectory and force is met with an obstruction by way of the walls of the tube that are attempting to follow a curve as my arms are restricting , to some extent the movement of the tube into a circle. A reactive force ( centripetal ) reacts with the lizard , and produces a resistive force ( equivalent of friction ) . But the forward inertia wins , and out pops the lizard. Then everything changes , as there is no longer restriction, reaction , and the motion is as previously described as tangential plus , some gained velocity in another direction . Now whether the names and directions of the forces is leading to ambiguity , I wonder ? Mike There is no outward force on the lizard. I have analyzed this and posted it already. The force is inward. The centripetal force here IS friction. The way you worded this raises the issue of whether you are using "reactive force" the way everyone else is.
Robittybob1 Posted May 20, 2015 Posted May 20, 2015 (edited) Remember that experiment with the ball on the string, and how when the string was cut, the ball went off at a tangent? (Not outwards). That would be the equivalent of your merry go round, if there was suddenly zero friction between it and the coin. At the opposite end of the scale, if the friction was "enough", the coin wouldn't move at all (relative to the merry go round). In between, where the friction exists, but isn't enough to counter the momentum of the coin, the coin will appear to slide outwards (relative to the merry go round), as it tries to follow it's straight line course. The exact amount of friction will determine the path - but as noted above it will be a spiral, relative to the merry go round. At no time will there be a real outwards (centrifugal) force. (Edit: there might be a very specific balance of speed (perhaps needing continuous variable adjustment) of merry go round rotation, friction and coin weight where you'd get that spiral to be an apparent radial - but that (if even possible) would be a special case, not a proof of a general principle.) Well, yeah, these experiments are easy. That's why modern science came to the conclusion it did about centrifugal vs. centripetal force. (Argument by authority is of course bogus, but do you really think this simple experiment would have the outcome you expect, and all of science has missed the conclusion you expect? Nobody has noticed? Really?) To someone looking from a central but stationary position with respect to the merry go round spinning beneath them, they might view the coin's path as a spiral but what I'm talking about is if you were to take multiple photos of this spiral path from this view point would the coin always be on the line we drew on the merry go round? I predict it will, for it only required a centripetal force to keep it in place, and that centripetal force would have acted along that exact same "radial line" And yet you spoke of an outward force. There is none. As soon as you cite that, your explanation fails. That's the rotating frame of reference I mentioned. It's invalid for the analysis of forces. Newton's laws (as written) cannot be applied in a rotating frame. So even if this worked, it is not evidence of an outward force. I have provided the analysis. I think your experiment will fail, though, because when the friction fails, the coin will slide in a transverse direction. But by all means do it. Alright, you seem to want me to prove you wrong. If I drew a radial line on the merry go round, how would I draw a transverse line from where the coin will start off from? (My two statements or questions definitely didn't deserve negative reputation points. Who is this mean person?) Edited May 20, 2015 by Robittybob1 -3
dimreepr Posted May 20, 2015 Posted May 20, 2015 To someone looking from a central but stationary position with respect to the merry go round spinning beneath them, they might view the coin's path as a spiral but what I'm talking about is if you were to take multiple photos of this spiral path from this view point would the coin always be on the line we drew on the merry go round? I predict it will, for it only required a centripetal force to keep it in place, and that centripetal force would have acted along that exact same "radial line" Alright, you seem to want me to prove you wrong. If I drew a radial line on the merry go round, how would I draw a transverse line from where the coin will start off from? (My two statements or questions definitely didn't deserve negative reputation points. Who is this mean person?) It’s not up to us to show you’re wrong; it’s up to you to show you’re right, over to you.
Robittybob1 Posted May 20, 2015 Posted May 20, 2015 It’s not up to us to show you’re wrong; it’s up to you to show you’re right, over to you. But what do you think will happen even before we run the experiment? Swansont has chosen a transverse line but I'm not sure what direction that is, but what about you? I have chosen the radial line. If you make a prediction I will check it out this weekend for I know where there is a merry go round.
dimreepr Posted May 20, 2015 Posted May 20, 2015 The point is it doesn't matter what I think “extraordinary claims demand extraordinary evidence”- David Hume.
Robittybob1 Posted May 20, 2015 Posted May 20, 2015 The point is it doesn't matter what I think “extraordinary claims demand extraordinary evidence”- David Hume. It is just simple physics, nothing extraordinary yet! Come on make a choice before the results are in please.
dimreepr Posted May 20, 2015 Posted May 20, 2015 (edited) The physics have been explained, ad nauseam, everything else is, by definition, extraordinary. Edited May 20, 2015 by dimreepr 3
swansont Posted May 20, 2015 Posted May 20, 2015 Alright, you seem to want me to prove you wrong. If I drew a radial line on the merry go round, how would I draw a transverse line from where the coin will start off from? (My two statements or questions definitely didn't deserve negative reputation points. Who is this mean person?) All that's required for a transverse component is a deviation from the radial line you draw.
Mike Smith Cosmos Posted May 20, 2015 Author Posted May 20, 2015 (edited) I think there might be a case for bringing into play here " the equivalence principle " There seems to constantly arise in all of the discussion about the forces and movements , that :- Something looks like xxxxxx but is not A xxxxxxx force appears to be there , but is not A mass,s movement looks like it is radial, but is not radial . Etc etc Instead of all this ' fictitious ' , housekeeping , concoctions . Is there not a case for simplifying by using the equivalence principal. Two things , if the behave the same , can be viewed as the same. Like Einstein's person in a lift in outer space or accelerating space ship and gravity . Behave the same , feel the same , so gravity is an acceleration , or however it goes. If centrifugal force ' feels' like there is a force there , looks like there is a force there, and makes things move as if there is a force there , Is there not a case for saying " there is a force there " even though there are mathematical and logical arguments to say " there is no force there " is there a way of simplifying to look more at how things ' appear ' to be? When I was teaching basic physics . I used to get students to be sealed in a household removal box, I had to teach about the basic forces .. Push , pull, twist, turn, stretch, squash ... The verbal results were interesting .. One such was they could not distinguish between Push.. And ..pull , turn had some interesting comment. Mike Ps on the internet , there seems gazillions of individuals that are discussing what they think " centrifugal force is " does this not indicate appearance and current physics reality seem to cause a confusing story line . Edited May 20, 2015 by Mike Smith Cosmos
swansont Posted May 20, 2015 Posted May 20, 2015 But what do you think will happen even before we run the experiment? Swansont has chosen a transverse line but I'm not sure what direction that is, but what about you? I have chosen the radial line. If you make a prediction I will check it out this weekend for I know where there is a merry go round. No, to clarify: I chose a transverse component, i.e. the motion will not be purely radial. If centrifugal force ' feels' like there is a force there , looks like there is a force there, and makes things move as if there is a force there , Is there not a case for saying " there is a force there " even though there are mathematical and logical arguments to say " there is no force there " is there a way of simplifying to look more at how things ' appear ' to be? No, there isn't. This isn't a poetry class, where you get to give your interpretation of a particular verse, or a matter of opinion. This is not subjective. There is only one right answer, and it has to align with the definitions and laws we have that apply to motion. You don't get points for doing it the wrong way.
Mike Smith Cosmos Posted May 20, 2015 Author Posted May 20, 2015 No, there isn't. This isn't a poetry class, where you get to give your interpretation of a particular verse, or a matter of opinion. This is not subjective. There is only one right answer, and it has to align with the definitions and laws we have that apply to motion. You don't get points for doing it the wrong way. Yes I appreciate what you are saying here, BUT , what if the current model , even though the maths works , for all that is currently required of devices that use these specific ,convoluted explanations. Perhaps on a par with how the maths was correct , but convoluted for planetary motion, . Then a new perspective , a simpler one , made everything far clearer , leading on to greater understanding. If we tried , everything in circular motion , worked in a simple way as if we were in the centre of the circle . And things pulled, pushed, traveled, accelerated etc all in the way they appear to do. Is there not a case for exploring a simple way which gives simple results ? Or is this just a pipe dream ? Mike
Strange Posted May 20, 2015 Posted May 20, 2015 Then a new perspective , a simpler one , made everything far clearer , leading on to greater understanding. I fail to see how it could be much simpler than it is. (I am just baffled that this, and related threads, have gone on for many hundreds of posts, with you and Rob failing to grasp even the simplest concepts.) 1
pzkpfw Posted May 20, 2015 Posted May 20, 2015 (edited) ... If centrifugal force ' feels' like there is a force there , looks like there is a force there, and makes things move as if there is a force there , Is there not a case for saying " there is a force there " even though there are mathematical and logical arguments to say " there is no force there " is there a way of simplifying to look more at how things ' appear ' to be? ... If the centrifugal force were real, there would be a real force outwards on something being kept moving in a circle. And if so, when that force is removed (e.g. that string being cut) that thing would move outwards, not at a tangent. (The only time you appear to see outwards movement is when you constrain that movement in your tube.) Just stand somewhere with bare feet. Feel your weight on your feet. It's true that your mass attracts the Earth, but you have much much less mass than it. So where does that feeling of weight on your feet come from? (Stand on sharp gravel if you need to be reminded of that weight.) Is that 80 kg coming from the Earth "pulling you down" or are you pulling the Earth up? Or stand against a cliff face, push hard on it. Yes, the cliff is feeling the force of your push - but where is the force coming from? If the cliff suddenly vanished - you'd fall forward. If you suddenly stopped pushing on the cliff, would it jump forward? A force felt in opposition to another force may seem interchangeable, but there's a reason why science labels some forces fictitious and some real. ... If we tried , everything in circular motion , worked in a simple way as if we were in the centre of the circle . And things pulled, pushed, traveled, accelerated etc all in the way they appear to do. Is there not a case for exploring a simple way which gives simple results ? ... There may well be times or scenarios when pretending centrifugal forces are real is useful. It's not uncommon to pick and choose reference frames for ease in some situations. For example, we would calculate most experiments on Earth pretending Earth is sitting still, not rotating, orbiting, etc. The only "problem" is when you start treating your frame of convenience as the "real" one, and ignore the real physics of what's going on. Edited May 20, 2015 by pzkpfw
Mike Smith Cosmos Posted May 20, 2015 Author Posted May 20, 2015 If the centrifugal force were real, there would be a real force outwards on something being kept moving in a circle. And if so, when that force is removed (e.g. that string being cut) that thing would move outwards, not at a tangent. (The only time you appear to see outwards movement is when you constrain that movement in your tube.)Just stand somewhere with bare feet. Feel your weight on your feet. It's true that your mass attracts the Earth, but you have much much less mass than it. So where does that feeling of weight on your feet come from? (Stand on sharp gravel if you need to be reminded of that weight.) Is that 80 kg coming from the Earth "pulling you down" or are you pulling the Earth up?Or stand against a cliff face, push hard on it. Yes, the cliff is feeling the force of your push - but where is the force coming from? If the cliff suddenly vanished - you'd fall forward. If you suddenly stopped pushing on the cliff, would it jump forward?A force felt in opposition to another force may seem interchangeable, but there's a reason why science labels some forces fictitious and some real. Yes I do understand what you are saying, I just wonder if there is not another perspective which might just handle the circular motion - centrifugal force " mental , instinctive , conflict , non intuitive " , that 'gets at ' quite a few people , not just me. On a side note Michael Kaku appeared on panorama a few days ago saying " gravity does not pull things down " but rather " space is pushing down " This probably brings into play Einstein's view of distortions in space , even though it is still valid , even though it infers two masses attracting one another by maths Newtons m1m2 G / r-squared. The distortions in space caused by the two masses gives a more intuitive picture ( the trampoline image ) . Could there just possibly be a simple paradigm with reference to centrifugal force , even though the maths works things out in a different way . ( as happened with Newtons maths and Einstein's general theory of relativity and the dented trampoline. ) ? Could there be ? Mike
pzkpfw Posted May 20, 2015 Posted May 20, 2015 ... Could there be ? So far, the answer is clearly no. Simply trying to reverse a bunch of science and going back to claiming centrifugal force is real is contributing nothing. You may as well try to argue Earth stands still and the Sun orbits us. 1
studiot Posted May 20, 2015 Posted May 20, 2015 Let us take a fresh look at why not. Let us consider a body moving in a circle (or part of one), of radius R. Now we postulate a force, C, pushing this body radially outwards. There are two possibilities for the source of this force. 1) Something external to the body. 2) Something within the body itself. No names no pack drill at this point, just go with the flow. The weight on a string clearly has no external agents acting on it so that should eliminate (1). So consider (2). Well if the body moves itself out to a large radius either A) It must increase or maintain its rotational speed. B) It must reduce its rotational speed. (B) leads us to the situation that the body is rotating but pushing forever outwards. As it does so it continually slows down its rotation until it stops alltogether. As soon as it stops the outward force ceases and the bodies Kinetic Energy has magically disappeared to nothing. So B violates the conservation ov energy. So what about (A) Well if the rotational speed remains constant or increases then the tangential velocity increases and the Kinetic Energy increases without limit, all internally from an object. So (A) also violates conservation of energy, in the other direction. Which leads to the conclusion such a force cannot exist.
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