swansont Posted May 6, 2015 Posted May 6, 2015 Why would tangential ever stop being tangential? I know radial is harder to define. Tangent is defined in terms of a circle. When there is no circle, how do you define a tangent?
DrP Posted May 6, 2015 Posted May 6, 2015 (edited) Rob - I don't think you have enough info to make that statement... it depends on why the disk is spinning... it was undefined. It could be driven at a constant speed with a motor or in the case of the roundabout it may have a pusher keeping it constant. Otherwise, of course it will slow (like the skater stretching out her arms), but that isn't really relevant your conversation is it? I mean, it's been going round in circles for 10 pages. Edited May 6, 2015 by DrP
swansont Posted May 6, 2015 Posted May 6, 2015 This gives rise to an outward circulating force. No, it doesn't.
Mike Smith Cosmos Posted May 6, 2015 Author Posted May 6, 2015 I think I can see what is happening here. A mass is travelling in a straight line having linear inertia . By an application of the appropriate orthogonal ( 90 degrees to ) force on the mass. This force can be made to be an inward facing force ( centripetal ) . The mass then moves in such a way as to follow a circle . The linear inertia is converted to an angular momentum. This would be some form of balanced , stable state at a set radius . If the mass is held by a stiff but flexible radius. Then if angular velocity is increased the mass will ( by an increase in centrifugal force travel up to a higher radius , until a new orbit is Achieved , whereby the system is again balanced , but at an extended new radius. Higher . Again with further input , and increase of orbital velocity , the mass would move outward ( Centrifugally ) under the influence of increased angular momentum. If the radius was then cut the mass would move from the circle edge , tangentially. It would seem that centripetal force pushes inwards and changes linear inertia ( straight line ) into a circle of exact size to reflect the linear inertia magnitude. It would seem the centrifugal forces are generated by increased circumferential speed. This in turn causes the mass to be under pressure to rise to a higher value of circular radius. With appropriately adjusted values of centripetal force , and in turn angular momentum. Mike
tar Posted May 6, 2015 Posted May 6, 2015 (edited) Thread, This link clarifies things for me a bit. http://en.wikipedia.org/wiki/Reactive_centrifugal_force I think, my experiences on the playground turnabout, and my muses about storing energy in concentric rings making up a flywheel, with pins that engaged the next outer ring as the speed of inner ring caused its pins to protrude into the catch groves of the next outer ring, are experiences and ideas that are dealing with reactive centrifugal force. SwansonT is only talking about inertial centrifugal force, which is an apparent or ficticious force that "looks" like a force is acting, from the point of view of an observer in the rotating frame, that does not exist when observed from an inertial frame. The ball looks like its curving under the influence of a force, but its really going straight, making the apparent force, a fictious force. On the other hand, a reactive centrifugal force really does enable a centrifugal clutch to operate, as each component of the restraint, that causes the acceleration of the body that wants to go in a straight line to follow a circular path applies an equal and opposite force to the "ring" or piece of the constraint closer to the center. Regards, TAR So one should not use the term centrifugal force without specifying if they are talking about reactive centrifugal force, that is countering the centripedal force at every integral of the constraint, or if they are talking about apparent, or inertial, or ficticious centrifugal force. Edited May 6, 2015 by tar
swansont Posted May 6, 2015 Posted May 6, 2015 I think I can see what is happening here. A mass is travelling in a straight line having linear inertia . By an application of the appropriate orthogonal ( 90 degrees to ) force on the mass. This force can be made to be an inward facing force ( centripetal ) . More precisely, that force is the inward facing force. The mass then moves in such a way as to follow a circle . The linear inertia is converted to an angular momentum. Not really. The system still has linear momentum. As has been mentioned already, discussion of angular momentum should take place in another thread. This would be some form of balanced , stable state at a set radius . If the mass is held by a stiff but flexible radius. Then if angular velocity is increased the mass will ( by an increase in centrifugal force travel up to a higher radius , until a new orbit is Achieved , whereby the system is again balanced , but at an extended new radius. Higher . Again with further input , and increase of orbital velocity , the mass would move outward ( Centrifugally ) under the influence of increased angular momentum. If the radius was then cut the mass would move from the circle edge , tangentially. There is no centrifugal force. So, no. It would seem the centrifugal forces are generated by increased circumferential speed. It may seem that way, but upon more careful inspection, it turns out that they are not. There are no centrifugal forces on the object in these scenarios. On the other hand, a reactive centrifugal force really does enable a centrifugal clutch to operate, as each component of the restraint, that causes the acceleration of the body that wants to go in a straight line to follow a circular path applies an equal and opposite force to the "ring" or piece of the constraint closer to the center. Regards, TAR So one should not use the term centrifugal force without specifying if they are talking about reactive centrifugal force, that is countering the centripedal force at every integral of the constraint, or if they are talking about apparent, or inertial, or ficticious centrifugal force. A reactive centrifugal force is exerted by the object, not on the object, and as such has no relevance to the discussion. The acceleration of an object is only the result of forces acting on it. An object moving in a circle feels a net inward force, which must be exerted by something (e.g. tension in a rope, gravity) One of the many problems in this misconception is that the centrifugal force is conjured from thin air. Nothing is exerting it, and yet it is claimed to be felt by the object. 1
Robittybob1 Posted May 6, 2015 Posted May 6, 2015 (edited) Tangent is defined in terms of a circle. When there is no circle, how do you define a tangent? But we did have a center point and a rotation about that point, so there are infinite radii and infinite tangents. The tangential line is not dependent on the rate of rotation, but radial length and center point. I like Mike's diagram and I'll wait to see where the thread goes before setting up another discussing something similar. More precisely, that force is the inward facing force. Not really. The system still has linear momentum. As has been mentioned already, discussion of angular momentum should take place in another thread. There is no centrifugal force. So, no. It may seem that way, but upon more careful inspection, it turns out that they are not. There are no centrifugal forces on the object in these scenarios. A reactive centrifugal force is exerted by the object, not on the object, and as such has no relevance to the discussion. The acceleration of an object is only the result of forces acting on it. An object moving in a circle feels a net inward force, which must be exerted by something (e.g. tension in a rope, gravity) One of the many problems in this misconception is that the centrifugal force is conjured from thin air. Nothing is exerting it, and yet it is claimed to be felt by the object. But didn't you say centrifugal force was the equal and opposite force to the centripetal force? What is the opposing force to the centripetal force otherwise? So can you have one and not the other? Idea! The moment the centripetal force stops the centrifugal force stops so the object has no forces on it from that time on and hence travels in a straight line. (That is one of Newton's Laws isn't it.) Edited May 6, 2015 by Robittybob1
pzkpfw Posted May 6, 2015 Posted May 6, 2015 (edited) ... A mass is travelling in a straight line having linear inertia . By an application of the appropriate orthogonal ( 90 degrees to ) force on the mass. This force can be made to be an inward facing force ( centripetal ) . The mass then moves in such a way as to follow a circle . ... Your language is a bit sloppy here, but close enough (heck, mine's not perfect). We can see there are two factors at play, the momentum (mass x velocity) of the object - it wants to go in a straight line - and the centripetal force. The resulting velocity (force is required to change momentum, which is a vector quantity) means the object travels in that circle. Given that, now think about what would happen if that centripetal force was just a little bit too small. What would the object do? ... Then if angular velocity is increased the mass will ( by an increase in centrifugal force travel up to a higher radius , until a new orbit is Achieved , whereby the system is again balanced , but at an extended new radius. Higher . Again with further input , and increase of orbital velocity , the mass would move outward ( Centrifugally ) under the influence of increased angular momentum. ... No! You suddenly introduce a centrifugal force when there's no need to. What's happened here, is that with the higher rotational speed, a higher centripetal force is needed to keep the mass in that circle. The desire of that mass to go in a straight line is stronger, as it's going faster, so more centripetal force is needed. It doesn't move out because some centrifugal force pushes it out, it appears to "move out" because there isn't enough centripetal force to keep it in. (What eventually happens depends on what's applying the centripetal force (spring, elastic, whatever)). ... It would seem that centripetal force pushes inwards and changes linear inertia ( straight line ) into a circle of exact size to reflect the linear inertia magnitude. ... If the object in question is moving in a circle, then more or less, yes. ... It would seem the centrifugal forces are generated by increased circumferential speed. This in turn causes the mass to be under pressure to rise to a higher value of circular radius. With appropriately adjusted values of centripetal force , and in turn angular momentum. ... No, there is no centrifugal (outwards) force. After all, where would it come from? Is the spring or elastic holding the mass suddenly pushing on that mass in a significant way? The mass wants to keep going straight, if the centripetal force is too small, the mass will try to. Edited May 6, 2015 by pzkpfw
Mordred Posted May 6, 2015 Posted May 6, 2015 Correct Newtons three laws of inertia. I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. alternatively worded, An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced force. II. The relationship between an object's mass m, its acceleration a, and the applied force F is F = ma. Acceleration and force are vectors (as indicated by their symbols being displayed in slant bold font); in this law the direction of the force vector is the same as the direction of the acceleration vector. III. For every action there is an equal and opposite reaction. (Action-reaction law) There is numerous variations on the wordings of those three laws, but they amount to the same. The wiki definitions are usually preferred in textbooks. http://en.m.wikipedia.org/wiki/Newton's_laws_of_motion
tar Posted May 6, 2015 Posted May 6, 2015 Pzkpfw, OK, the centripedal force is too small, and the thing tries to go in a tangent, instead of straight line, but it sure "looks" like the thing is trying to go radially outward. I took a disk of clay and stuck three verical toothpicks in it with three balls, small medium and large on it, and attempted to spin the disc in a horizonal orientation on a pen. More of a start and stop, and I could "feel" the inertia of the large ball trying to not get moving...but the large ball wound up tipping the toothpick radially outward. I then made three same size balls and repeated the experiment. They toothpicks were too deep in the clay, provided enough centripedal force, and the toothpicks did not tip. I put the toothpicks in the clay only half as deep as before and repeated the experiment. They all three tipped outward. One radially outward, one a little behind radially, and one a little ahead of radially. There seems to be another aspect to this discussion to consider, and that is the force that starts the disc spinning and overcomes the inertia of the stationary disc. The force I applied to the clay was a repeated tangential force on the circumference of the disc. Each pulse coaxed a slowing disc from its slowing condition, to a pulse that coaxed it in its circular rotation. Once the toothpick started to tip, the ball was "outside" its original circle and its original tangent line and its moment of inertia, wanting to "stay" out there, seems to cause the toothpick to tip even more easily outward. Mike, Here is a picture illustrating what I was mentioning earlier about the centripedal/reactive centrifugal force NOT being exactly counter gravity, in New York. The one toothpick points toward/away from the center of the Earth. The other toothpick points toward/away from the axis of rotation somewhere between the center of the Earth, and the North Pole.
swansont Posted May 6, 2015 Posted May 6, 2015 But we did have a center point and a rotation about that point, so there are infinite radii and infinite tangents. The tangential line is not dependent on the rate of rotation, but radial length and center point. I like Mike's diagram and I'll wait to see where the thread goes before setting up another discussing something similar. "Infinite tangent"? If there is no circle, there is no tangent. The line is still a line, but don't call it a tangent when there is no circle. But didn't you say centrifugal force was the equal and opposite force to the centripetal force? What is the opposing force to the centripetal force otherwise? So can you have one and not the other? I did say that. I said other things, too, which you are ignoring. The only forces we care about are the ones exerted on the object, so the reaction centrifugal force is irrelevant. It isn't exerted on the object. Idea! The moment the centripetal force stops the centrifugal force stops so the object has no forces on it from that time on and hence travels in a straight line. (That is one of Newton's Laws isn't it.) But if the centrifugal force and centripetal canceled, you'd still have to have straight line motion. An object moving in a circle has a centripetal force on it as the net force (which has some source, as I said). There is nothing more to the story. Anything you add beyond this point is fiction. Complicating the scenario will not change the physics. The best course of action IMO is to stop trying to run with the ball, and learn Newton's laws of motion before doing anything else. Because you three (Rb1, Mike and tar) obviously have not learned how to apply Newton's laws of motion. Any conclusions you arrive at are likely to be wrong. (At best, they will be accidentally right, i.e. right for the wrong reason) OK, the centripedal force is too small, and the thing tries to go in a tangent, instead of straight line, but it sure "looks" like the thing is trying to go radially outward. The tangent is the straight line, and is not directed radially outward. It is actually perpendicular to that. 1
tar Posted May 7, 2015 Posted May 7, 2015 had some upload issues 1. SwansonT, Right, the tangent is the straight line. And there is no force directed radially outward, according to F=Ma, except my toothpick tipped radially outward, for some reason. Just trying to figure the reason. Regards, TAR
pzkpfw Posted May 7, 2015 Posted May 7, 2015 (edited) Pzkpfw, OK, the centripedal force is too small, and the thing tries to go in a tangent, instead of straight line, but it sure "looks" like the thing is trying to go radially outward. ... I don't think anybody is arguing against what it seems like. The term centrifugal, e.g. as in "centrifugal clutch", wouldn't exist otherwise. It just turns out that's not (i.e. outwards force) what's really happening. ... ... except my toothpick tipped radially outward, for some reason. Just trying to figure the reason. Regards, TAR You've made a complex scenario - with the toothpick standing upright in the clay at the start. The centripetal acceleration attempting to keep the blob going in a circle ends up putting a rotation of that toothpick (in the plane of the rotation: the base of the toothpick isn't in line with the blob at the end of it), and the clay holding it gives way. It still isn't evidence of centrifugal force. Edited May 7, 2015 by pzkpfw 1
tar Posted May 7, 2015 Posted May 7, 2015 Pzkpfw, No, not evidence of a force that does not exist...but the clay ball at the end of toothpick is not unlike my 10 year old head on the playground roundabout. Earlier, it was suggested that the human instrument, was not sufficient to measure forces and the direction thereof. I am only submitting evidence that the toothpick tipped outward, and looking at the path of the tip, evidenced by the slot in the clay, where it stood and fell, the slot is a radial line, and I only applied a tangential force to the circumference of the ring. Somehow, the inward force, the centripedal force and the inertia of the blob, both the stationary inertia, and the tangential inertia, conspired to move the blob radially outward. No force was applied in that direction, but that is the direction the toothpick, tipped. Regards, TAR
pzkpfw Posted May 7, 2015 Posted May 7, 2015 Pzkpfw, No, not evidence of a force that does not exist...but the clay ball at the end of toothpick is not unlike my 10 year old head on the playground roundabout. Earlier, it was suggested that the human instrument, was not sufficient to measure forces and the direction thereof. I am only submitting evidence that the toothpick tipped outward, and looking at the path of the tip, evidenced by the slot in the clay, where it stood and fell, the slot is a radial line, and I only applied a tangential force to the circumference of the ring. Somehow, the inward force, the centripedal force and the inertia of the blob, both the stationary inertia, and the tangential inertia, conspired to move the blob radially outward. No force was applied in that direction, but that is the direction the toothpick, tipped. Regards, TAR Yeah, you're still not getting it. The blob seems to have moved radially "outward" from the base, but that's not what happens. The blob has tried to go in a straight line while the base has gone inward (the circle). 1
Mordred Posted May 7, 2015 Posted May 7, 2015 (edited) I think it may be time to step back and review linear force, and how Newtons laws of inertia applies, before handling the angular momentum case. Let's start with a mass 10 kg on a frictionless surface. Until sufficient force is applied to move the mass of 10 kg the object will remain at rest. F=ma, calculate The force needed to accelerate 10kg 10 m/s^2. If that force is applied for 10 seconds, what is its final velocity? Now keeping in mind in the above frictionless surface, how much force will it take to bring the same object moving at the final velocity will take to bring said object to rest? Part b assuming your still at the final velocity, and the object is moving at 90 degree, during its motion a new force of 100 Newtons is applied at 0 degrees what is the objects final direction ? In degrees. The reason I want you guys to think about the above is that we've been going around in circular and repeated explanations, and we're not getting anywhere. So lets step back to more simple problems before handling the angular case. The above isn't intended as insult, it's a step back and think the problem through correctly Now approach a 10 kg pendulum, at rest, apply 1000 Newtons of force at 90 degrees for one second. Which direction and velocity should the ball on the pendulum move? What force is preventing the pendulum to move in the same direction as the above ? Here is some key relations. [latex]a_c=\frac{v_2}{r}[/latex] [latex]f_c=a_c r[/latex] [latex]f_c=m\frac{v^2}{r}[/latex] Here is angular velocity http://en.m.wikipedia.org/wiki/Angular_velocity this site covers basic torque calcs http://www.engineeringtoolbox.com/motion-formulas-d_941.html Edited May 7, 2015 by Mordred
Mike Smith Cosmos Posted May 7, 2015 Author Posted May 7, 2015 (edited) TAR . THANKS! For the model demo! Great , you have demonstrated it . brilliant , you are my friend for life ! Probably a dubious accolade ! http://www.scienceforums.net/uploads/monthly_05_2015/post-15509-0-26827800-1430955133.jpg Wow! You guys have been busy while I have been asleep. . I like it! . like it a lot ! EUREKA , .....BINGO! .... It is perfectly clear now this WHOLE THING is about the CONVERSION of . . . LINEAR MOTION . TO . CIRCULAR MOTION and Visa Versa ( other way round) CIRCULAR MOTION. . TO. . LINEAR MOTION . See Diagram :- Mike Ps . Probably to be separate thread :- ...{{{{ --------- The Conversion of Linear Inertia To. Angular momentum . ---------}}}}}}}} Robotitybob. Are you going to do it or am I to do it ? It was suggested by Swansont . That it would make a good or mandatory , separate thread ? . Mike Hey . Ho ! Tum-- Tiddly -- Tum ..! Today's the day ' we break the Bank '. at Monty Carlo.... Tum-- Tiddly -- Tum ..! Or did I just Reinvent the Wheel ? ... Mike Tum-- Tiddly -- Tum ..! . . Tum-- Tiddly -- Tum ..! . . Today's the day ' we break the Bank ' at Monty Caaarlloo.... Tum-- Tiddly -- Tum ..! Edited May 7, 2015 by Mike Smith Cosmos -1
Robittybob1 Posted May 7, 2015 Posted May 7, 2015 (edited) "Infinite tangent"? If there is no circle, there is no tangent. The line is still a line, but don't call it a tangent when there is no circle. I did say that. I said other things, too, which you are ignoring. The only forces we care about are the ones exerted on the object, so the reaction centrifugal force is irrelevant. It isn't exerted on the object. But if the centrifugal force and centripetal canceled, you'd still have to have straight line motion. An object moving in a circle has a centripetal force on it as the net force (which has some source, as I said). There is nothing more to the story. Anything you add beyond this point is fiction. .... We also said the equal and opposite forces are not acting on the same object so they can never cancel each other out. So while the centripetal force is acting on the mass, the mass travels in a circle around the central point. The centrifugal force is acting on the center. So we are in agreement on that, in that there is only one force acting on the object. I agree the centrifugal force is not acting on the object. An arc (part of a circle) would have tangents so you don't need a circle to have a tangent.A pendulum would have tangents, so just part of a curve will do. Google definition of tangent: tangentˈtan(d)ʒ(ə)nt/Submit noun 1. a straight line or plane that touches a curve or curved surface at a point, but if extended does not cross it at that point. 2. a completely different line of thought or action. "Loretta's mind went off at a tangent" 3. MATHEMATICS the trigonometric function that is equal to the ratio of the sides (other than the hypotenuse) opposite and adjacent to an angle in a right-angled triangle. adjective adjective: tangent 1. (of a line or plane) touching, but not intersecting, a curve or curved surface. "this curve is tangent to the average cost curve" Edited May 7, 2015 by Robittybob1
swansont Posted May 7, 2015 Posted May 7, 2015 I agree the centrifugal force is not acting on the object. Which is why it's not a relevant part of the discussion.
Robittybob1 Posted May 7, 2015 Posted May 7, 2015 Which is why it's not a relevant part of the discussion. You could be right about that too, but then we'd have to back to the OP and see where the idea of centrifugal force came into the discussion.
swansont Posted May 7, 2015 Posted May 7, 2015 You could be right about that too, but then we'd have to back to the OP and see where the idea of centrifugal force came into the discussion. Keeping up with the discussion is such a burden… had some upload issues 1.pinkdisc3s.jpg SwansonT, Right, the tangent is the straight line. And there is no force directed radially outward, according to F=Ma, except my toothpick tipped radially outward, for some reason. Just trying to figure the reason. Regards, TAR You spun it. How did it tip radially outward, in the frame of the table on which you placed the device? Radially outward means no change in angle, i.e. the motion is purely radial. But if you are spinning it, there a change in angle. "Increasing in radius" ≠ "radially outward motion" The object increases in its distance from center because of its linear motion, as I showed in my previous drawing.
tar Posted May 7, 2015 Posted May 7, 2015 SwansonT, A reactive centrifugal force is taken into consideration when a propeller designer calculates the stresses and strains within the material. I read that inorder to analyze this, the engineer breaks the propeller into concentric rings of material, with the one ring acting upon both the ring on the inside and the ring on the outside. The reactive centrifugal force acts on the ring toward the inside, and the centripetal force acts on the ring to the outside. This analysis is not unuseful when considering the boy on the round-about. The position of the fluid in his inner ears is what is informing him of which way gravity is, and which way he is being "pulled". Which way he will "fall" if he lets go. This is where the rigid body analysis of the situation is somewhat limited, because it does not consider the position of the fluid in the inner ear of the boy. The motorcycle rider we were talking about before leans in the turn to put the bike between her and the direction her inner ears is telling her is down, the direction she will fall if she does not keep the bike and her ears balanced and normal to this "pull". The fluid in the inner ear is also trying to fly off on a tangent. Like the water at the bottom of the bucket on the end of the rope, it will settle at the bottom of the bucket, and the bucket's bottom is not pointed along the tangent, it is pointed normal to the tangent. It is pointed radially outward. Regards, TAR Thread, I have not yet looked at pictures of children on a circulating roundabout, but my specuation is, going by my memories as a child, that a not unusual position would be "braced against an outward pull" with the top of the head facing toward the axis of rotation and closer to horizonal the faster the round about is going. My speculation is that the body wants to be "upright" in respect to gravity, and where the fluid in the inner ears is, is the direction this 'down' is thought to be. Regards, TAR
swansont Posted May 7, 2015 Posted May 7, 2015 The fluid in the inner ear is also trying to fly off on a tangent. Like the water at the bottom of the bucket on the end of the rope, it will settle at the bottom of the bucket, and the bucket's bottom is not pointed along the tangent, it is pointed normal to the tangent. It is pointed radially outward. Where it is and which direction it moves are not the same thing. Neither the force nor the motion is radially outward.
tar Posted May 7, 2015 Posted May 7, 2015 SwansonT, The radial/tangential direction is somewhat ambiguous, looking at some of the videos on the internet of motorbikes being used to spin small roundabouts. The people have their legs on the other side of the center of rotation, and their shoulder blades and arms near the circumference. Each part of their body is describing a different circle, so there are an infinite amount of tangent lines to consider, even at one instant. All these infinitely numbered tangent lines, however, are still normal to the radial line that would describe the center of the person's body. What is interesting to me, is that the tangent lines can be drawn in both the direction of motion, and extend back in the other direction. The normal to these tangent lines is the radial line. When the young man slides over the bar in one of the videos I watched, his body is extended out radially from the roundabout. That is, it appears as if his feet clear the ring, at the same time. It happens very fast, and he stays in relatively the same orientation to the rider that remains on the ride, as he flies off and winds up on the ground, "outward" from the center of rotation. So, what combination of forces, moments of inertia and cavity positioning, makes the fluid in the inner ear gravitate to the bottom of the cavity? Regards, TAR
swansont Posted May 7, 2015 Posted May 7, 2015 The radial/tangential direction is somewhat ambiguous, No, they are not. The radial direction is directed radially outward from the center, through the point, and the tangential is perpendicular to the radial, touching the circle at only the one point. They are uniquely determined. Ambiguity sets in when you try and redefine what is meant by radial and tangential. Or try and complicate the problem. One reason we use point masses in our examples. What is interesting to me, is that the tangent lines can be drawn in both the direction of motion, and extend back in the other direction. Lines are infinitely long. Interesting, perhaps, but hopefully not surprising. It happens very fast, and he stays in relatively the same orientation to the rider that remains on the ride, as he flies off and winds up on the ground, "outward" from the center of rotation. Outward, but not by moving radially outward, as has been discussed multiple times. Absolutely nobody has claimed that you won't end up further from the center of rotation when you get released from a rotating system. The false claims that are being rebutted are that (1) there is a force on you directed away from the center, i.e. radially out — there is none, and (2) that the motion is radially out — it isn't.
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