Klaynos Posted May 22, 2015 Posted May 22, 2015 Rolling balls are certainly not frictionless else they wouldn't roll, they'd slide. Yes the situations are different, yours is more complicated, given this thread is suggest a simpler example might be better.
Robittybob1 Posted May 22, 2015 Posted May 22, 2015 (edited) Rolling balls are certainly not frictionless else they wouldn't roll, they'd slide. Yes the situations are different, yours is more complicated, given this thread is suggest a simpler example might be better. Good point. Ball bearings reduce friction not because they are frictionless but because they roll. http://www.scienceforums.net/topic/88420-centrifugal-forces-appear-to-act-opposite-to-gravity-how-is-this-possible/page-19#entry867736 I remember that post now. A rolling ball will have an extra angular momentum to consider. Is this what you used to see? Edited May 22, 2015 by Robittybob1
tar Posted May 22, 2015 Posted May 22, 2015 Thread, University Physics Sixth Edition Written by Francis W. Sears, Mark W. Zemansky and Hugh D. Young Published By Addison-Wesley Publishing Company Copy on my shelf-Reprinted with Corrections May 1983 The section on motion in a plane did not in any way teach me that there was such a force as centrifugal force. I was wrong to suggest that I had been taught differently than what is being described as established Physics in this thread. Any misconceptions are my own, and not assignable as faults in “our” understanding of motion. From the section on motion in a plane (copied without permission.) “Some readers may wish to add to the forces shown in Fig.5-12 an outward, “centrifugal” force, to “keep the body out there,” or to “keep it in equilibrium.” (“Centrifugal” means “fleeing a center.”) Let us examine this point of view. First, the body doesn’t stay there! A moment later it will be at a different position on its circular path…” “Those who wish to add a force to “keep the body in equilibrium” forget that the term equilibrium refers to a state of rest,or of motion in a straight line with constant speed. Here, the body is not moving in straight line, but in a circle. It is not in equilibrium, but has an acceleration toward the center of the circle and must be acted on by a resultant or unbalanced force to produce this acceleration. In this example there is no outward force on the body!” Regards, TAR 3
studiot Posted May 22, 2015 Posted May 22, 2015 Francis Sears (with and without Zemansky) books always offered a clarity of understanding missing IMHO from their great rivals and successors at this level, (Resnick and Halliday). +1
Robittybob1 Posted May 22, 2015 Posted May 22, 2015 Thread, University Physics Sixth Edition Written by Francis W. Sears, Mark W. Zemansky and Hugh D. Young Published By Addison-Wesley Publishing Company Copy on my shelf-Reprinted with Corrections May 1983 The section on motion in a plane did not in any way teach me that there was such a force as centrifugal force. I was wrong to suggest that I had been taught differently than what is being described as established Physics in this thread. Any misconceptions are my own, and not assignable as faults in “our” understanding of motion. From the section on motion in a plane (copied without permission.) “Some readers may wish to add to the forces shown in Fig.5-12 an outward, “centrifugal” force, to “keep the body out there,” or to “keep it in equilibrium.” (“Centrifugal” means “fleeing a center.”) Let us examine this point of view. First, the body doesn’t stay there! A moment later it will be at a different position on its circular path…” “Those who wish to add a force to “keep the body in equilibrium” forget that the term equilibrium refers to a state of rest,or of motion in a straight line with constant speed. Here, the body is not moving in straight line, but in a circle. It is not in equilibrium, but has an acceleration toward the center of the circle and must be acted on by a resultant or unbalanced force to produce this acceleration. In this example there is no outward force on the body!” Regards, TAR I haven't got that text any longer but you will just about guarantee that it is drawn with an immoveable center of rotation. It is this central point that the object's centrifugal force acts on the same center that is providing the centripetal forces. If there is allowed to be unbalanced forces in our discussion, what happens if the central point loses its immovable character? Then the centrifugal force over balances the centripetal part. I don't like the unbalanced concept for didn't Newton talk of equal and opposite forces, so you should see a balance at all times. With gravitational forces the objects move wrt each other in binary patterns orbiting around a barycenter. Even in the best set-up the immoveable central point will "move" to some degree unbelievable as that sounds (even I struggle with that).
swansont Posted May 22, 2015 Posted May 22, 2015 I haven't got that text any longer but you will just about guarantee that it is drawn with an immoveable center of rotation. It is this central point that the object's centrifugal force acts on the same center that is providing the centripetal forces. If there is allowed to be unbalanced forces in our discussion, what happens if the central point loses its immovable character? Then the centrifugal force over balances the centripetal part. I don't like the unbalanced concept for didn't Newton talk of equal and opposite forces, so you should see a balance at all times. With gravitational forces the objects move wrt each other in binary patterns orbiting around a barycenter. Even in the best set-up the immoveable central point will "move" to some degree unbelievable as that sounds (even I struggle with that). Sorry, what centrifugal force is acting on the rotating object? You haven't identified how this is provided (which is all that we're discussing at this point). No discussion other than you or Mike telling us what is providing that alleged force, and no introducing new scenarios: just an object that's moving in a circle (a ball on a string was the scenario, if you want a more descriptive system)
Phi for All Posted May 22, 2015 Posted May 22, 2015 No discussion other than you [Rob] or Mike telling us what is providing that alleged force, and no introducing new scenarios: just an object that's moving in a circle (a ball on a string was the scenario, if you want a more descriptive system) ! Moderator Note We're going to do it this way, this time. All other posts will be trashed. This thread has gone round in circles too long, we need resolution.
Robittybob1 Posted May 22, 2015 Posted May 22, 2015 Thanks. "what is providing that alleged force. ...... just an object that's moving in a circle (a ball on a string was the scenario)." Can you put tension on that string without two forces, one being applied at each end? If one force is called the centripetal force what is the other called?
studiot Posted May 22, 2015 Posted May 22, 2015 Can you put tension on that string without two forces, one being applied at each end? If one force is called the centripetal force what is the other called? The force in the string at the other end is called the anchoring, tying or fixing force. Are you quite clear which end is which and which direction the forces act in a string?
swansont Posted May 22, 2015 Posted May 22, 2015 Thanks. "what is providing that alleged force. ...... just an object that's moving in a circle (a ball on a string was the scenario)." Can you put tension on that string without two forces, one being applied at each end? If one force is called the centripetal force what is the other called? We're discussing the force on the ball.
Robittybob1 Posted May 22, 2015 Posted May 22, 2015 We're discussing the force on the ball. OK you tie a string to a ball and the other end to a pivot point. Where does the centripetal force come from?
swansont Posted May 22, 2015 Posted May 22, 2015 OK you tie a string to a ball and the other end to a pivot point. Where does the centripetal force come from? It's provided by the tension in the string. Which is physically attached to the ball. (We know it's inward because you can't push with a string)
Robittybob1 Posted May 22, 2015 Posted May 22, 2015 It's provided by the tension in the string. something missing there. You have to have the ball in transverse motion as well. The pivot point is an immovable object (a fixed point). It isn't capable of pulling the string or making the ball move.
Strange Posted May 22, 2015 Posted May 22, 2015 The pivot point is an immovable object (a fixed point). It isn't capable of pulling the string or making the ball move. In which case the ball isn't moving: it is lying on the floor with the string loosely connecting it to the pivot point. Why do you keep making these ludicrous diversions?
Robittybob1 Posted May 23, 2015 Posted May 23, 2015 (edited) In which case the ball isn't moving: it is lying on the floor with the string loosely connecting it to the pivot point. Why do you keep making these ludicrous diversions? Exactly. That is how the experiment starts off. the ball isn't moving: it is lying on the floor with the string loosely connecting it to the pivot point. It isn't a diversion, I am trying to get Swansont to tell me where the centripetal force comes from; progressing from that situation. What caused the tension in the string that he mentioned #462? That is where that angle of motion comes into the equation for if you throw the ball just in a radial direction that won't produce circular motion either. So it is more than just giving the ball initial momentum but a momentum component perpendicular to the radials, and that is the basis of angular momentum the "mv" part and then multiply that by the length of the radius or in total L = mvr where v = tangential velocity component of the initial momentum vector. Edited May 23, 2015 by Robittybob1
Strange Posted May 23, 2015 Posted May 23, 2015 Exactly. That is how the experiment starts off. I am trying to get Swansont to tell me where the centripetal force comes from; progressing from that situation. What caused the tension in the string he mentioned? This entire thread has been about a weight been swung in a circle on the end of string. And now you ask about a ball that is not moving? I can only assume you are posting random, irrelevant questions to keep the thread going and wind people up.
swansont Posted May 23, 2015 Posted May 23, 2015 Exactly. That is how the experiment starts off. No, that is before the experiment starts. In the situation under discussion, the ball is already moving in uniform circular motion. How it got moving (and who paid for it, what pattern is on the ball and what brand of string) is/are irrelevant. Now, will you finally address the question: what is physically causing the alleged centrifugal force?
Robittybob1 Posted May 23, 2015 Posted May 23, 2015 No, that is before the experiment starts. In the situation under discussion, the ball is already moving in uniform circular motion. How it got moving (and who paid for it, what pattern is on the ball and what brand of string) is/are irrelevant. Now, will you finally address the question: what is physically causing the alleged centrifugal force? OK I have told you how the ball achieves its angular momentum so where does the tension in the string come from?
StringJunky Posted May 23, 2015 Posted May 23, 2015 OK I have told you how the ball achieves its angular momentum so where does the tension in the string come from? Momentum. The forces that make up the string constitutes the centripetal force. In the absence of the string it will go on a straight line journey. The centripetal force exerted by the string is what makes the ball travel in a circle.
Robittybob1 Posted May 23, 2015 Posted May 23, 2015 (edited) Momentum. The forces that make up the string constitutes the centripetal force. In the absence of the string it will go on a straight line journey. The centripetal force exerted by the string is what makes the ball travel in a circle. Thanks, but can you use the word "momentum" in a sentence scientifically explaining how the string gets tension? Does the tension vary proportionally to momentum? The centripetal force is going to be Fc = mv^2/r where as momentum is just mv, so just looking at it the difference is a factor of v/r. v/r is that something like a frequency? v = 2 Pi r/T, t= period, divide both sides by r and we get v/r = 2 Pi/T, v/r = constant /T seconds, so can we call that a frequency? Edited May 23, 2015 by Robittybob1
Klaynos Posted May 23, 2015 Posted May 23, 2015 Where the centripetal force comes from on the string is irrelevant for the question of where a centrifugal force acting on the back comes from. If you can't answer the question admit it.
Robittybob1 Posted May 23, 2015 Posted May 23, 2015 Where the centripetal force comes from on the string is irrelevant for the question of where a centrifugal force acting on the back comes from. If you can't answer the question admit it. I would like to answer the question but first I've got to understand the question. You are the first to describe the "centrifugal force acting on the back". Where is the back? Since there is tension in the string I can only conceive of a force at each end of the string, so in your view which end of the string is the centripetal force acting? It surely must originate at the pivot point and that force is transmitted to the object via the tension in the string. So that leaves only one place where the centrifugal force can originate and that is at the other end of the string, in other words within and by the mass itself.
imatfaal Posted May 23, 2015 Posted May 23, 2015 v/r = w Angular Velocity. But - please try to stick to the questions. It is clear to all observers that you are clutching at straws. As someone (I think Studiot) suggested to Mike about a thousand posts, a few threads, and months ago - draw a diagram, a nice free-body diagram. Your system is the ball; not the ball and pivot, not the ball, pivot, and air, not the ball, pivot, air, park-bench, and small dog.
Klaynos Posted May 23, 2015 Posted May 23, 2015 I would like to answer the question but first I've got to understand the question. You are the first to describe the "centrifugal force acting on the back". Where is the back? On the ball, sorry autocorrect on my phone. Since there is tension in the string I can only conceive of a force at each end of the string, so in your view which end of the string is the centripetal force acting? It surely must originate at the pivot point and that force is transmitted to the object via the tension in the string. So that leaves only one place where the centrifugal force can originate and that is at the other end of the string, in other words within and by the mass itself. We're talking about the ball do the centripetal force acting on the ball comes from the string and as we're talking about the ball it is at the ball end. Where is any possible centrifugal force acting on the ball coming from?
Robittybob1 Posted May 23, 2015 Posted May 23, 2015 (edited) v/r = w Angular Velocity. But - please try to stick to the questions. It is clear to all observers that you are clutching at straws. As someone (I think Studiot) suggested to Mike about a thousand posts, a few threads, and months ago - draw a diagram, a nice free-body diagram. Your system is the ball; not the ball and pivot, not the ball, pivot, and air, not the ball, pivot, air, park-bench, and small dog. I was wondering if it was angular velocity. Thanks for the clarification. I can't accept that the whole system is only the ball. It must be all that is within the circle that the ball turns. Who decides such matters? If you look at the ball only how much of the circle do you analyse? Just a small portion to be sure it is travelling in a circle! OK if that was the case there is only one force acting on the ball, but .... Edited May 23, 2015 by Robittybob1 -1
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