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Thread: nonconservative work

  1. #1 nonconservative work 
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    This isn't homework. I'm reviewing physics after many years of neglect. As with most of my posts, I made this problem up.

    Let object A have mass and object B have mass . One of A's surfaces is flat, as is one of B's. These flat surfaces are in contact and slide relative to each other in a straight line for a distance, x, experiencing kinetic friction. The kinetic friction, of magnitude f, is constant and is the only force acting on A or B in the direction of motion, and the only other forces acting on A and B are the normal forces pressing the surfaces together. The normal forces are assumed to be equal and opposite, so that the only acceleration of the objects is in the direction of friction. No net force acts on the center of mass.

    During a finite time interval, the speed of each surface relative to the other decreases from to due to friction. The nonconservative work done by each surface on the other is since each surface experiences the same displacement and the magnitude of the frictional force exerted by each surface on the other is the same since the two frictional forces comprise a force/reaction force pair.

    Therefore the change in kinetic energy of A, as seen from B's restframe is equal to the change in kinetic energy of B, , as seen from A's restframe, since we are assuming that no other forces accelerate the objects.

    Therefore , an absurd result.

    Something's wrong and it's driving me crazy! Please help me find the error in this reasoning, thanks.


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  3. #2  
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    Quote Originally Posted by inkliing View Post
    This isn't homework. I'm reviewing physics after many years of neglect. As with most of my posts, I made this problem up.

    Let object A have mass and object B have mass . One of A's surfaces is flat, as is one of B's. These flat surfaces are in contact and slide relative to each other in a straight line for a distance, x, experiencing kinetic friction. The kinetic friction, of magnitude f, is constant and is the only force acting on A or B in the direction of motion, and the only other forces acting on A and B are the normal forces pressing the surfaces together. The normal forces are assumed to be equal and opposite, so that the only acceleration of the objects is in the direction of friction. No net force acts on the center of mass.

    During a finite time interval, the speed of each surface relative to the other decreases from to due to friction. The nonconservative work done by each surface on the other is since each surface experiences the same displacement and the magnitude of the frictional force exerted by each surface on the other is the same since the two frictional forces comprise a force/reaction force pair.

    Therefore the change in kinetic energy of A, as seen from B's restframe is equal to the change in kinetic energy of B, , as seen from A's restframe, since we are assuming that no other forces accelerate the objects.

    Therefore , an absurd result.

    Something's wrong and it's driving me crazy! Please help me find the error in this reasoning, thanks.
    The situation is not symmetrical, you need to apply a force to B (absent in your calculations) in order to get it to start moving wrt A. That force, while present in frame A, is not present in frame B (since there is no such force acting on A, the only force acting on A is the "drag" force due to friction). In addition, neither A nor B are inertial frames, so there is no reason to believe that the speed of A wrt B is the same as the speed of B wrt A.
    In order to solve your problem, you need to introduce a third frame, an inertial one and to judge the motion of A and B wrt to that frame.


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  4. #3  
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    Thank you! It was staring me in the face.
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  5. #4  
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    Quote Originally Posted by inkliing View Post
    Thank you! It was staring me in the face.
    you are welcome, I am glad I could help
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