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B Collision of two objects

  1. Mar 26, 2017 #1
    Suppose there are two objects, denoted by A and B. When they collide with each other with the condition that A is stationary and B is in motion, B exerts an action force on A due to which A exerts a reaction force on B. Thus the net force exerted on A is FactionAB and the net force exerted on B is FreactionBA. Same is the case when A is in motion and B is at rest. But when A and B collide when they are in motion, the net force exerted on A is FactionAB + FreactionAB and the net force exerted on B is FactionBA + FreactionBA.
    Question: Is my reasoning correct for the case when A and B collide when they are in motion? Is the net force exerted on A and B when they collide when they are in motion greater in magnitude than the net force exerted on A and B when they collide when only one of them is in motion?
  2. jcsd
  3. Mar 26, 2017 #2


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    The force depends how fast they are moving relative to each other.

    In fact, all three scenarios are essentially the same: the same collision in different frames of reference. If you wanted to analyse a collision where both objects are moving it's a good idea to change your reference frame to one in which only one object is moving.

    Finally, for this reason, there is only really one pair of action -reaction forces in each case.
  4. Mar 26, 2017 #3
    @PeroK, Is it possible to explain that there is one pair of action-reaction forces without changing the frame of reference?
  5. Mar 26, 2017 #4


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    No, you got confused by the physically meaningless "action / reaction" terminology.

    There is one pair of equal but opposite forces, because there is one interaction. Forget the "action / reaction" labels. The case where both move shows why they are meaningless.
  6. Mar 27, 2017 #5
    As A.T says, there is only one force exerted by A on B and only one force exerted by B on A. These two forces are equal in magnitude and opposite in direction. This does not depend on which is moving and which is not. This is so in all cases.
  7. Mar 27, 2017 #6
    "action" an "reaction" are only logical concepts, who is who is relative, if we take the reference frame of one of the objects we can consider the moving object is "acting" and the stopped object "reacting" but in a third reference frame we would only have two objects colliding, the forces must be opposite so that the momentum is conserved.
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