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Please Help ! Collision eating my head!

  1. Nov 1, 2013 #1
    1. The problem statement, all variables and given/known data


    In the figure shown, a ball of mass m collides perpendicularly on a smooth stationary wedge of mass M, kept on a smooth horizontal plane. If the coefficient of restitution of collision is e, then determine the velocity of the wedge after collision.

    2. Relevant equations

    can we conserve momentum when the collision is partially elastic ?



    3. The attempt at a solution
    i wrote→ mV°sinθ= MV + meV°sinθ
    is this right??
     

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  2. jcsd
  3. Nov 1, 2013 #2

    CWatters

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    Momentum is conserved in both Elastic and Inelastic collisions.
     
  4. Nov 1, 2013 #3
    yes but how in this case??
     
  5. Nov 1, 2013 #4
    this is partially elastic collision, value of e is not known to you...
     
  6. Nov 1, 2013 #5

    haruspex

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    The ball's velocity after impact is not eV0.
    What is the definition of the coefficient of restitution?
     
  7. Nov 2, 2013 #6
    this is e......
     

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  8. Nov 2, 2013 #7

    haruspex

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    OK, except that there should be a minus sign because the relative velocity will reverse.
    In the present problem it's a bit tricky to apply because of the different directions involved. You know the relative velocity before collision, but what should you take as the relative velocity afterwards? Any thoughts?
     
  9. Nov 2, 2013 #8
    yes.... the relative velocity will be respect to ground frame.....!!!!!!!!
     
  10. Nov 2, 2013 #9

    haruspex

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    Relative velocity doesn't need a frame. Or, if you prefer, it's the velocity of the one object in the frame of the other.
     
  11. Nov 3, 2013 #10

    haruspex

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    I don't know whether this is still of interest to you, but I have concluded that you cannot solve it using the definition of CoR in the usual way. There is no appropriate relative velocity after the collision. Instead, use the way in which the CoR affects the recovered energy.
    Treating the collision as compression of a spring, there comes a point at which compression is a maximum. At that point, the ball and the wedge have the same velocity in the direction of travel of the ball. You can work out (but it's not easy) what fraction of the original KE is now stored as PE. The affect of the CoR being < 1 is that not all of the PE is recovered again as KE. The fraction that is recovered is e2. This allows you to work out how much energy is lost. Fwiw, I get lost energy = ##\frac{(1-e^2)M}{M+m\sin^2(\theta)}##
     
  12. Nov 3, 2013 #11
    ya that seems to be a bit tricky........but how this gonna help me??
     
    Last edited: Nov 4, 2013
  13. Nov 4, 2013 #12
    this question is very hard....!!!!!!!
     
  14. Nov 4, 2013 #13

    tiny-tim

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    Hi Kishlay! :wink:

    I don't see the difficulty …

    Call the speeds after v and V.

    Write out the coefficient of restitution formula for v and |V - v|.

    Do conservation of momentum in the horizontal direction (because there are no external forces in that direction) …

    what do you get? :smile:
     
  15. Nov 4, 2013 #14

    haruspex

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    I very much doubt that is valid in the context of this problem. It ignores the contribution of the reaction from the ground.
     
  16. Nov 4, 2013 #15

    tiny-tim

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    the question doesn't give a coefficient of restitution between the block and the ground, so i think we're intended to apply the formula only to the interaction between the block and the ball
     
  17. Nov 4, 2013 #16

    haruspex

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    I made that assumption already (which I should have made explicit). If you think the relative velocity ratio can be applied, please state exactly what two velocities you would take for after the collision.
    As far as I am aware, Newton's Experimental Law is for two-body collisions. Newton found that for a given pair of objects the relative velocity ratio was constant. Initially this was just head-on collisions, but it's reasonably obvious how to extend it to oblique ones.
    In the CoM frame of reference, one can deduce that the fraction of energy conserved is e2. Thinking about the physics, it becomes clear that this is the fundamental fact: of the PE stored in the compression phase, a fixed fraction is released back to become KE. So NEL is a consequence, not a physical fundamental.
    It follows that the method I used to solve the problem is reliable and valid in the OP context. If you think you have a way to do it using velocity ratios it will need to produce the same answer.
     
  18. Nov 4, 2013 #17

    ehild

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    The ratio of the final to initial KE in the COM reference is exactly the square of the ratio between the speeds.

    ehild
     
  19. Nov 4, 2013 #18

    haruspex

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    Yes, that is the test I am proposing of whether the two selected speeds are valid, but what two speeds are you going to pick?
     
  20. Nov 4, 2013 #19

    ehild

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    Speed of approach is vo
    Speed of separation is ##|\vec v_{ball} -\vec V_{wedge}| ## with velocity vectors after the impact.

    ehild
     
  21. Nov 4, 2013 #20

    haruspex

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    That doesn't even work for an oblique two ball collision, let alone the three body case we have here.
     
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