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Mastering Physics: Ball hits wall Elastically

  1. Feb 24, 2005 #1
    First off let me say Im completely stumped. I dont remember teacher teaching this, and its definitely not in book(I went through it 3 or 4 times) and its not even the same book mastering physics uses...

    The Question is :
    What is the final angle that the ball's velocity vector makes with the negative y axis?

    The Given are:
    A ball of mass m moving with velocity v_i strikes a vertical wall. The angle between the ball's initial velocity vector and the wall is theta_i. The duration of the collision between the ball and the wall is Deltat, and this collision is completely elastic. Friction is negligible, so the ball does not start spinning. In this idealized collision, the force exerted on the ball by the wall is parallel to the x axis.

    an image cna be found here: http://session.masteringphysics.com/problemAsset/1010992/25/MLM_e2.jpg

    I have no clue on how to solve this... all I do know however is that physics is not my subject heh. Can anyone help lead me in right direction? (keep in mind I am seriously not good in physics [Math yes] physics no so try and keep it dumb if you can? I want to know how to get the answer..well I have to know actually

    Edit: Ok I know this much I have to find the Y component and X component and stick them together thing is... how? I was thiking it has to be sin(Theta_i) to get the piece that is rquired for the distance the ball is from the wall?
    Last edited: Feb 24, 2005
  2. jcsd
  3. Feb 24, 2005 #2

    Doc Al

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    Staff: Mentor

    Since the collision is elastic, you know energy is conserved. Hint: Is momentum conserved? (Remember that momentum is a vector; consider x and y components separately.)
  4. Feb 24, 2005 #3
    The momentum would be conserved wouldnt it? Well not completely though? Wouldnt a lot be lost because of the wall? I dont understand where you are pointing me with this? I know I have to treat x and y comp seperately but I dont truely know what teh heck an x and y component is in this prob maybe thats where my problem lies?
  5. Feb 24, 2005 #4

    Doc Al

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    Staff: Mentor

    I'll rephrase my question: Is momentum conserved in the x-direction? ... the y-direction? (Why?)

    Also: What's the final speed?

    Now write an equation for conservation of momentum. (Use a little trig to find the x and y components of the velocity.)
  6. Feb 24, 2005 #5


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    Staff Emeritus
    Science Advisor

    As Doc Al pointed out, momentum is a VECTOR. Initially, the momentum vector is pointed TOWARD the wall (because the ball is moving toward the wall). Is it pointed that way after the ball bounces? But it is true that energy is conserved.
  7. Feb 24, 2005 #6


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    Homework Helper

    Don't forget to deal with gravity. It is an elastic collision, but the magnitude of the velocity before and after the collision is not the same (deal with the x and y components separately).
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