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Homework Help: Physics Coursework: Oblique Collisions. Tips needed!

  1. Mar 21, 2013 #1
    1. The problem statement, all variables and given/known data

    I'm currently doing a coursework project for physics on oblique collisions. I have done two different experiments, both with preliminaries etc.. The first experiment involved suspending a hockey ball (henceforth known as the striking ball) on a bifilar pendulum and releasing it from a height to collide with a stationary hockey ball (the struck ball). The struck ball was on the edge of a circle with arcs of concentric circles, centred on the middle of the ball, drawn every two centimetres. This is shown here http://i.imgur.com/OFM7pqx. I varied the labeled distance *x* and recorded each run with a video camera.

    My final experiment used a ramp instead of a pendulum to start the collision and I used steel ball bearings instead of hockey balls. This is shown in the picture here- http://i.imgur.com/ystEiJT. The set up is broadly similar but this time I was able to calculate the velocity and angle of both balls' movement post-collision. The way I calculated the velocity was measuring distances traveled in a set number of frames (I knew the camera fps) and using the scale to convert to real distance and then calculate speed.

    2. Relevant equations

    Used GPE formula and equated it to KE to work out the speed of the striking ball, having known the height of release. Also used momentum formulae to compare pre and post collision momenta.

    3. The attempt at a solution
    In my analysis I'm comparing the pre- and post-collision momenta of the balls combined. The results seem broadly as expected - no resultant momentum in the y (up and down on screen) plane and near the same momentum in the x plane. I'm attributing the discrepancy in the x-plane to it not being perfectly elastic+friction. I wanted to look at the oblique collision formula but couldn't get my head around it. I do have the angles at which the balls traveled after collision but for some reason the graphs look terrible. This is most likely due to the regretful fact I have a small number of results.

    **I have a few questions if anyone can answer them:**

    * What should I be expecting the angle of the struck ball's movement to look like as the displacement x increases?
    * Is it possible to calculate the CoR?
    * Is there anything else I can likely analyse? (I have mass, speed, and angle of movement)
    * Are there any good websites that I might be able to read about the theory or expected results in this sort of collision?
    * Any other tips?

    If you need any more info to answer any questions I am happy to supply it. Thanks for reading.
  2. jcsd
  3. Mar 21, 2013 #2


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    An important consideration in oblique collisions is friction. Without friction, the struck ball will move along the line of centres of the balls at instant of collision. Just do a bit of geometry to relate that to displacement.
    With friction, the struck ball will pick up more momentum in the forward direction of the striking ball, correspondingly slowing that ball. Also, both balls will acquire spin. Depending on your set-up, the spin could result in a subsequently curved trajectory.
    For an elastic collision without friction, I believe the two balls move off afterwards at right angles. Not sure whether that applies with friction too.
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