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Homework Help: 2 dimensional elastic collision

  1. Nov 28, 2006 #1
    Hey, I'm having problems with this:

    A particle m with initial speed V1 collides with a particle at rest with mass 3m. After collision, particle with mass m is seen moving 30 degrees from its initial direction. What is the angle of motion of the 2nd particle after collision?

    I can get three equations from momentum and energy for three unknowns but I can't solve it. Help?
  2. jcsd
  3. Nov 28, 2006 #2
    Show me your equations.
  4. Nov 28, 2006 #3

    V1 = v1cos30 + 3VcosC


    v1sin30 = 3VsinC


    V1^2 = v1^2 + 3V^2,

    where C is the unknown angle, v1, V are the final velocities of the m and 3m masses.
  5. Nov 28, 2006 #4
    That's not exactly how I'd go about it. oops nevermind lol.
  6. Nov 28, 2006 #5
    Well you see your equation 1 gives you what V1 is. So you can plug that into equation 3. Now you are only left with 2 uknowns, V and C. Now equation 2 gives you a way of getting rid of V. So now you only have 1 equation and 1 unknown, C.

    btw, conservation of momentun does not use angles.

    I should say conservation of momentum does not have to use angles. So if it helps, go ahead and use the total conservation of momentum equation.
    Last edited: Nov 28, 2006
  7. Nov 28, 2006 #6
    that's wrong. Ill explain tomorrow why. Anyways thax.
  8. Nov 28, 2006 #7
    Your equation in the y direction needs a negative sign.

    Also, you will use angles.
  9. Nov 28, 2006 #8
    Please can someone actually help?
  10. Nov 28, 2006 #9


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    Staff Emeritus
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    Gold Member

    Your equations seem correct (C will then be a negative angle). What's your problem in solving them ?

    You need to solve for the quantities v1, V and C and you have 3 equations.

    Try to eliminate v1 and V using the first two equations to write everything as a function of C in the third one, and you'll have an equation for your angle C.
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