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Homework Help: 2D momentum collision question

  1. Jan 19, 2012 #1
    1. The problem statement, all variables and given/known data
    The drawing shows a collision between two balls. Ball A has a mass of .030kg and is moving along the x axis with a velocity of +5.5 m/s. It makes a collision with ball B, which has a mass of .055kg and is initially at rest. The collision is not head-on. After the collision, the two balls fly apart with the angles shown in the drawing below
    What is the final velocity of ball A and B?

    ... .... | Ball A
    ... .... | 65°
    . A ---B---
    ... .... | 37°
    ... .... | Ball B

    Can't get above picture to look right, going to take a picture of it and save it in the post

    A and B show the start position and Ball A and Ball B shows where they finish
    2. Relevant equations
    M1V1 + M2V2 = M1V1'+ M2V2'
    Using inverse trig?

    3. The attempt at a solution
    I saw basically the same problem in Khan's academy and in there he had one of the two ball's final velocity given. I have no real idea how to solve for two variables but here was my try:
    Momentum in X initial= 5.5 * .03 = .165
    Momentum in Y initial= 0
    m1v1+ m2v2= m1v1'+ m2v2'
    .030*5.5 + 0 = .03v1' +.055v2'

    Attached Files:

    Last edited: Jan 19, 2012
  2. jcsd
  3. Jan 19, 2012 #2
    You should have three equations,

    Energy before = energy after

    Momentum in x direction before = momentum in x direction after

    Momentum in y direction before = momentum in y direction after
  4. Jan 19, 2012 #3
    Ok, well the energy before and after equation is:
    (.03*5.5)+ (0.055*0)= (.03*V1')+(.055*V2')
    Momentum in X before:
    Momentum in the Y before:
    Momentum final for X must equal .165
    Momentum final for Y must equal 0
    I do not really understand how I can use this to find two velocities without knowing atleast one of them. How do I use the angles given to help me figure this out?
  5. Jan 19, 2012 #4
    Write it out, if you see it written out it might click, if not it did not take too much time. I don't think you need the energy equation. You have two unknowns, the magnitudes of the velocity of each mass after collision so you only need two equations in the unknowns.
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