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Homework Help: Billiard Ball Collision

  1. Oct 23, 2004 #1
    A ball of mass m = 2.1 kg moving with a speed of v0 = 15.2 m/s strikes an identical ball which was initially at rest. After the collision, the incoming ball (ball 1) goes off at q1 = 28° relative to its original direction and the struck ball (ball 2) moves off at q2 = 38° as shown in the diagram.

    a) What is the magnitude of the initial momentum of the two ball system?

    Ans, Used momentum m1v1 = m2v2 , Got the answer
    b) What is the ratio of the balls' final speeds?
    Since there is no vertical momentum before the collision, the net vertical momentum must be zero after the collision.
    Since the vertical momentum must be zero after the collision, what does that tell us about the ratio of vertical momentum of the two balls?

    c) What is the final speed of the incoming ball? v1

    d)What is the final speed of the struck ball? v2

    Need help with b , c , and d .

    Attached Files:

  2. jcsd
  3. Oct 23, 2004 #2
    If i read your question correct then c and d are to be solved before b ...

    Ok have u derived any equation whatsoever for the system ?? If so post it , cuz note that the system has two unknowns v1 and v2 and u should have two equations for this.

    -- AI
  4. Oct 23, 2004 #3
    How do I do that ?

    For the x direction would it be,

    v1 cos theta1 and

    for the y direction v2 sin (-theta2)

    This is what I know, just a vauge idea, but I dont now if the eqns are correct .

    Please guide.!!!!!
    Appreciate ur help!111
  5. Oct 23, 2004 #4
    resolve the velocity vectors v1 and v2 along x and y directions
    find the momentum equation along x - direction
    find the momentum equation along y - direction

    these form the two equation in two unknowns...
    solve these to find v1 and v2 ...

    -- AI
  6. Oct 23, 2004 #5
    There is only 1 vector pointing in the x and y directions

    would it be

    v1 cos theta1 + v1 sin theta2 = 0


    v2 cos theta2 + v2sin theta2 = 0

    How do i go about finding momentum... Please help
  7. Oct 23, 2004 #6
    Yes i think u have the equations on the right track

    though ur second question is at err,
    it should be v1sintheta1 = v2sintheta2
    (can u see why??)
    Now solve these two equations ...
    obtain v1 and v2 ..

    -- AI
  8. Oct 25, 2004 #7
    I have been trying this for a while now, but the ratio I am getting is incorrect.

    Here is what I did:

    Resolving components:

    For the x- direction:

    v1 cos theta1 + (-v1 cos(0)) = 0


    -v2 cos (0) + v2 cos theta2 =0

    For the y-direction:

    v2 sin ( 90 - theta1) + (-v2 sin (90- theta2)) = 0

    This is what I know so far,

    Any insights would be appreciated!!!
  9. Nov 1, 2011 #8
    [if] m1 = m 2 → θ1 + θ2 [should be] = 90°.
    28°+38° = 66°
  10. Nov 2, 2011 #9
    if diagram is correct m2 should be ≈ 1
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