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Two dimensional momentum

  1. Dec 25, 2003 #1
    I have a problem with this question,

    1. Two rolling golf balls of the same mass collide. The velocity of one ball is initially 2 m/s [E] After the collision, the velocities of the balls are 2.49 m/s [62.8° North of West] and 2.37 m/s [69.2° South Of East]. What are the magnitude and direction of the unknown velocity?

    Basicall I did:

    For x component:

    V1 + V2 = V1'cos(180°-62.8°) + V2'cos(360°-69.2°)

    I solved for V2

    I did the same for the y component then I took the x and y component values and did the pythagoras theorem to get the angle and value...The value at the back of the book is 3 m/s and moving W for the initial velocity of the other ball...My answer was 4.7 m/s [20° South of West]

    Whats my mistake?
  2. jcsd
  3. Dec 26, 2003 #2


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    You certainly have the right idea. If you show more work I might be able to tell you where the problem is.
  4. Dec 26, 2003 #3
    I've got two answers and one of them is at the back of book. How do I know which ball moves where?
  5. Dec 26, 2003 #4


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    Are you assuming conservation of energy (perfectly elastic collision)? If so you can use V12+ V22= V1'2+ V2'2.
  6. Dec 26, 2003 #5


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    It's not necessary here.
    He's got two unknowns, and two equations from conservation of momentum. The only unknowns are the x and y components of one ball's inital velocity, and the other inital and both of the final velocities are given.
  7. Dec 26, 2003 #6
    Im assuming its a perfectly elastic collision since its two golf balls
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