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Homework Help: Elastic Collisions 1D

  1. Jun 25, 2008 #1
    1. The problem statement, all variables and given/known data
    a .450kg ice puck moving east with a speed of 3 m/s has a head on collision with a .9 kg puck initially at rest. Assuming a perfectly elastic collision what will be the speed and direction of each object after the collision?

    2. Relevant equations
    v1 +v1f = v2+v2f
    .5*m1*v1i^2 + .5*m2*v2i^2 = .5*m1*v1f^2 + >5*m2*v2f^2
    3. The attempt at a solution
    I've used the first equation listed and rearranged it for V2f in order to solve for V1f, and plugged that back into the second equation without luck. I've googled the solution to elastic collisions but that only confuses me more. Which equation am I supposed to plug this into? Once i have V1f can i use the first equation to solve for V2f, or do i need to go through the whole process again? It's a bit of algebra so i may have gotten lost somewhere in there. COuld someone just give me a play by play of what i'm supposed to do here to do this the short way, as I know the long way is using the 2nd and 3rd equations and thats WAY more algebra. All help is appreciated! Thanks in advance!
  2. jcsd
  3. Jun 25, 2008 #2


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    Your approach is almost correct, though I don't think the first equation is generally applicable.

    You have two unknowns (final speed for both objects) so you do need two equations to solve them both. One is conservation of momentum (second equation) and the other is conservation of energy (third equation). Try using those.
  4. Jun 25, 2008 #3
    Try this too... generally, for elastic collisions, the relative velocity of two objects before a collision is reversed after the collision.

    You could also try working in another frame... imagine that you were actually on the moving puck, and that the stationary puck is actually moving towards you at 3 m/s. After the collision, the (initially stationary) puck should be moving away from you at 3 m/s as well.
  5. Jun 25, 2008 #4
    I got it! Thanks for the help. I went through it again, and was able to solve it using the conservation of momentum eq after plugging in the velocities. Got 1 m/s to the left for the first object and 2m/s to the right for the second. time to fry bigger fish :D
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