Elastic Collision

[collegelife101]

Two platelets collide head-on. Platelet 1 has a momentum p to the right. Platelet 2 has a momentum of 2p to the left. What is the final momentum of Platelet 1?

The correct answer was 2p left. I understand that an elastic collision means there is not change in kinetic energy, so are we assuming here that all of the energy from platelet 2 is transferred to 1, leading to a momentum of 2p to the left? Likewise, is platelet 2 now moving in the p left direction?

How do we know that the momentum doesn't change in a different way, with different velocities than what they started out with, but where you still have no change in KE?

Thanks!

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[syoung]

Think that's the definition of elastic collision. This would be different if it were inelastic.

 

[NextStepTutor_1]

lets say the mass of both platelets is m, then the velocities are given by v1 and 2*v1 for the two platelets. Total momentum is p to the left. When the two balls collide elastically, they both experience the same force by Newton's third law. Then, remember that impulse = f * change in time. So that the momentum of the balls change by the same amount. Thus, the only way that momentum is conserved is then if (p - x) + (-2p + x) = -p. Solve for x(impulse) and we get x = 3p. So there is only one way to get the balls to move. Hope that helps!