A cart of mass M1 = 4.00 kg and initial speed = 4.00 m/s collides head on with a second cart of mass M2 = 2.00 kg at rest. Assuming that the collision is elastic, find the speed of M2 after the collision.

i keep getting 2.3 m/s
what am i doing wrong?

2 answers

Show your work and we'll tell you. Did you assume that both total momentum and kinetic energy were conserved?

The center of mass moves at velocity 2/3* 4.00 = 8/3 m/s. In that "CM" coordinate system, M2 approaches with velocity -8/3 m/s and leaves with velocity +8/3 m/s. In earth-fixed coordinates, its speed is 8/3 + 8/3 = 16/3 = 5.33 m/s.
The speed of M1 after collision is -(4 - 8/3) + 8/3 = 4/3 = 1.33 m/s.

You may verify that these numbers are correct by plugging them into the momentum and KE conservation equations.
okay I think I get it now, I was going at it in a different way.
thanks so much.