Two Carts Colliding
A cart of mass M1 = 4.00 kg and initial speed = 5.00 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.
(in m/s)

For elastic collison like this use conservation of momentum and conservation of energy:


m_av_{1a} + m_bv_{1b}= m_av_{2a}+m_2v_{2b} \\
\frac 1 2 m_a v_{1a}^2 + \frac 1 2 m_b v_{1b}^2 = \frac 1 2 m_a v_{2a}^2+\frac 1 2 m_b v_{2b}^2


Two equation and two unknowns - take it away!

oh yeah, before.. I did only the first one, and I thought it's the answer..
so thanks for the 2nd equation :)