A 10.0 kg cart and a 15 kg cart are locked together with a compressed spring between them. They are then released so that the spring pushes the two carts apart. The 15 kg cart is moving at 3.0 m/s afterward. How fast is the 10.0 kg cart moving?

Answer 1

Total momentum will remain zero.

15*3.0 + 10*V = 0
V = -4.5 m/s

The minus sign means that it goes in the opposite direction from the 15 kg cart.