A body of mass 2kg is travelling in a straight line along the x-axis. It collides with a second body of mass 3kg which is moving at -2m/s. The two bodies move off together at 3m/s. What is the initial velocity of the first body?

2u-32=53=15, 2u=15+6. Therefore, u=10.5m/s. This is a standard type of question, it is simply about conservation of momentum. Momentum (p) is equal to massvelocity. Initially, the first body has momentum 2u kgm/s as its mass is 2kg and its velocity is u. The second body has momentum -6 kgm/s. After the collision, the two bodies move off together at the same speed so the final momentum is 15 kgm/s. We can then rearrange to find u.