Question

a boy of mass 50kg running at 5m/s jumps on to a 20 trolley travelling in the same direction at 1.5m/s. Find their common velocity.

Answer 1

Use momentum conservation.

initially both system ( boy and trolly ) are different. Get their momentum.
Finally when boy get on trolly both becomes one system with mass as sum of boy and trolly having some common velocity.Get it's momentum.
Then equalise total initial momentum and final momentum.