Question

A boy with mass 50 kg running at 5 m per second jump onto a 20 kg trolley traveling in the same direction at 1.5 m second what is their common velocity

Answer 1

This is a problem of momentum conservation.

Treat boy and trolley as a part of system. Hence, all the forces between them will be internal forces, which cannot change the momentum of the system.

Initial momentum of the system = 50 × 5 + 20 × 1.5

Initial momentum of the system = 280 kg m/s

Let the final common velocity be v.

Final momentum = (50 + 20) × v = 70 v

Applying momentum conservation,

Initial momentum = Final momentum

70v = 280

v = 4 m/s

Hence, the common velocity of boy and trolley = 4 m/s