Question

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

Answer 1

Mass of the boy, m₁ = 50 kg

Speed of boy, u₁ = 5 m/s

Mass of trolley m₂ = 20 kg

Speed of trolley u₂ =1.5 m/s

Combined mass of boy and trolley, m = 20+ 50 = 70 kg

Combined velocity v

Acc. to the law of conservation of momentum

m₁ u₁ + m₂ u₂ = mv

50 x 5 + 20 x 1.5 = 70 x v