Question

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

Answer 1

Mass of the boy , $m_1=50kg$
Speed of boy,$u_1=5m/s$
Mass of trolley $m_2=20kg$
Speed of trolley $u_2=1.5m/s$
Combined mass of boy and trolley, $m=20+50=70kg$
Combined velocity v
Acc.to the low of conservation of momentum
$m_1{u}_1+m_2{u}_2=mv$
$50\times 5+20\times 1.5=70\times v$
$v=\frac{250+30}{70}=4m/s$