Question

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

Answer 1

MV = m(boy) . v(boy) + m(trolley) . v(trolley) ; 
M= net mass in the end i.e boy+ trolley , V= Final velocity of boy + trolley.

Going by this, 
MV = 50 * 5 + 20 * 1.5
MV = 280
so, V= 280/M, But M = 50 + 20 =70kg

Hence V = 4m/s

Answer 2

Mass of boy = m1 = 50 kg

Mass of trolley = m2 = 20 kg

Initial velocity of boy = u1 = 5 ms-1

Initial velocity of trolley = u2 = 1.5 ms-1

Final velocity =

v = m1u1 + m2u2 \ m1 + m2

50 × 5 + 20 × 1.5 ÷ 50 + 20

= 4ms-1

HOPE IT HELPS YOU!!!