Question

A man weighing 80 kg is standing in a trolley weighing 320 kg. The trolley is resting on frictionless horizontal rails. If the man starts walking on the trolley with a speed of 1 m / s, then after 4 sec his displacement relative to the ground will be

• A. 5 m
• B. 4.8 m
• C. 3.2 m
• D. 3.0 m

Answer 1

Answer: (C)

3.2 m

Answer 2

If the man starts walking on the trolley in the forward direction then whole system will move in backward direction with same momentum.

Momentum of man in forward direction = Momentum of system (man + trolley) in backward direction

⇒ $80 \times 1 = ( 80 + 320 ) \times v$ ⇒ $v = 0.2 \mathrm {~m} / \mathrm { s }$

So the velocity of man w.r.t. ground $1.0 - 0.2 = 0.8 \mathrm {~m} / \mathrm { s }$

$\therefore$ Displacement of man w.r.t. ground