Question

A wagon weighing $1000$ kg is moving with a velocity ${\text{50 km/h}}^{-1}$ on smooth horizontal rails. A mass of $250$ kg is dropped into it. The velocity with which it moves now is?

• A. ${\text{2.5 km h}}^{-1}$
• B. ${\text{20 km h}}^{-1}$
• C. ${\text{40 km h}}^{-1}$
• D. ${\text{50 km h}}^{-1}$

Answer 1

The correct option is C ${\text{40 km h}}^{-1}$

Applying conservation of linear momentum : $P_i=P_f$

$M_{wagon}{V}_{wagon}+M_{body}{V}_{body}=(M_{wagon}+M_{body})V$

where $V$ is the velocity of the combined system.

Or $1000\times 50+250\times 0=(1000+250)V$

$⟹V=40km/h$

Correct answer is option C.