Question

A wagon weighing 1000 kg is moving with a velocity 50 km/h on smooth horizontal rails. A mass of 250 kg is dropped into it. The velocity with which it moves now is

• A. 12.5 km/hour
• B. 20 km/hour
• C. 40 km/hour
• D. 50 km/hour

Answer 1

The correct option is C

40 km/hour

Initially the wagon of mass 1000 kg is moving with velocity of 50 km/h

So its momentum $=1000\times 50\frac{kg\times km}{h}$

When a mass 250kg is dropped into it. New mass of the system = 1000+250 = 1250 kg

Let v is the velocity of the system.

By the conservation of linear momentum : Initial momentum = Final momentum

$1000\times 50=1250\times v$

$v=\frac{50,000}{1250}=\frac{40km}{h}.$