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

Answer: (C)

40 km/hour

Answer 2

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$= 1000 + 250 = 1250kg$

Let v is the velocity of the system.

By the conservation of linear momentum : Initial momentum = Final momentum ⇒ $1000 \times 50 = 1250 \times v$

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