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. Calculate the final velocity of the trolley.
(2 Marks)

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
[1 Mark]
$1000\times 50=1250\times v$
$v=\frac{50,000}{1250}=40km/h$
[1 Mark]