A wagon weighing 1000kg is moving with a velocity 50km/h on smooth horizontal rails.A mass of 250kg is dropped on it. The velocity with which it starts to move is:A. 12.5km/hB. 20km/hC. 40km/hD. 50km/hE. 30km/h

To find the velocity with which the wagon starts to move after the mass is dropped on it, we can use the principle of conservation of momentum.

The initial momentum of the wagon before the mass is dropped on it is given by the product of its mass and velocity:

Initial momentum of wagon = mass of wagon * velocity of wagon

= 1000 kg * 50 km/h

= 50000 kg km/h

When the mass of 250 kg is dropped on the wagon, the total mass of the system becomes 1000 kg + 250 kg = 1250 kg.

Let's assume the final velocity of the system (wagon + dropped mass) is v.

The final momentum of the system is given by the product of the total mass and the final velocity:

Final momentum of system = total mass * final velocity

= 1250 kg * v

According to the principle of conservation of momentum, the initial momentum of the system should be equal to the final momentum of the system:

Initial momentum of wagon = Final momentum of system

50000 kg km/h = 1250 kg * v

Simplifying the equation, we find:

v = 50000 kg km/h / 1250 kg

v = 40 km/h

Therefore, the velocity with which the system (wagon + dropped mass) starts to move is 40 km/h.

So, the correct answer is C. 40 km/h.