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

Newton's 3 Laws of Motion are as follows:

1st Law (Inertia)

It states that an object remains in its current state of rest or uniform motion unless acted upon by an external force. (eg you would keep on sliding on ice, because there is very little friction to reduce your speed)

2nd Law 

It states that a body experiencing a force f experiences an acceleration a given by f = ma (where m = mass of body)

3rd Law

It states that for every action, there is an equal and opposite reaction and these action and reaction forces act on different bodies. (eg when you walk, you push the ground backward, The ground pushes you forward with an equal force, propelling you forward)

PLEASE APPROVE MY ANSWER!!