Question

An object is thrown up vertically. Does it's momentum remain conserved? Explain.

Answer 1

F = dP/dt where P is linear momentum and F is external force.

Now, if F = 0 then dP/dt = 0 which leads to the result that dP = 0, on integrating both sides we get, P = constant which means it is conserved.

But when there is external force applying on body, then its momentum can't be constant anymore.

Here in this case of throwing ball upwards, the ball is in action of many external forces like weight, air drag, etc which leads to change the momentum of the ball with time.

Hence, law of conversation of momentum is holds only when no external force acting on the body.