what are he internal and external forces acting on the earth explain in 500 or more words
Answers
Linear momentum is conserved only if there are no external forces. But there is an external force acting, the force of gravity, so the vertical component will not be conserved for a "real" collision by which I mean one which lasts for some nonzero time. But, you have stated that the struck particle remains at rest during the collision; the only way this can be true is if the collision is instantaneous. So the momentum will be conserved because the collision lasts for zero time. The reason this is not a physically possible collision is that it requires the struck particle to instantaneously acquire its velocity which requires an infinite force. Nevertheless, these kind of problems with an external force present are often approximated as having momentum conserved if the collision happens very quickly. Another example is shooting a bullet into a block of wood sitting on a horizontal surface which has friction; only if the collision happens very quickly will momentum be approximately conserved.