Hi. I am studying physics on my own from scratch, so far so good, though I've run into this concept I am struggling to understand. If we push a large rock over a cliff, it falls because of the pull of the Earth's gravity on it. This force is its weight and it makes the rock accelerate towards the Earth. Its weight does work and the rock gains kinetic energy. It also gains momentum downwards. Now, according to the book I use - Cambridge International AS and A level Physics - something must be gaining an equal amount of momentum in the opposite (upward) direction. it is the Earth, which starts to move upwards as the rock falls downwards. When the rock hits the ground, its momentum becomes zero. At the same instant, the Earth also stops moving upwards. The rock's momentum cancels out the Earth's momentum. At all times during the rock's fall and crash-landing, momentum has been conserved. _____ I understand the principle of conservation of momentum, but I don't quite understand how it relates to this. I am guessing it will have something to do with gravity, but even then, why there has to be an equal amount of momentum in the opposite direction? Thank you for an explanation.