Conservation of momentum is a very important law. A rather interesting idea popped in my head earlier pertaining to this. Since momentum is conserved, wouldn't it be theoretically possible to lift off of the ground by hitting the ground fast enough with your hand? (Assuming you don't break something or get hurt.) If you're on a slippery surface or on something with wheels you can push off of a wall and slide in the opposite direction, so wouldn't something similar work vertically? Momentum is always conserved in an isolated system, isolated being the key word. This means that for momentum to be conserved there must be no net force on the system before or after the collision occurs. In the vertical direction, there is the force of gravity acting downwards on the system, so momentum is not conserved. So this crazy idea would not work. The reason momentum is conserved in the horizontal plane on the ground is because that force of gravity is canceled out by the normal force from the ground (or whatever surface it's on) making the net force before and after zero.