A body is thrown horizontally from a cliff with an initial horizontal speed. It hits the ground at a distance X from its initial position. It is said that the displacement = horizontal speed * time. When the body falls, gravity acts upon it. So why don't we consider gravity ? Any proof that the displacement would be the same as above if the same body would have moved with the same initial speed on a flat horizontal path for the same time period ? Of course in the 2nd situation, it would continue to move infinitely.