You know what happens when a ball is dropped. It accelerates downward, gaining speed at a rate of 9.8 m/s each second. What would happen if the same ball rolled off a table?

To answer this question you should define a coordinate system. One axis points down, the other, perpendicular to the first, points in the horizontal direction of the rolling ball. The force of gravity acts in the downward direction, causing the ball to accelerate downward, but there is no force in the horizontal direction, so the ball’s horizontal speed would not change. Because only the downward acceleration affects its fall, it would hit the ground at the same time that a dropped ball would. The path the ball takes is a parabola.

Galileo’s Principle of Relativity, not Einstein’s, can help you understand this result. Galileo imagined a sailor dropping a ball from a high mast on a moving sailboat. The sailor would see the ball drop straight down, but an observer on the shore would see the ball having a horizontal velocity equal to the velocity of the boat. Therefore this observer would see the ball’s parabolic path. But both would agree when the ball hit the deck of the boat at the same time. Galileo said that the laws of physics are independent of relative motion. This statement is called the principle of relativity.