The fact that gravity is present as a universal force was presented with some simple examples. Newton's observations were presented starting with the concept of inertia, motion in a straight line with constant velocity when no external force acts on the moving object.This led to a review of acceleration being a change in velocity or direction, which requires a force to act on the object. A force is responsible for acceleration. The "falling apple" story regarding Newton and the motion of the moon, led Newton to state that both are accelerating, requiring a force. Newton stated that the apple was acceleration toward the Earth and that the Moon was also falling toward the Earth. Both are being pulled by gravity. Examples were used to define tangential velocity and its role in causing the moon, or any other body, to circle indefinitely.