According to Newton's law of inertia, an object already moving will continue to move in a straight line at a constant speed unless acted on by an outside force. Thus, to make an object move in a circular path, an outside force must act on the object. Centripetal force is the force that pushes or pulls an object inward so that it will move in a circular path. The word centripetal means to seek the center.

When you whirl a stone tied to a string in a circle, you must constantly pull on the string to keep the stone from flying off in a straight line. The force the string applies to the object is the centripetal force. Centripetal force operates in other scenarios as well, acting, for example, on a car to make it travel around a curve. In this case, the force is produced by the friction between the tires and the pavement. Likewise, Earth's gravity exerts a centripetal force on a communications satellite that prevents it from flying off into space by keeping it in orbit.

In the initial demonstration of this segment from ZOOM, involving a cylindrical container, as the ball rolls around the inside of the cylinder walls, a centripetal force continually redirects it to the center of the cylinder. If you were to create an opening in the cylinder through which the ball could escape, the ball would fly outward along a line tangent to the cylinder and would then continue in the same direction of travel as at the moment it exited the cylinder. When this container is held upside down, the centripetal force is not strong enough to counteract gravity, which pulls the ball downward and out of the cylinder.

With the second container, a pitcher with a wide body that curves inward near the lip, the ball does not drop out of the opening. As with a cyclist riding on a banked racetrack, the downward force of gravity is matched by the upward force provided by the surface over which the ball (or the cyclist) travels. Centripetal force acts on the ball as it rolls around inside the pitcher, putting the ball in position to be supported by the pitcher's curved surface, which is what really keeps it from falling out.