centripetal force

Our Living Language  : In a popular carnival ride, people stand with their backs against the wall of a cylindrical chamber. The chamber spins rapidly, the floor drops out, but the riders remain pressed against the wall without falling. Although the riders may insist they stay aboard because of an outward force pushing them against the wall, the reality is the opposite: the riders are subject to an inward, or centripetal, force. As the ride spins, it forces the riders to travel in a circle. According to Isaac Newton's law of inertia, objects in motion tend to travel in a straight line at constant speed unless acted on by an external force. To make an object travel along a curved path, a force must keep the object moving toward the center of curvature—in this case the axis of rotation. The wall of the ride's cylindrical chamber accomplishes this by pushing the riders toward the center (with the friction between the riders and the wall holding the riders up). The force of the Earth's gravity acts as a centripetal force on orbiting objects, such as the Earth's Moon, which is constantly being accelerated toward the center of the Earth, as in free fall. The Moon has enough inertia not to plummet into the Earth but not so much that it can escape the Earth's pull, and thus it will orbit almost indefinitely.