If an object moves in a circle, either a planet, a satellite, or even a ball on a string twirled around, then there is no change in speed as it moves around the circle, so there can be no force in direction of motion. There must be a force, because the direction of the ball is changing, so its velocity is changing. The force must be perpendicular to the motion.

Try to make a ball move in a circle. This works best with a ball the size of a soccer ball or basketball on a smooth (wood or tile) floor. Start the ball moving, then kick it gently in the direction perpendicular to its motion. Try another kick or two. Note that its direction changes in the direction of the kick—the momentary force you placed on the ball. If you could exert a constant force that is always perpendicular to the motion the ball would move in a circle. Note that the direction of the force is always toward the center of the circle. Such a force is called “centripetal,” or center seeking.

The force required to keep an object moving in a circle depends on three quantities: the mass of the object, the speed of the object, and radius of the circle. The force must be larger for larger mass, greater speed, and smaller radius of the circle.