A centripetal force is one that makes a body move in a curved path. Gravity is an example of a centripetal force in Newtonian gravity (see Newtonian gravity), making a planet orbit a star by continually accelerating the planet toward the star at the orbitals center. Without this centripetal force, the planet would fly off into space in a straight line.
When you whirl a tennis ball over your head on a string, the ball feels a centripetal pull force. The centripetal force is often confused with the centrifugal (outward) force, which can be a fictitious force. It accounts for the sense of being pushed outward when looping-the-loop on a roller coaster.
The centrifugal force can also be a reaction force to a centripetal force, according to Newtons third law of motion (see newton's laws of motion). In the case of a tennis ball on a string, the whirling ball exerts an outward centrifugal force on the person spinning it.
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