5.1: Centripetal Force
Topic outline
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A centripetal force is any force that makes an object move in a circular motion. A centripetal force can involve any of the classical or electromagnetic forces, such as gravitational force (weight), normal force, tension, and friction.
For example, gravitational force acts as a centripetal force on a planetary scale because it causes planets to orbit in a circle. However, gravitational force is not a centripetal force on the Earth's surface because gravity makes objects fall straight down toward the Earth's center, not in a circle. A normal force can act as a centripetal force, such when a roller coaster does a loop-da-loop. Friction can act as a centripetal force, such as when it causes cars to turn corners on a road.
The equation for centripetal acceleration is
, where 
is object speed and 
is radius (distance from center). -