# Pair of Centripetal force

We often says, forces do exist in a pair...... So what must be the pair of centripetal force? Obviously centrifugal is not the pair, because it is fictitious force and describe in non inertial frame.

A very common misconception amongst physics students is that the centripetal force is a "type of force" like the normal force, or frictional force, or force of gravity. The centripetal force is not a type of force. Rather, the centripetal force is the force that is required for an object to remain in circular motion at a particular radius and speed.

In different scenarios, different forces may provide the centripetal force for an object in circular motion. For planets orbiting a star, the centripetal force is provided by the gravitational force from the star on the planet, and therefore the action/reaction pair is the force from the planet on the star.

For a tether ball swinging around at the end of a rope tied to a pole, the centripetal force is provided by the force of tension from the pole on the ball, and the action/reaction pair is the force of tension from the ball on the pole.

For particles in a cyclotron, the centripetal force is provided by the magnetic force from the bending magnets, and therefore the action/reaction pair is the magnetic force exerted by the particles on the magnets.

It also seems that you may not be sure of what makes two forces an action/reaction pair as described by Newton's 3rd law. Two forces are only an action/reaction pair if:

• there are two and only two objects involved
• each force is on a different object
• they are in opposite directions
• they are the same type of force (they’re really two parts of one interaction)
• they happen at the exact same time