Like stalactites and stalagmites in geology, centripetal force and centrifugal force are often confused in physics. Both forces are important in describing circular motion. However, when making calculations, you usually only need to worry about centripetal force.
Circular Motion
A body in circular motion may have a constant speed, but since it is constantly changing direction, it's velocity (a vector with both magnitude and direction) is constantly changing. When velocity is constantly changing, that means there is acceleration. In the case of circular motion, the direction of acceleration is the center of the circular path. This acceleration is called centripetal acceleration.
A special type of circular path is an orbit. In an orbit, the circling body is constantly falling towards the center but never reaches it.
Since acceleration is always a part of circular motion, and bodies have mass, a force must also be involved.
Definition of Centripetal Force
The word centripetal is from Latin and means "towards the center". The direction of the force is towards the center. Centripetal force "glues" a body to a circular path. Centripetal force describes how the force acts, not what force it is - so it could be from gravity or electromagnetic, or even the nuclear forces if the circular path is on the subatomic scale.
Definition of Centrifugal Force
The word centrifugal is also from Latin, and it means "flee from the center". Newton's Third Law states that all forces have an equal and opposite reaction. Centrifugal force is a reaction to a centripetal force, and that's why you only need to consider centripetal force.
