ABSTRACT
The term frictional force is used to describe the force that arises when two bodies are in contact with one another and that opposes the motion of one body relative to the other. Think of a crate resting on the floor. If you try to slide the crate over the floor you need to exert a big enough force to overcome the frictional force that exists between the crate and the floor before any motion can occur. Also, when you have got the crate moving, you have to keep on applying a force to overcome friction in order to keep it moving at a steady pace. The term static friction is used when the bodies are at rest and the frictional force is opposing attempted motion; the term kinetic friction is used when the bodies are moving with respect to each other and the frictional force is opposing motion with constant velocity.
