Friction

Related to the normal force is the frictional force. The two are related because

they are both due to the fact that the body is in contact with the surface. Whereas the normal force was perpendicular to the surface, the frictional force is parallel. Furthermore, friction opposes motion, and so its vector always points away from the direction of movement. 

Friction is divided into two types-static and kinetic. These are represented by Ff, with a further subscript 's' for static friction, and a subscript 'k' for kinetic friction, . As its name suggests, static friction occurs when the body is not moving with respect to the surface. It is the force which makes it difficult to start something moving. On the other hand, kinetic friction occurs when the body is sliding over the surface. (Of course rolling objects experience friction as well.) This is the force which causes objects to slow down and eventually stop. Friction is usually approximated as being proportional to the normal force. The proportionality constant is called the coefficient of (static or kinetic) friction. The coefficient is represented as ms for static friction, and mk for kinetic friction; the numerical value of m depends on the nature of the surface with which the body is in contact. 

We've added (kinetic) friction to our free body diagram.


 

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