How do you Find Friction?

     Friction is the force that resists the relative motion or tendency when two substances are in contact. There are two types of friction generally mentioned. One is static friction and the other is kinetic friction.Static friction is greater than kinetic friction. 

     To find static friction, it equals the product of normal force (Fn) and static friction coefficient (Fs). F=Fn x Fs.   Fs is usually given in questions, and the normal force (Fn) is usually what students need to look for. (Depends on the orientation of the object, there are different ways to find the normal force. For example, Fn equals -m x g only when the gravitational force is in opposite direction to the normal force. Otherwise students need to consider about angles and use sin, cos to find it.)

     To find kinetic friction, the formula is quite similar. F= Fn x Fk .Fn is still normal force. Again, there are different ways to find Fn depends on different cases. Fk is kinetic friction coefficient. It is different from Fs and do not get confused. Generally Fs is greater than Fk for the SAME object and that is why static friction is usually greater than kinetic friction of the SAME object.   

     There is also one thing that needs attention. We can only find kinetic friction when the object is moving at constant velocity. If the object given is accelerating, we CANNOT find kinetic friction of the object. And we can only find static friction when the object is not moving, or has no moment.

     (The symbols Fk, Fn, Fs, N mentioned above vary depending on different textbooks, publishers, professors and teachers.)