How to identify the direction of relative motion for friction

[ngkamsengpeter]

Direction of friction force is oppose relative motion of the surface .
What is relative motion of the surface mean and how to identify the direction of the relative motion of the surface .

 

[Doc Al]

Friction forces always oppose slipping of the surfaces. When sliding does occur, it's easy to find the direction of the friction. If surface B exerts friction on surface A, then you can find the direction of the friction force on A by viewing things from the reference frame of B. If A moves to the left with respect to B, then the friction force on A acts to the right. (Of course, the friction force that A exerts on B is equal and opposite to the friction force that B exerts on A.)