Does kinetic friction always oppose the direction of motion?
No it does not. In some cases friction can cause motion. To illustrate this we will consider a body M1 of length L placed on a frictionless surface and a smaller body M2 placed on top of M1 on the leftmost edge of it. The coefficient of kinetic friction between M1 and M2 is μk and the positive direction of motion is to the right. If we start applying a force F on M2 of big enough magnitude, it will start moving on top of M1 towards the rightmost edge of it. The forces acting on M1 on the x-axis will be the positive force F we are applying and a negative frictional force FF12 applied on M2 by M1.. According to Newton's Third Law, there must be an opposite - equal in magnitude and opposite in direction - force acting on M1 by M2, ie the reaction of FF12. This positive frictional force, FF21, is the only force acting on M1 on the x-axis, and therefore causes it to move towards the right.
