In this force concept question, we are given a block moving on a rough horizontal surface, a constant force pulling to the right on the block and a constant velocity along the surface, and we show how to find the friction force when the velocity is constant.
We don't need to know anything about the coefficient of kinetic friction here! The point is that constant velocity means acceleration is zero. Zero acceleration means the net force is zero, and finally a net force of zero means the leftward friction force has the same magnitude as the applied force pulling to the right on the block!
The main lesson here is that given an object moving at constant velocity, we know immediately that all the forces balance on that object, so the friction force balances the applied force.
