1. We consider the downward motion with a constant velocity. It means that there is no acceleration and gravity is balanced by the friction force (see Eq. 1). It allows to calculate the coefficient of friction.
2. When one projects a block up the plane, the friction force changes its direction to opposite (downward). The balance of forces breaks and a block decelerates. Here we calculate the work done by friction: it equals initial kinetic energy mu^2/2 (see Eq. 5).
not understood....plz explain