You are lowering two boxes, one on top of the other,
down a ramp by pulling on a rope parallel to the surface of the
ramp (Fig. E5.33). Both boxes move together at a constant speed
of 15.0 cm>s. The coefficient of kinetic friction between the ramp
and the lower box is 0.444, and the coefficient of static friction
between the two boxes is 0.800. (a) What force do you need to
exert to accomplish this? (b) What are the magnitude and direction of the friction force on the upper box?