Chapter 6, Problem 6Q

In Fig. 6-14, a block of mass m is held stationary on a ramp by the frictional force on it from the ramp. A force $\stackrel{\to }{F}$, directed up the ramp, is then applied to the block and gradually increased in magnitude from zero. During the increase, what happens to the direction and magnitude of the frictional force on the block?

Figure 6-14 Question 6.