Answer
Initially, the frictional force is directed up the ramp. When force F increases from zero, the frictional force flips and is directed downwards. As the magnitude of force F increases, the magnitude of the frictional force increases.
Work Step by Step
When the block is stationary on the ramp, friction is acting against gravity, parallel to the ramp. When force F is applied, the frictional force flips sides and acts, against force F, down the ramp. As force F increases, the frictional force increases with it.
