Initially, the force of friction is acting upwards, parallel to the ramp. When force F is increased from zero, the friction force continues to act in the same direction, however its magnitude increases.
Work Step by Step
Initially, the frictional force acts against gravity (upwards, parallel to the ramp) in order to keep it stationary. As the force pulls down the ramp, the frictional force only increases in magnitude. No change in direction of the force occurs.