Final answer:
The correct answer to what happens to the force of friction when a block slides down an incline is d. Depends on the incline angle, as well as other factors such as the nature of the incline's surface and the state of motion of the block.
Step-by-step explanation:
When a block is sliding down an incline, the force of friction acting on the block can increase, decrease or remain the same, depending on several factors, including the nature of the surfaces in contact and whether the block is speeding up or moving with constant velocity.
If an object is to slide down an inclined plane at constant velocity, then the magnitude of the component of the weight along the slope must equal the magnitude of the frictional force acting against it. This is due to the fact that when the forces are balanced, there is no net force to cause acceleration, leading to constant velocity movement. The coefficient of kinetic friction is generally independent of the relative motion and therefore does not change with speed.
The direction of the force that friction applies on the moving block across a horizontal floor is always opposite to the direction of the block’s motion, regardless of whether the block is just starting to move or already moving.