In a free-body diagram for a block sliding to the right on a horizontal surface, how does kinetic friction affect the block's motion when no other horizontal forces act on it?

• A. It acts upward and reduces the normal force, changing only the vertical motion.
• B. It has no effect unless the surface is inclined, so the block keeps moving at constant speed.
• C. It acts opposite the direction of motion and causes the block to slow down (acceleration to the left).
• D. It acts in the direction of motion and causes the block to speed up (acceleration to the right).

Answer: (C)

Verified step by step guidance

1. Identify the forces acting on the block: since the block is sliding on a horizontal surface, the main forces are the gravitational force downward, the normal force upward, and kinetic friction opposing the motion.
2. Recall that kinetic friction always acts opposite to the direction of the relative motion between the surfaces in contact. Since the block is sliding to the right, kinetic friction acts to the left.
3. Understand that kinetic friction is a horizontal force that opposes the motion, so it will cause a deceleration (negative acceleration) of the block in the horizontal direction.
4. Note that kinetic friction does not affect the vertical forces directly; it does not act upward or downward, so it does not change the normal force or vertical motion.
5. Conclude that with no other horizontal forces acting, kinetic friction causes the block to slow down by exerting a force opposite to its motion.