In a free-body diagram of a 5-kg block on a horizontal surface pulled by a constant horizontal force with kinetic friction μ_k, which forces act on the block?

In a free-body diagram of a 5-kg block on a horizontal surface pulled by a constant horizontal force with kinetic friction μ_k, which forces act on the block?

• A. Gravity downward, normal upward, applied force to the right, and kinetic friction to the left.
• B. Gravity downward, normal upward, applied force to the right, and static friction to the left.
• C. Gravity downward, normal upward, applied force to the right, and kinetic friction to the left.
• D. Gravity downward, applied force to the left, normal upward, and kinetic friction to the right.

Answer: (C)

The key idea is that a block on a horizontal surface experiences four forces: gravity pulling downward, the normal force from the surface pushing upward, the applied force acting horizontally in the pull direction, and friction arising from contact with the surface. Since the problem specifies kinetic friction, the block is sliding, so friction acts opposite the direction of motion. If the pull is to the right, the friction force must point to the left. The normal force balances gravity when there’s no vertical acceleration, so it points upward with magnitude equal to mg. Therefore, the free-body diagram includes gravity downward, the normal force upward, the applied force to the right, and kinetic friction to the left.

The key idea is that a block on a horizontal surface experiences four forces: gravity pulling downward, the normal force from the surface pushing upward, the applied force acting horizontally in the pull direction, and friction arising from contact with the surface. Since the problem specifies kinetic friction, the block is sliding, so friction acts opposite the direction of motion. If the pull is to the right, the friction force must point to the left. The normal force balances gravity when there’s no vertical acceleration, so it points upward with magnitude equal to mg. Therefore, the free-body diagram includes gravity downward, the normal force upward, the applied force to the right, and kinetic friction to the left.