• A heavy wooden box is lying at rest on a rough horizontal floor. A student applies a gradually increasing horizontal force to push it. Which of the following statements is correct regarding the frictional force acting on the box before and after it starts moving?

Answer: C. Static friction increases to match the applied force up to a maximum limit, and once the box moves, kinetic friction takes over and is slightly less than the maximum static friction.

Before an object starts moving, static friction acts on it. Static friction is a self-adjusting force; as you push harder, the friction increases equally to keep the object at rest. This continues up to a maximum limit called limiting friction. Once the applied force overcomes this limit, the object begins to slide. At this point, kinetic (sliding) friction takes over. Kinetic friction is always slightly less than the maximum static friction because it takes less force to keep an object moving .

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