Which statement correctly distinguishes static friction from kinetic friction?

• A. Static friction occurs during motion; kinetic friction stops motion.
• B. Static friction prevents motion up to a limit; kinetic friction acts during motion and is usually smaller than static friction.
• C. Static friction and kinetic friction are the same.
• D. Kinetic friction prevents motion until threshold is exceeded.

Answer: (B)

Static friction is the force that resists the start of sliding between two surfaces. It can adjust to match the applied force up to a maximum value, so as long as your push is not larger than that limit, the object stays at rest. The maximum static friction depends on how hard the surfaces are pressed together (the normal force) and a coefficient that represents their roughness.

Once the push exceeds that threshold, the surfaces begin to slide, and kinetic friction takes over. This friction acts opposite the direction of motion while the object is moving, and it typically has a smaller, roughly constant magnitude compared with the maximum static friction.

So the best distinction is: static friction prevents motion up to a limit; kinetic friction acts during motion and is usually smaller than static friction. For example, a book on a table won’t slide until your push passes the maximum static friction; once it starts sliding, the resistance drops to a lower, steady kinetic friction.