Which statement best describes static friction and kinetic friction?

Which statement best describes static friction and kinetic friction?

• A. Static friction prevents motion when the object is moving; kinetic friction when at rest.
• B. Static friction prevents motion when the object is on the threshold of slipping; once motion occurs, kinetic friction governs the sliding motion.
• C. Static friction equals μk N; kinetic friction equals μs N.
• D. Static friction always equals μs N; kinetic friction is greater than static friction.

Answer: (B)

Friction behaves in two distinct ways: static friction acts to prevent initiation of motion, and kinetic (dynamic) friction acts when the object is already sliding. Static friction adjusts its value up to a maximum of f_s,max = μ_s N to balance the applied tangential force. The object stays at rest as long as the push is less than that maximum. When the push exceeds f_s,max, the object begins to slip, and then kinetic friction takes over, with a roughly constant value f_k = μ_k N, which is typically less than f_s,max. That’s why the statement describing static friction as preventing motion up to the threshold of slipping, and then kinetic friction governing the sliding motion, is the best description. The other options misstate when static or kinetic friction act or treat them as fixed values rather than up to a maximum (or typically smaller) maximum.

Friction behaves in two distinct ways: static friction acts to prevent initiation of motion, and kinetic (dynamic) friction acts when the object is already sliding. Static friction adjusts its value up to a maximum of f_s,max = μ_s N to balance the applied tangential force. The object stays at rest as long as the push is less than that maximum. When the push exceeds f_s,max, the object begins to slip, and then kinetic friction takes over, with a roughly constant value f_k = μ_k N, which is typically less than f_s,max.

That’s why the statement describing static friction as preventing motion up to the threshold of slipping, and then kinetic friction governing the sliding motion, is the best description. The other options misstate when static or kinetic friction act or treat them as fixed values rather than up to a maximum (or typically smaller) maximum.