Static friction plays what role in motion?

• A. Static friction prevents motion from starting until a threshold force is exceeded.
• B. Static friction always equals kinetic friction.
• C. Static friction only acts when objects are sliding.
• D. Static friction is independent of surface roughness.

Answer: (A)

Static friction resists the start of motion between two surfaces in contact. It can adjust to match the applied force up to a maximum value, which is μs times the normal force. If the push is smaller than that maximum, nothing moves and the friction force simply balances the effort. Once the push reaches the maximum static friction, any additional force causes the surfaces to slide, and friction becomes kinetic, usually smaller in magnitude. So static friction’s main role is to prevent motion until a threshold is exceeded. It isn’t always equal to kinetic friction, it can act even when there’s no sliding, and it depends on the surface properties through the friction coefficient.