Which type of friction acts when surfaces are at rest relative to each other?

Which type of friction acts when surfaces are at rest relative to each other?

• A. Static friction.
• B. Kinetic friction.
• C. Rolling friction.
• D. Air resistance.

Answer: (A)

Static friction is the force that resists motion when two surfaces are pressed together but not sliding. Its magnitude can rise from zero up to a maximum value determined by the normal force and the coefficient of static friction. This friction prevents movement until the applied force exceeds that maximum; once movement begins, kinetic friction takes over. Rolling friction applies to rolling objects, and air resistance acts on objects moving through air. So when surfaces are not moving relative to each other, static friction is at work.

Static friction is the force that resists motion when two surfaces are pressed together but not sliding. Its magnitude can rise from zero up to a maximum value determined by the normal force and the coefficient of static friction. This friction prevents movement until the applied force exceeds that maximum; once movement begins, kinetic friction takes over. Rolling friction applies to rolling objects, and air resistance acts on objects moving through air. So when surfaces are not moving relative to each other, static friction is at work.