What is static friction and how does it differ from kinetic friction?

What is static friction and how does it differ from kinetic friction?

• A. Static friction prevents motion up to a limit; kinetic friction acts when an object slides, usually less than static friction
• B. Static friction acts when an object slides, while kinetic friction stops motion
• C. Static friction always equals kinetic friction
• D. Static friction and kinetic friction are unrelated

Answer: (A)

Friction comes in two forms depending on whether the object is not yet moving or already sliding. Static friction is what resists the start of motion; it can take on a range of values up to a maximum, increasing as you push harder until the object just begins to move. Once motion has started, the friction that opposes the sliding is called kinetic friction. It is typically smaller than the maximum static friction and acts at roughly a constant magnitude for a given pair of surfaces and normal force. This choice is correct because it captures both ideas: static friction prevents motion up to a limit, and kinetic friction acts once sliding occurs and is usually less than the maximum static friction. The other statements are not accurate: static friction does not only act when an object slides, nor does kinetic friction stop motion on its own in all situations, and static and kinetic friction are related concepts describing different contact states.

Friction comes in two forms depending on whether the object is not yet moving or already sliding. Static friction is what resists the start of motion; it can take on a range of values up to a maximum, increasing as you push harder until the object just begins to move. Once motion has started, the friction that opposes the sliding is called kinetic friction. It is typically smaller than the maximum static friction and acts at roughly a constant magnitude for a given pair of surfaces and normal force.

This choice is correct because it captures both ideas: static friction prevents motion up to a limit, and kinetic friction acts once sliding occurs and is usually less than the maximum static friction. The other statements are not accurate: static friction does not only act when an object slides, nor does kinetic friction stop motion on its own in all situations, and static and kinetic friction are related concepts describing different contact states.