Static friction is a force that works against moving a stationary object while kinetic friction works __________ against keeping the object in motion.

Static friction is a force that works against moving a stationary object while kinetic friction works __________ against keeping the object in motion.

• A. With less force than static friction
• B. With more force than static friction
• C. With the same force as static friction
• D. At zero force

Answer: (A)

Static friction resists the start of motion with a maximum force μsN. Once an object is sliding, the friction that opposes keeping it in motion is kinetic friction, which has a magnitude μkN and is typically smaller than μsN (μk < μs). Because the resistance to motion once moving is smaller, the force needed to keep the object moving is less than the force needed to overcome static friction to start it. So kinetic friction works with less force than static friction against keeping the object in motion. A common way to see this is that starting to move a heavy object takes more push than keeping it moving, since μs is larger than μk.

Static friction resists the start of motion with a maximum force μsN. Once an object is sliding, the friction that opposes keeping it in motion is kinetic friction, which has a magnitude μkN and is typically smaller than μsN (μk < μs). Because the resistance to motion once moving is smaller, the force needed to keep the object moving is less than the force needed to overcome static friction to start it. So kinetic friction works with less force than static friction against keeping the object in motion. A common way to see this is that starting to move a heavy object takes more push than keeping it moving, since μs is larger than μk.