Which statement correctly describes kinetic friction?

• A. Its direction is the same as the motion.
• B. Its magnitude is independent of the normal force.
• C. Its direction is opposite to the direction of motion; magnitude is mu_k * F_N.
• D. It is always equal to static friction.

Answer: (C)

Friction during sliding is described by Coulomb friction: it resists motion between surfaces. The key idea is that kinetic friction points opposite to the direction of relative motion, and its magnitude is proportional to the normal force pressing the surfaces together: F_k = μ_k F_N. This means the frictional force grows as you press harder (increase F_N) and, in the ideal model, does not depend on how fast you slide or on the contact area. It’s the force that keeps the motion from continuing unimpeded.

Before motion starts, friction can take on whatever value up to μ_s F_N to prevent movement; once sliding begins, the friction switches to the kinetic form with magnitude μ_k F_N, which is typically less than μ_s F_N.

So, the statement that friction acts opposite to motion and has magnitude μ_k times the normal force correctly captures how kinetic friction behaves.