Which statement is true about kinetic friction on a slipping object?

Which statement is true about kinetic friction on a slipping object?

• A. Its direction is the same as the velocity.
• B. Its magnitude is independent of the normal force.
• C. Its direction is opposite to the velocity; magnitude is mu_k * F_N.
• D. It equals the normal force.

Answer: (C)

Kinetic friction on a slipping object acts to oppose the relative motion between the surfaces in contact. That means the friction force points opposite the direction of the velocity along the contact surface. Its strength follows f_k = μ_k N, so the friction depends on how hard the surfaces press together (the normal force) and the kinetic-friction coefficient, not on how fast the object is sliding. On a horizontal surface with only gravity, N ≈ mg, so f_k = μ_k mg. So this statement captures both the opposite direction to motion and the correct dependence of magnitude on the normal force. The other ideas—friction pointing in the same direction as motion, or its magnitude being independent of the normal force, or friction equaling the normal force—don’t fit the standard model of kinetic friction.

Kinetic friction on a slipping object acts to oppose the relative motion between the surfaces in contact. That means the friction force points opposite the direction of the velocity along the contact surface. Its strength follows f_k = μ_k N, so the friction depends on how hard the surfaces press together (the normal force) and the kinetic-friction coefficient, not on how fast the object is sliding. On a horizontal surface with only gravity, N ≈ mg, so f_k = μ_k mg.

So this statement captures both the opposite direction to motion and the correct dependence of magnitude on the normal force. The other ideas—friction pointing in the same direction as motion, or its magnitude being independent of the normal force, or friction equaling the normal force—don’t fit the standard model of kinetic friction.