Newton's Laws of Motion Practice Test 2026 - Free Motion Theory Practice Questions and Study Guide

When two surfaces slide past each other, kinetic friction acts to oppose that sliding motion. The friction force points opposite the velocity of the moving surface relative to the other, so if the block moves to the right, friction points to the left. Its magnitude is approximately μ_k times the normal force (μ_k N) and, for a given contact, stays roughly constant as speed changes. Kinetic friction exists only when sliding occurs; static friction handles any attempt to move when there’s no sliding.

So the statement that friction always acts opposite the direction of motion is the best description. The idea that it increases with velocity isn’t correct in the usual model, since friction’s strength doesn’t depend on speed. The idea that friction equals μ_k N times velocity is also incorrect, as it’s μ_k N, not multiplied by speed. And friction isn’t limited to rough surfaces; microscopic surface irregularities create friction even on surfaces that seem very smooth.