Forces and Newton's Laws

The Three Laws

Newton's first law: constant velocity (including rest) means zero net force. The second law quantifies dynamics: $F_{net} = ma$ The third law pairs equal and opposite forces acting on different bodies — they never cancel within one free-body diagram.

The Common Forces

Weight is $mg$ downward. The normal force is perpendicular to the surface and adjusts to circumstances — it is not automatically . Friction opposes relative sliding: Static friction takes whatever value is needed up to its maximum and exceeds kinetic friction. Tension is uniform in an ideal rope. For circular motion, some real force (tension, gravity, friction, normal) supplies the centripetal requirement .