Newton's Laws and Free-Body Diagrams

Kinematics describes motion; dynamics explains it. The foundation of dynamics is Newton's three laws, which link the forces acting on an object to how its motion changes. Alongside the laws, the free-body diagram is the essential problem-solving tool that turns a physical situation into a solvable set of equations.

Newton's first law

Newton's first law states that an object continues at rest or moves with constant velocity unless acted on by a resultant (net) external force. This is the law of inertia: objects resist changes to their state of motion. Mass is the measure of inertia. The first law reframes our intuition; motion does not require a continuous force to sustain it, only a force to change it. A puck sliding on frictionless ice would glide forever. In everyday life friction and dr