Laws of motion
Also known as: Newton's laws of motion, Newton's laws
The laws of motion are Newton's three principles describing how forces affect movement: objects resist changes in motion (inertia), net force equals mass times acceleration (F = ma), and every action has an equal and opposite reaction.
The laws of motion are the three principles Isaac Newton formulated to describe the relationship between forces and the motion of objects. Together they form the foundation of classical mechanics — the physics of everything from a rolling ball to an orbiting satellite.
Newton's first law (the law of inertia) states that an object at rest stays at rest, and an object in motion continues at constant velocity, unless acted on by a net external force. Newton's second law quantifies what a net force does: it produces an acceleration proportional to the force and inversely proportional to the object's mass, summarized as F = ma. A 10 N net force on a 2 kg object produces an acceleration of 5 m/s². Newton's third law states that forces come in pairs: when object A pushes on object B, object B pushes back on A with a force equal in magnitude and opposite in direction.
Applying the laws correctly means accounting for all the forces acting on an object — gravity, the normal force, tension, friction, applied pushes and pulls — usually with a free-body diagram. Key subtleties trip students up: a net force of zero means constant velocity (not necessarily rest), and third-law force pairs act on different objects, so they never cancel each other for a single object.
The AP Physics 1 exam devotes an entire unit to the laws of motion. Expect free-body diagram construction, F = ma problems in one and two dimensions, and conceptual questions probing inertia and action-reaction pairs.
Key takeaways
- Newton's first law: objects maintain their state of rest or uniform motion unless a net external force acts on them.
- Newton's second law: net force equals mass times acceleration (F = ma).
- Newton's third law: forces occur in equal and opposite pairs acting on different objects.
- Free-body diagrams are the essential tool for identifying forces and applying the laws correctly.