Newton’s Laws of Motion

Key ideas

• Aristotle was wrong. Forces are not needed to keep an object moving. What stops moving objects is friction.
• First law. An object stays at rest or moves in a straight line at constant speed unless a resultant force acts on it.
• Translational equilibrium. No resultant force: at rest or constant velocity. On Earth it always comes from two or more balanced forces.
• Second law. a ∝ F and a ∝ 1/m, so F = ma (constant mass). 1 N accelerates 1 kg by 1 m s⁻².
• Mass is inertia. Mass measures an object’s resistance to a change of motion.
• Third law. If A exerts a force on B, B exerts an equal and opposite force on A: same type, different bodies.
• Impact time. A longer collision means a smaller deceleration and force: sand pits, air bags, crumple zones.

Common mistakes

• ✗A moving object needs a force pushing it.✓Constant velocity means zero resultant force.
• ✗Impact force equals weight.✓It depends on the duration of the collision.
• ✗Weight and normal force are a third-law pair.✓Same body, different types: not a pair.
• ✗In F = ma, F is the applied force.✓F is the resultant force.
• ✗Equal and opposite forces have equal effects.✓Damage depends on area, duration and material.

Equations

Graphs to know

Worked example

Key figures