Newton’s three laws of motion
First law: inertia
An object stays at rest, or keeps moving at constant velocity, unless a resultant force acts on it.
Second law: F = ma
F = m × a
The resultant force on an object equals its mass times its acceleration. A 2 kg trolley pushed with a 6 N resultant force accelerates at 3 m/s².
Third law: action and reaction
When object A exerts a force on object B, B exerts an equal and opposite force on A. The two forces act on different objects, so they never cancel each other out.
Exam tip: Always say "resultant force". Forces can be present without causing acceleration if they balance.