Newton’s three laws of motion: a quick static-GK guide
Newton’s laws describe inertia, the relation between net force and acceleration, and equal-and-opposite interaction forces. They are common physics fundamentals in school science and exams.
Newton’s first law describes inertia, the second relates net force to mass and acceleration, and the third says interacting bodies exert equal and opposite forces on each other.
The three laws in plain language
The first law says an object remains at rest or moves at constant velocity unless a net external force acts on it. This is the law of inertia: motion does not need a continuous push when no net force changes it.
The second law relates net force to mass and acceleration, commonly written F = ma for constant mass. The third law says forces between two interacting bodies are equal in magnitude and opposite in direction, acting on different bodies.
Common exam traps
A heavier object needs more net force than a lighter object to achieve the same acceleration. Equal-and-opposite forces do not cancel on one object because they act on different bodies.
For numerical questions, check units: force is measured in newtons, mass in kilograms and acceleration in metres per second squared.
Editorial note
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Frequently asked questions
What is Newton’s second law commonly written as for constant mass?
F = ma, where F is net force, m is mass and a is acceleration.