Newton’s laws of motion · Year 13
Newton’s three laws explain how forces change motion. Without a resultant force nothing changes; a resultant force causes acceleration (F = ma); and forces always come in equal and opposite pairs acting on different objects. Together they explain everything from seatbelts to rocket launches.
Key facts
- First law: an object stays still or moves at constant velocity unless a resultant force acts (inertia).
- Second law: resultant force = mass × acceleration, F = ma.
- Third law: if A pushes on B, B pushes on A with an equal and opposite force of the same type.
- Third-law pairs act on different objects, so they never cancel each other out.
- Find the resultant force first, then use F = ma.
- In a lift accelerating upwards: T − mg = ma.