MathsGradeUp logoMathsGradeUp
PhysicsInternational GCSEEdexcel & Cambridge (CIE)Years 10–11

IGCSE Physics Newton's Laws of Motion Notes

Newton's three laws explain how forces change motion. Learn each one and when to apply it and force questions become reliable marks.

Practise this topic free Ask Study Coach

Key concepts

First law
An object stays at rest or moves at constant velocity unless acted on by a resultant force (inertia).
Second law
Resultant force equals mass times acceleration: F = ma. Force and acceleration are in the same direction.
Third law
For every action there is an equal and opposite reaction — the two forces act on different objects.
  • Inertia is an object's resistance to a change in its motion; more mass means more inertia.
  • Terminal velocity is reached when drag balances weight, giving zero resultant force.
  • Newton's third-law pairs are the same type of force, equal in size, opposite in direction, on different objects.

Key formulas

Newton's second law

F = m × a

Weight

W = m × g

Worked example

A 1200 kg car experiences a driving force of 3600 N and 600 N of resistive force. Find the acceleration.

  1. 1Find the resultant force: F = 3600 − 600 = 3000 N.
  2. 2Write Newton's second law: F = m × a.
  3. 3Rearrange for a: a = F ÷ m = 3000 ÷ 1200.
  4. 4Calculate: a = 2.5 m/s².

Answer: a = 2.5 m/s²

Common mistakes

✗ Thinking a moving object needs a constant force to keep moving.

✓ By the first law, no resultant force is needed to keep constant velocity — only to change it.

✗ Pairing forces on the same object as a third-law pair.

✓ Third-law pairs act on different objects, e.g. car pushes road, road pushes car.

✗ Using driving force instead of resultant force in F = ma.

✓ Subtract resistive forces first to get the resultant.

Quick check

Try these, then reveal the answers.

State Newton's first law.Reveal answer

An object stays at rest or moves at constant velocity unless acted on by a resultant force.

At terminal velocity, what is the resultant force?Reveal answer

Zero — drag balances weight.

Third-law pairs act on how many objects?Reveal answer

Two different objects.

Practise more questions with Study Coach feedback

Exam tips

  • Name which law you are using in an explanation to secure the mark.
  • Always find the resultant force before applying F = ma.
  • For terminal velocity, explain the balance of weight and drag.

Related practice

Practise this topic

Exam-style questions with instant Study Coach feedback.

Flashcards

Quick-fire recall to lock in the key facts.

Worksheets

Printable practice you can work through offline.

Past papers

Full papers with examiner-grade marking.

Study Coach

Ask for a step-by-step explanation any time.

All Physics

Every Physics topic across every exam board.

Related topics

Forces & MotionMomentum

Key terms explained

Short, exam-ready definitions with a formula, a worked example and an examiner tip.

What is acceleration?What is conservation of energy?What is electric current?What is frequency?What is kinetic energy?What is moment of a force?Full glossary

Frequently asked questions

What is inertia?+

The tendency of an object to resist a change in its motion; the greater the mass, the greater the inertia.

Why do falling objects reach terminal velocity?+

As speed increases, drag increases until it equals weight, giving zero resultant force and constant velocity.

Create a free account to practise newton's laws of motion with Study Coach feedback

Try 5 questions now, get instant examiner-grade marking and ask the Study Coach to explain anything. Free during our early launch period — no credit card required.

Create free account Try the Study Coach

Related on MathsGradeUp

Keep studying

Practise it