GCSE Physics Newton's Laws of Motion Notes
Newton's three laws explain how forces change motion. Learn each law with an example and you can tackle almost any forces question.
Key concepts
- First law (inertia)
- An object stays at rest or moves at constant velocity unless a resultant force acts on it.
- Second law
- The acceleration of an object is proportional to the resultant force and inversely proportional to its mass: F = ma.
- Third law
- When two objects interact, they exert equal and opposite forces on each other.
- Inertia is the tendency of an object to resist changes to its motion; more mass means more inertia.
- Third-law force pairs act on different objects, so they never cancel out.
- Doubling the resultant force doubles the acceleration for the same mass.
Key formulas
Newton's second law
F = m × a
resultant force = mass × acceleration.
Worked example
A 1200 kg car accelerates at 2 m/s². What resultant force is needed?
- 1Write the equation: F = m × a.
- 2Substitute the values: F = 1200 × 2.
- 3Multiply: F = 2400.
- 4Add the unit: F = 2400 N.
Answer: F = 2400 N
Common mistakes
✗ Thinking a moving object needs a constant force to keep moving.
✓ With no resultant force it keeps moving at constant velocity (first law).
✗ Saying third-law pairs cancel out.
✓ They act on different objects, so they never cancel on a single object.
✗ Forgetting mass is inversely related to acceleration.
✓ For the same force, a heavier object accelerates less.
Quick check
Try these, then reveal the answers.
Which law explains why you lurch forward when a car stops suddenly?Reveal answer
Newton's first law (inertia).
A 500 N force acts on a 250 kg object. Find its acceleration.Reveal answer
a = F ÷ m = 500 ÷ 250 = 2 m/s²
State Newton's third law.Reveal answer
When two objects interact they exert equal and opposite forces on each other.
Exam tips
- Name the law and give a matching example — many questions ask for both.
- For third-law pairs, always say which object exerts the force on which.
- Use F = ma with the resultant force, not one individual force.
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Frequently asked questions
What is inertia?+
Inertia is an object's resistance to a change in its motion. The greater the mass, the greater the inertia, so the harder it is to speed up, slow down or change direction.
Why don't Newton's third-law pairs cancel out?+
The two forces act on different objects, so they cannot cancel when you consider the motion of a single object.
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