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