Curriculum fit: GCSE Physics and Combined Science (forces and motion). US: middle and high school physical science (forces and interactions).
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What Is It?
Newton's three laws describe how forces affect the motion of objects. They explain why seat belts are needed, how rockets work and why it is harder to push a heavy object than a light one.
The key idea is that a resultant force changes an object's motion, and the bigger the mass, the smaller the acceleration for the same force.
Key Formulas and Ideas
- First law: an object stays at rest or moves at constant velocity unless a resultant force acts on it (inertia).
- Second law: resultant force = mass × acceleration (F = ma).
- Third law: when object A pushes on B, B pushes back on A with an equal force in the opposite direction.
- Force is measured in newtons (N), mass in kg and acceleration in m/s².
Worked Examples
F = Ma
A 1200 kg car accelerates at 3 m/s². What resultant force acts on it?
- F = ma.
- = 1200 × 3 = 3600.
✓ Answer 3600 N
Finding Acceleration
A resultant force of 24 N acts on a 6 kg trolley. Find its acceleration.
- a = F ÷ m.
- 24 ÷ 6 = 4.
✓ Answer 4 m/s²
Third Law Pairs
A book rests on a table. Identify the force pair for the weight of the book.
- The Earth pulls the book down (weight).
- By the third law, the book pulls the Earth up with an equal force.
✓ Answer The book's pull on the Earth
Common Mistakes to Avoid
- Thinking the third-law forces act on the same object. They act on different objects.
- Saying a moving object needs a resultant force to keep moving.
- Using mass in grams in F = ma.
- Confusing weight and mass.
Practice Questions
Frequently Asked Questions
If forces are equal and opposite, why does anything move?
Because the paired forces act on different objects. Movement depends on the resultant force on a single object.
Why do seat belts help?
In a crash the car stops, but passengers keep moving by inertia. Seat belts apply a force to slow them gradually.
Keep Going With These Guides
Resultant Forces and Free Body Diagrams (Net Force)
Add forces to find the resultant (net) force, draw free body diagrams and calculate weight, with worked examples and practice questions.
KS3GCSEMiddle SchoolHigh SchoolMomentum and Collisions: P = Mv and Conservation of Momentum
Calculate momentum, solve collision problems with conservation of momentum and link force to change in momentum, with worked examples.
GCSEHigh SchoolSpeed, Velocity and Acceleration: Formulas and Examples
Calculate speed and acceleration, read distance-time and velocity-time graphs and use v² = u² + 2as, with worked examples.
KS3GCSEMiddle SchoolHigh SchoolSources and Review
Reviewed by StudyMorrow editorial review on 5 Nov 2026.