Newton's Laws of Motion

Use Newton's three laws to explain why objects stay still, speed up, slow down and push back.

About 3 min read
  • State Newton's three laws of motion
  • Use F = ma to calculate force, mass or acceleration
  • Identify equal and opposite force pairs
Before you start

You know that forces are measured in newtons, and that speed tells you how fast an object moves.

Newton's first law

An object stays at rest or keeps moving at a steady speed in a straight line unless a resultant force acts on it. A book on a table stays still because the forces on it balance. A passenger jerks forward when a bus stops because their body tends to keep moving.

Definition: resultant force

The single force that has the same effect as all the forces acting on an object.

Newton's second law

A resultant force makes an object accelerate. A bigger force gives more acceleration, while a bigger mass gives less.

Newton's second law

force (N) = mass (kg) × acceleration (m/s²)

Newton's third law

When object A pushes on object B, object B pushes back on object A with an equal force in the opposite direction. These two forces act on different objects, so they do not cancel each other.

Calculate acceleration

A trolley of mass 20 kg is pushed with a resultant force of 50 N. Find its acceleration.

Maths
F = ma
a = F ÷ m
a = 50 ÷ 20 = 2.5 m/s²

Answer: 2.5 m/s²

Balanced does not mean stopped

Balanced forces mean no acceleration. The object can be still, or it can keep moving at a steady speed.

Exam focus
Say it like this“The resultant force is zero, so the speed stays constant.”
Not like this“There are no forces, so it keeps going.” — Forces may still act, but they balance.
Common mistakeSaying that third-law pairs cancel. They act on different objects.

Practice

A 1000 kg car accelerates at 3 m/s². Find the resultant force.
F = ma = 1000 × 3 = 3000 N.
A 12 N resultant force gives a 4 m/s² acceleration. Find the mass.
m = F ÷ a = 12 ÷ 4 = 3 kg.
A swimmer pushes water backwards. What does the water do?
It pushes the swimmer forwards with an equal force.
Going further
A 2 kg box is pulled with 10 N against 4 N of friction. Find its acceleration.
Resultant force = 10 − 4 = 6 N, so a = 6 ÷ 2 = 3 m/s².

Quick check

Newton's laws quick check

What happens to acceleration if the resultant force doubles and mass stays the same?

In F = ma, acceleration is directly proportional to force at fixed mass.

A cyclist moves at a steady speed in a straight line. What is the resultant force?

Steady velocity means no acceleration, so the forward and backward forces balance.

Earth pulls you down with 500 N. What is the matching third-law force?

A third-law pair has the same force type and acts on the two interacting objects.
Key takeaways
  • With no resultant force, velocity stays constant.
  • A resultant force causes acceleration: F = ma.
  • Forces come in equal and opposite pairs.
  • Third-law pairs act on different objects.
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