GCSE · Physics · AQA · Spec 8463

Newton's First Law

Nothing moves or stops by itself — Newton's First Law explains why.

What you need to know

  • If the resultant force on an object is zero, a stationary object stays stationary.
  • If the resultant force on an object is zero, a moving object continues at the same speed in the same direction (constant velocity).
  • A non-zero resultant force is needed to change an object's speed or direction of motion.
  • Higher tier: inertia is the tendency of an object to remain in its state of rest or uniform motion.

The big picture

Newton's First Law states that an object will remain stationary or continue moving at a constant velocity unless acted upon by a resultant force. A resultant force is the single combined force that results from all forces acting on an object. If the resultant force is zero, nothing changes — a still object stays still, and a moving object keeps moving at the same speed in the same direction. To change an object's speed or direction, you must apply a non-zero resultant force.

TONIGHT'S REVISION

Newton's First Law

Zero resultant force means no change in motion — stationary objects stay still, moving objects keep going.

Physics · Forces

Newton's First Law — Forces and Motion

Compare three scenarios: what changes when the resultant force is zero vs non-zero?

Resultant force · F = ma
Net forceZero — forces are balanced
WeightNormalDrive forceFrictionDragF net →
A book sits still on a flat table. Weight acts downwards; the normal contact force acts upwards. The resultant force is zero, so the book stays stationary. Newton's First Law: zero resultant force means no change in motion.

Selected scenario

Book on a Table

Net force

Zero — forces are balanced

Motion

At rest

At rest. Forces balance — no acceleration.

Exam line: For AQA, always state the resultant force AND link the zero/non-zero resultant explicitly to whether velocity changes.
Watch out: Forces still act even when the resultant is zero — balanced forces are not the same as no forces.

Stop and Think

Read carefully — this is the most common exam misconception on this topic.

A train moves in a straight line at a constant speed of 50 m/s. The engine exerts a forward force of 8000 N. Which statement correctly applies Newton's First Law?

Zero Resultant Force — Constant Velocity

Both arrows are the same length — equal and opposite forces → zero resultant.

car (constant velocity)Driving Force (F)Resistive Forces (R)Weight (W)Normal Contact Force (N)

Tap a force to see what it does.

Higher

Higher tier: Inertia

?

Reason it through

Why does a more massive object resist changes to its motion more strongly?

Link 1 of 3

First link · your turn

What property of an object determines how hard it is to start moving or stop it?

2
Locked — reveal the link above first
3
Locked — reveal the link above first

Relationship matrix

Tap any cell to reveal it. Tap a column header to read one property down every item.

Resultant forceAre forces present?Does motion change?
Object at reste.g. book on table
Object at constant velocitye.g. car at steady speed
Object changing velocitye.g. braking car

Each cell hides a short answer and the reason behind it. Predict before you tap.

Cross-subject · Exam skill

AQA Command Words — Newton's First Law

Know what each command word is asking before you write a word.

ExplainBloom · understanding

01Definition

Give reasons or a mechanism — link the cause to the effect using physics.

02Mark-band signal

Usually 2–3 marks: name the resultant force, then link it to Newton's First Law and the resulting motion.

03Sentence stems

  • Explain why a car moving at constant velocity has a resultant force of zero.
  • Explain what would happen to the car's velocity if the driving force increased.
  • Explain, using Newton's First Law, why passengers lurch forwards when a bus brakes.

04Common mistake

Describing what happens without saying WHY — 'the car keeps moving' scores zero; 'the resultant force is zero so Newton's First Law means velocity is unchanged' scores the mark.

Exam line: On AQA 8463 Paper 2, Newton's First Law questions almost always use 'Explain' or 'Use Newton's First Law to explain' — practise giving: resultant force → link to law → motion outcome, in that order.
Watch out: The command word 'state' never needs an explanation. Adding one won't gain marks but can lose them if you accidentally contradict your stated fact.

Key points

1Zero resultant force = no change in motion — the object stays still or keeps moving at constant velocity.
2Constant velocity means both constant speed AND constant direction.
3Forces can still act on an object even when the resultant is zero — they just balance out.
4A change in speed OR direction always means a non-zero resultant force is acting.
5Higher tier: more massive objects have greater inertia and resist changes in motion more strongly.

Worked example

Problem

A car travels along a straight, level road at a steady 30 m/s. The engine provides a driving force of 2000 N forwards. What must the total resistive force be, and what does Newton's First Law tell us about the car's motion?

🧠

Memory hook

Think of a hockey puck on ice: once you hit it, it would slide forever if there were no friction. No resultant force, no change — Newton's First Law in action.

★ Exam tip

On AQA Paper 2, 'explain why the object moves at constant velocity' questions expect you to state explicitly that the resultant force is zero AND link this to Newton's First Law by name. One mark is typically awarded for 'forces are balanced / resultant force is zero' and another for connecting this to unchanged velocity — don't write one without the other.

⚠ Watch out

Thinking that a moving object needs a constant force to keep it moving — it doesn't. A zero resultant force keeps an object moving at constant velocity; force is only needed to CHANGE the motion.

✓

Check yourself

A book sits on a table. Gravity pulls it down. Is there a force balancing gravity? What does Newton's First Law tell you must be true about the resultant force?

Flashcards

(22)
State Newton's First Law.
An object remains stationary or continues at constant velocity unless acted upon by a non-zero resultant force.
What happens to a stationary object if the resultant force on it is zero?
It remains stationary.
What happens to a moving object if the resultant force on it is zero?
It continues to move at the same speed in the same direction (constant velocity).
What is meant by constant velocity?
Constant speed AND constant direction — both must be unchanged.
What does a non-zero resultant force cause?
A change in the object's speed or direction (i.e. a change in velocity).
Can forces act on an object even when the resultant force is zero?
Yes — multiple forces can act, but if they balance out the resultant is still zero.
A car moves in a straight line at constant speed. What can you conclude about the resultant force?
The resultant force is zero — driving force equals resistive force.
Higher tier: What is inertia?
The tendency of an object to remain in its state of rest or uniform motion — its resistance to changes in motion.
Higher tier: Which property of an object determines its inertia?
Mass — a more massive object has greater inertia and is harder to accelerate or decelerate.
What is a resultant force?
The single force that represents the combined effect of all forces acting on an object (found by adding forces, accounting for direction).
A skydiver falls at terminal velocity. What is the resultant force on them?
Zero — weight downwards equals air resistance upwards, so the resultant force is zero and velocity is constant.
Why does a ball rolled along the floor eventually stop, even though Newton's First Law says it should keep going?
Because friction and air resistance act as resistive forces, producing a non-zero resultant force that slows the ball down.
Does Newton's First Law apply in space where there is no air resistance?
Yes — an object moving in deep space with no forces acting on it will continue at constant velocity indefinitely.
A box is pushed across a floor at constant velocity. What does this tell you about friction?
The friction force equals the pushing force, so the resultant force is zero — consistent with Newton's First Law.
What two things must be constant for an object to have constant velocity?
Both speed and direction must be constant.
If an object is changing direction (e.g. going around a bend at constant speed), is the resultant force zero?
No — changing direction means the velocity is changing, so there must be a non-zero resultant force acting.
Higher tier: Why is it harder to push a loaded shopping trolley than an empty one, using Newton's First Law?
The loaded trolley has greater mass, so it has greater inertia — a larger resultant force is needed to change its state of motion.
A book rests on a table. Name the two forces acting on it and state whether the resultant is zero.
Weight (gravity) acts downward; normal contact force acts upward. They are equal and opposite, so the resultant force is zero.
On which AQA Physics paper is Newton's First Law assessed?
Paper 2.
A cyclist pedals at constant speed on a flat road. The driving force from pedalling is 150 N. What is the total resistive force?
150 N — because constant speed means zero resultant force, so resistive forces must equal the driving force.
What is the difference between speed and velocity?
Speed is how fast an object moves (scalar); velocity is speed in a given direction (vector).
Higher tier: Give a real-life example that demonstrates inertia.
Passengers lurch forwards when a bus brakes sharply — their inertia keeps them moving forwards while the bus decelerates.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

Learn Newton's First Law properly — interactive practice, marked questions and flashcards.

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