# Newton's First Law and Inertia — GCSE Physics
Before Isaac Newton, people believed that a force was needed to keep an object moving. Aristotle thought objects would naturally slow down and stop. Newton's revolutionary insight was the opposite: objects keep doing what they're doing unless a force changes that. This is Newton's First Law of Motion — one of the most important principles in all of physics.
1. Newton's First Law of Motion
An object at rest stays at rest, and an object in motion continues moving at constant speed in a straight line, unless acted upon by a resultant (unbalanced) force.
This can be broken into two parts:
Part 1: Stationary Objects
If an object is not moving and the resultant force on it is zero, it will remain stationary.
Example: A book on a table — weight pulls it down, normal force pushes it up. These forces are balanced (resultant = 0), so the book stays put.
Part 2: Moving Objects
If an object is moving and the resultant force on it is zero, it will continue moving at the same speed in the same direction (constant velocity).
Example: In deep space, far from any planets, if you throw a ball, it will travel in a straight line at constant speed forever — there's no friction, air resistance, or gravity to slow it down.
The Key Implication
Forces are not needed to keep an object moving. Forces are needed to:
- Start an object moving (change from rest)
- Stop an object (change from moving to rest)
- Speed up or slow down an object
- Change the direction of an object's motion
All of these involve a change in velocity — which is acceleration.
2. What Is Inertia?
Inertia is the tendency of an object to resist changes to its state of motion.
- An object at rest has inertia — it resists being moved
- An object in motion has inertia — it resists being stopped or having its direction changed
Inertia and Mass
The greater the mass of an object, the greater its inertia.
- A shopping trolley full of groceries (large mass) is harder to start moving and harder to stop than an empty one (small mass)
- A bowling ball (large mass) is harder to accelerate than a tennis ball (small mass)
Inertial mass is defined as the measure of how difficult it is to change the velocity of an object:
Where:
- = inertial mass (kg)
- = force (N)
- = acceleration (m/s²)
A larger mass requires a greater force to achieve the same acceleration.
3. Real-World Applications
3.1 Seatbelts
When a car suddenly stops in a crash:
- The car decelerates rapidly due to the collision force
- The passengers' bodies continue moving forward (Newton's First Law — their bodies want to keep moving at the same speed)
- Seatbelts provide the force needed to decelerate the passengers with the car, preventing them from hitting the dashboard or windscreen
3.2 Headrests
In a rear-end collision:
- The car accelerates forward suddenly
- The passengers' heads tend to stay where they are (inertia)
- This causes the head to snap backwards relative to the body — whiplash
- Headrests push the head forwards with the rest of the body
3.3 Satellite Orbits
Satellites in orbit around Earth are continuously changing direction (moving in a circle). Although their speed may be constant, their velocity is always changing. Gravity provides the centripetal force that changes their direction.
3.4 Tablecloth Trick
When you pull a tablecloth quickly from under plates:
- The plates have inertia — they resist the change in motion
- The friction between the cloth and plates acts for such a short time that the force is not enough to significantly move the plates
- The plates stay (roughly) in place
3.5 Passengers on a Bus
When a bus suddenly brakes:
- The bus decelerates (unbalanced force from brakes)
- Standing passengers continue moving forward (their inertia keeps them moving)
- They lurch forward
When the bus turns sharply:
- The bus changes direction
- Passengers' bodies continue in the original direction
- They feel pushed to the outside of the turn
4. Common Misconceptions
| Misconception | Reality |
|---|---|
| "You need a force to keep moving" | No — you only need a force to change motion. Without friction, objects would move forever. |
| "If something is moving, there must be a net force" | No — constant velocity means balanced forces (zero resultant). |
| "Heavier objects are harder to move because of gravity" | Partly true, but even in space (no gravity), heavier objects are harder to accelerate because of inertia. |
| "When a car is moving at constant speed, driving force is greater than friction" | No — at constant speed, driving force equals friction (balanced forces). |
5. Newton's First Law and Circular Motion
Newton's First Law tells us that an object moves in a straight line unless acted on by a force. So if an object moves in a circle, there must be a resultant force — even if the speed is constant.
This force is called the centripetal force, and it always points towards the centre of the circle.
Examples:
- Planet orbiting the Sun: gravity provides centripetal force
- Car turning a corner: friction between tyres and road provides centripetal force
- Ball on a string: tension in the string provides centripetal force
Worked Example: Example 1
Question: A football is stationary on the ground. Explain, using Newton's First Law, why the ball does not move.
The ball has two forces acting on it: weight acting downwards and the normal contact force from the ground acting upwards. These forces are equal and opposite, so the resultant force is zero. According to Newton's First Law, an object at rest remains at rest when the resultant force is zero. Therefore, the ball does not move.
Worked Example: Example 2
Question: A train moves at a constant speed of 120 km/h along a straight track. The driving force from the engine is 45,000 N. Calculate the total resistive force acting on the train. Explain your answer.
The train is at constant speed in a straight line — this means constant velocity, so the resultant force is zero (Newton's First Law).
The resistive forces (friction + air resistance) must be exactly equal to the driving force for the forces to be balanced.
Worked Example: Example 3
Question: Explain why passengers in a car feel pushed back into their seats when the car accelerates quickly.
When the car accelerates, the seat pushes the passenger's body forward. However, the passenger's body has inertia — it tends to remain in its current state of rest (or motion). The body resists the forward acceleration, and the passenger feels pressed into the seat as the seat pushes them forward.
Worked Example: Example 4
Question: Two objects, A (mass 2 kg) and B (mass 10 kg), are on a frictionless surface. The same force of 20 N is applied to each. Which object has greater inertia? Which accelerates faster?
Object B has greater inertia because it has greater mass (10 kg > 2 kg).
Acceleration of A: m/s² Acceleration of B: m/s²
Object A accelerates faster (5 times faster) because it has less inertia (less mass).
7. Practice Questions
- State Newton's First Law of Motion. (2 marks)
- A skater glides across smooth ice at constant speed in a straight line. The friction is negligible. Explain why the skater continues to move even though no force is being applied. (2 marks)
- A car is travelling at 30 m/s on a motorway. The driving force is 2400 N. State whether the car is accelerating, decelerating, or travelling at constant speed if the total resistive force is: (a) 2400 N (1 mark) (b) 1800 N (1 mark) (c) 3000 N (1 mark)
- Explain, using the concept of inertia, why it is dangerous not to wear a seatbelt. (3 marks)
- Two objects have masses of 5 kg and 20 kg. If the same force is applied to both, which accelerates more and why? (3 marks)
Answers
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Frequently Asked Questions
Does Newton's First Law mean objects never stop?
In theory, yes — if no forces act, an object in motion will keep moving forever. In practice, there is almost always friction or air resistance, which are unbalanced forces that slow objects down. In the vacuum of space, objects do travel indefinitely.
Is Newton's First Law just a special case of Newton's Second Law?
You could argue that: if and , then (no change in velocity). But Newton stated the First Law separately because it establishes the concept of inertial reference frames and overturns the ancient idea that forces are needed for motion.
What happens if forces are balanced but not zero individually?
The individual forces still act (e.g., weight still pulls you down, the ground still pushes you up), but the resultant is zero. The object behaves as if no net force acts on it — it maintains constant velocity.
Is mass the same as inertia?
Mass is the measure of inertia. More mass = more inertia = more resistance to changes in motion.
Summary
- Newton's First Law: An object remains at rest or at constant velocity unless acted on by a resultant force
- Balanced forces (resultant = 0) → no change in motion
- Unbalanced forces → acceleration (change in speed or direction)
- Inertia is the resistance to change in motion; it depends on mass
- More mass → more inertia → harder to accelerate
- Real-world applications: seatbelts, headrests, satellite orbits, tablecloth trick