Extension: Newton's First Law, Inertia
You'll be able to apply Newton's First Law to explain everyday situations.
In 1970, Victoria became the world's first place to make seatbelts compulsory, cutting road deaths by around 18% in just one year, all thanks to one law of physics.
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Q1 · You're riding a skateboard and it suddenly stops. What happens to you?
Q2 · Why do you need to push a shopping trolley to get it moving, and then brake to stop it?
● Know
- Newton's First Law: objects stay at rest or in constant motion unless acted on by an unbalanced force
- Inertia = an object's resistance to change in motion
- Inertia increases with mass
● Understand
- Why seatbelts and headrests work (inertia)
- Why heavier objects are harder to start and stop
- Why motion stays constant when the net force is zero
● Can do
- Apply Newton's First Law to explain everyday situations
- Compare inertia of objects with different masses
- Explain why loaded trucks need longer stopping distances
- Newton's First Law
- Inertia
- At rest
- Constant velocity
- Unbalanced force
- An object's tendency to resist any change in its motion
- An object stays at rest or at constant velocity unless acted on by an unbalanced force
- A non-zero net force that causes a change in motion
- The state of not moving, zero velocity
- Moving at the same speed in the same direction, no change in motion
Imagine you're sitting in a bus that brakes sharply: your body lurches forward even though nothing pushed you, you kept moving because nothing stopped you. That lurch is inertia in action. Newton showed that objects always keep doing what they're already doing, and it takes a force to change that.
Newton's First Law (also called the Law of Inertia):
"An object at rest stays at rest, and an object in motion stays in motion at the same speed and in the same direction, unless acted on by an unbalanced force."
What this means in practice:
- A stationary book will stay still forever unless something pushes or pulls it
- A moving object keeps the same speed and direction unless an unbalanced force acts on it
- In an idealised space model with zero net force, a spacecraft keeps the same velocity without its engine on; real spacecraft still experience gravity
On Earth, friction and air resistance often oppose motion. An object slows when the net force opposes its velocity; if other forces balance that resistance, its velocity stays constant. Newton's key idea was that motion changes only when the net force is not zero.
Text description
- At rest: a book on a table. Normal force up and gravity down; net = 0 N, so it stays still (no change).
- Moving: a spacecraft at constant velocity. No forces, no change: it moves forever at the same speed.
- Inertia: the car brakes and stops, but the body keeps going; the seatbelt applies a force to stop the body.
- Newton's First Law: no change in motion without an unbalanced force; objects resist change (inertia).
Inertia is the tendency of an object to resist any change in its motion. Two things about that sentence are easy to get wrong. Inertia is not a force, so it never appears as an arrow on a force diagram; it is a property that objects simply have, in the same way that they have a colour or a volume. And inertia is not "wanting to stay still" — an object already moving resists being stopped exactly as stubbornly as a still object resists being started.
What decides how much inertia an object has is its mass, and nothing else. Not its size, not its material, not how fast it is going. The reason is worth spelling out, because it makes the rule impossible to forget: mass is a count of how much matter is there, and changing the motion of matter takes force. Double the matter and you have doubled the amount that has to be got moving, so the same push produces half as much change. That is why more mass means more inertia in both directions at once, harder to start and harder to stop, rather than just "heavy things are hard to lift".
Try it mentally. Flick a five-cent coin across a table and it shoots off, because a few grams of matter is all your finger has to shift. Give an iron pot of 5 kg the identical flick and it barely twitches, because the same small force is now being asked to change the motion of a thousand times as much matter. Nothing about your finger changed between the two attempts, which is what makes this a clean test: the only variable was the mass, so the difference in the result has to be down to the mass.
Practical applications:
- Seatbelts: when a car stops suddenly, inertia keeps your body moving forward at the car's original speed. The seatbelt applies the unbalanced force needed to stop you too.
- Headrests in cars: in a rear-end collision, the car seat pushes your body forward but your head lags behind due to its inertia, headrests prevent this "whiplash".
- Crash test dummies: simulate the inertia of a human body so engineers can design safer cars without injuring real people.
Australian road safety, inertia at work:
- Australia's seatbelt laws began in Victoria in 1970, the first compulsory seatbelt law in the world. This law was designed specifically around Newton's First Law: a moving body in a stopping car keeps moving unless restrained.
- A fully loaded B-train truck (up to 68 tonnes) vs an empty sedan (about 1.5 tonnes): the truck has about 45 times more inertia. At highway speed, it needs a stopping distance of up to 280 m, almost 3 football fields. This is why highway signs warn about truck stopping distances.
- Speed limits in school zones are a First Law story too, but it is worth telling it accurately. Dropping to 40 km/h does not reduce anyone's inertia, because inertia depends on mass alone and nobody gets lighter in a school zone. What changes is how much motion has to be taken away in a crash. A passenger's body keeps travelling at whatever speed the car was doing until something applies a force to stop it, so at 40 km/h there is far less motion for a seatbelt, or for a child on the footpath, to have to absorb.
Sports:
- A cricket ball has more mass (and therefore more inertia) than a tennis ball, it's harder to change the cricket ball's trajectory mid-flight. Pace bowlers rely on the ball maintaining its line.
- The tablecloth trick pull the cloth out from under the dishes very fast. The dishes' inertia keeps them in place because the pull is so brief that not enough horizontal force acts on them.
- Seatbelt
- Headrest
- Truck stopping distance
- Tablecloth trick
- Large mass = large inertia = needs huge distance to stop
- Provides the force to stop the body when the car stops suddenly
- Dishes stay still because the brief pull doesn't overcome their inertia
- Prevents head from lagging behind in a rear-end collision
A spacecraft is launched and then its engines are turned off completely. For this idealised model, assume the net force on it is zero, neglecting gravity and air resistance. Predict what happens to the spacecraft's speed and direction over the next 100 years. Use Newton's First Law in your explanation.
How close was your prediction?
Excellent, you correctly applied Newton's First Law: no force = no change in motion.
Remember: objects don't "naturally slow down", they slow down because forces act on them. With no forces, motion is forever unchanged.
Setup: Place a playing card or stiff piece of cardboard over a cup. Put an egg (or ball) on top of the card, centred over the cup's opening.
The trick: Flick the card out from under the egg as fast as possible (horizontally). If done quickly enough, the egg drops straight into the cup.
Your task:
- Before doing it: predict what will happen and explain using Newton's First Law.
- After doing it (or imagining it): explain why the egg drops into the cup using the words "inertia", "unbalanced force", and "Newton's First Law".
- Why does it matter HOW FAST you flick the card? What would happen if you pulled the card slowly?
Classify each situation as: (A) Inertia keeping something still when motion is applied around it, or (B) Inertia keeping something moving when the force is removed or the object it's attached to stops.
| Situation | A or B? | Brief explanation |
|---|---|---|
| Dishes staying on the table when a tablecloth is yanked out | ||
| Passengers lurch forward when a bus brakes suddenly | ||
| A satellite coasting through space after its rocket turns off | ||
| Coffee staying still in the cup when you quickly slide the cup along the table | ||
| A cricket ball continuing on its path after it leaves the bowler's hand | ||
| You sliding forward in your seat when a car accelerates from rest |
Q1. A soccer ball rolls across a grass field and gradually slows down. Explain why it slows down and what Newton's First Law would predict in an ideal model with zero net force. (3 marks)
Q2. A car is travelling at 60 km/h when the brakes are applied. Explain what Newton's First Law predicts will happen to the driver if no seatbelt is worn. (3 marks)
Q3. Compare the inertia of a loaded semitrailer with an empty sedan. Explain why this difference matters for road safety in Australia. (4 marks)
Answers
▾MCQ 1
B Newton's First Law specifically requires an unbalanced force (net force ≠ 0) to change motion. Having mass (A) does not itself cause a change in motion. Individual forces can act while balancing each other. A resting object with only balanced forces still doesn't move.
MCQ 2
C Inertia depends on mass. A bowling ball is the heaviest of the four options (typically 4–7 kg vs golf ball ~0.05 kg, tennis ball ~0.06 kg, soccer ball ~0.4 kg). More mass = more inertia.
MCQ 3
B Your body was moving forward at the bus's speed. When the bus stops suddenly, the braking force acts on the bus (and your seat), but nothing immediately provides the same braking force to your body. Your body's inertia keeps it moving forward while the bus stops beneath you.
MCQ 4
D Newton's First Law: with no unbalanced force, motion continues unchanged forever, same speed, same direction. Objects do not naturally slow down (A); a net force opposite to their velocity is needed. In the stated zero-net-force model, velocity remains constant.
MCQ 5
B Seatbelts are designed around Newton's First Law (the law of inertia). When the car stops, the passenger's body tends to keep moving forward (inertia). The seatbelt applies the unbalanced force needed to decelerate the passenger along with the car.
Short Answer 1
Model answer: The ball slows down because friction between the ball and the grass acts opposite to the direction of motion. This is an unbalanced backward force that reduces the ball's speed. In an ideal model with zero net force, Newton's First Law predicts that the ball's velocity would remain constant: it would keep moving at the same speed in the same direction. Removing friction alone would not rule out other unbalanced forces.
Short Answer 2
Model answer: Newton's First Law states that a moving object keeps moving unless an unbalanced force acts on it. The car's brakes apply a force to the car, bringing it to a stop. However, if no seatbelt is worn, there is no force acting directly on the driver's body. The driver's inertia keeps their body moving forward at 60 km/h even as the car stops. The driver would continue forward, striking the steering wheel, dashboard or windscreen at speed, potentially causing severe or fatal injury. A seatbelt provides the unbalanced force to stop the driver along with the car.
Short Answer 3
Model answer: A loaded semitrailer can weigh up to 68 tonnes; an empty sedan weighs about 1.5 tonnes. Since inertia depends on mass, the semitrailer has approximately 45 times more inertia. This means it requires much greater force and distance to stop, up to 280 m at highway speed, compared to about 50–70 m for a car. For road safety in Australia, this means: (1) trucks need much greater following distances; (2) merging in front of a truck is dangerous because it cannot slow quickly; (3) Australian highways post signs warning about truck stopping distances; and (4) transport regulations limit truck speeds in part due to their inertia and consequent stopping distances.
The hook at the start of this lesson asked why Australia made seatbelts compulsory in Victoria in 1970, the world's first such law. The answer is all about inertia!
Explain why a seatbelt saves lives using Newton's First Law. Then compare it to what happens when a skateboard suddenly stops and you keep moving. Use the terms inertia, Newton's First Law, and unbalanced force in your answer.
- Newton's First Law: objects stay at rest or at constant velocity unless acted on by an unbalanced force. This is the law of inertia.
- Inertia depends on mass, more mass = more inertia = harder to start and stop. It is a property, NOT a force.
- Seatbelts, headrests, and Australian heavy vehicle regulations are all designed around Newton's First Law. Victoria's 1970 seatbelt law was the first in the world.