Why do you lurch forward when a bus suddenly brakes? Why does a spinning coin keep spinning? The answer was figured out by Isaac Newton over 300 years ago — and it changed how we understand motion forever.
Imagine a book sliding across a table.
Write down your answers before reading on:
Isaac Newton's revolutionary insight into motion
In 1687, Isaac Newton published his Principia Mathematica, which included three laws of motion that transformed physics. Newton's first law of motion states:
An object will remain at rest or continue to move at a constant velocity in a straight line unless acted upon by an unbalanced external force.
This means that no force is needed to keep an object moving — force is only needed to start, stop or change the motion.
The resistance of matter to changes in motion
Inertia is the tendency of an object to resist changes to its motion. It is not a force itself — it is a property of matter.
The more mass an object has, the more inertia it has. A truck has more inertia than a bicycle, which is why it takes much longer to stop.
How inertia affects your daily life
You experience inertia constantly:
How understanding inertia saves lives
Seatbelts are one of the most important safety applications of Newton's first law. In a car crash, the car stops suddenly but the passengers' bodies tend to keep moving forward at the car's original speed.
A seatbelt applies a force to the passenger, bringing them to a stop safely over a longer period of time. Without a seatbelt, passengers can hit the windscreen, dashboard or steering wheel — or be thrown from the vehicle.
This is why seatbelt laws exist in every Australian state and territory.
Correcting a 2000-year-old misunderstanding
Misconception: "A moving object needs a force to keep it moving."
Correction: This was Aristotle's view and was accepted for nearly 2000 years. Galileo and Newton showed that in the absence of friction, an object would keep moving forever. On Earth, friction and air resistance are the real reasons moving objects slow down — not a lack of force.
"Inertia is a type of force." No — inertia is a property of matter, not a force. It describes how objects resist changes to motion.
"Heavier objects fall faster because they have more inertia." No — in a vacuum, all objects fall at the same rate. Inertia affects how hard it is to stop a moving object, not how fast it falls.
Road safety: Australia's Towards Zero strategy aims to eliminate road deaths. Understanding inertia is central to this — crumple zones, seatbelts and airbags all work by managing the inertia of occupants during collisions. The Monash University Accident Research Centre (MUARC) conducts world-leading research into vehicle safety.
Train safety: Trains have enormous inertia due to their mass. Australian rail networks use long braking distances because a fully loaded freight train can take over a kilometre to stop. Understanding inertia helps design safer railway crossings and signalling systems.
Space exploration: The Australian Space Agency contributes to missions where Newton's first law is essential. In space, with negligible friction, spacecraft coast for millions of kilometres without using fuel — a direct application of inertia.
1. What does Newton's first law state?
2. What is inertia?
3. Which object has the greatest inertia?
4. Why do you lurch forward when a bus brakes suddenly?
5. In deep space with no forces, what happens to a moving object?
1. Explain Newton's first law using the example of a passenger in a car that suddenly stops. Include the concept of inertia in your answer. 4 MARKS
2. Why is it easier to stop a bicycle than a truck moving at the same speed? Use the concept of inertia. 4 MARKS
3. Describe the common misconception that 'force is needed to keep things moving.' Explain why this is incorrect and what actually causes moving objects to slow down on Earth. 4 MARKS
Go back to your Think First answer. Has your understanding changed?
C — Newton's first law states that an object remains at rest or continues at constant velocity unless acted on by an unbalanced external force.
B — Inertia is the tendency of an object to resist changes to its motion. It depends on mass.
D — A truck has the greatest mass and therefore the greatest inertia, making it hardest to stop.
B — When the bus brakes, your body tends to keep moving forward due to inertia. The seat or seatbelt applies a force to stop you.
C — In deep space with negligible forces, a moving object would continue moving at constant speed indefinitely due to inertia.
Model answer: When a car suddenly stops, the passenger's body continues moving forward at the car's original speed due to inertia. Inertia is the tendency of the body to resist changes to its motion. The seatbelt applies a backward force to the passenger, bringing them to a stop safely over a longer time. Without a seatbelt, the passenger would hit the windscreen or dashboard because no force acts on them to stop their forward motion.
Model answer: It is easier to stop a bicycle than a truck at the same speed because the bicycle has less mass and therefore less inertia. Inertia is the resistance of an object to changes in its motion. The truck's greater mass means it has greater inertia, so a larger force or longer time is needed to bring it to a stop. This is why trucks need much longer braking distances than bicycles.
Model answer: The common misconception, held by Aristotle for nearly 2000 years, is that a continuous force is needed to keep an object moving. Galileo and Newton showed this is incorrect: in the absence of friction, an object would keep moving forever at constant velocity. On Earth, moving objects slow down because of friction and air resistance — these are forces that oppose motion. Once you remove friction (as in space), no force is needed to maintain motion.
Run, jump and glide through inertia-based challenges! Test your knowledge of Newton's first law while dodging obstacles.
Tick when you have finished all activities and checked your answers.