Inertia and mass
A bus brakes hard and you move suddenly forward. No force pushed you forward. Why did you move?
Loading lab…
Step 1 of 3
- Bus and rider both move at 10 m/s.
- The bus brakes. A force acts on the bus.
- A moment later the rider still moves. The bus has stopped.
Your body was moving with the bus. When the bus stopped, your body kept moving.
The rule
Inertia is an object's tendency to resist a change in its motion. A larger mass has more inertia.
The brakes act on the bus. No force pushes you forward, so your body keeps moving. A seat belt pushes on you and stops you.
This tendency is inertia. Mass measures how much inertia an object has. Mass is measured in kilograms.
| Case | More inertia |
|---|---|
| An empty cart and a full cart | The full cart |
| A 2 kg ball and a 5 kg ball | The 5 kg ball |
| A ball at rest and the same ball moving fast | Equal. The mass is the same |
Watch out!
Inertia is not a force. It is a feature of any object with mass. It does not push anything.