Mass
Mass is the quantity of matter in an object. It is a scalar quantity, measured in kilograms (kg), and it does not change when the object is moved to a new place. The mass of a bag of rice is the same in Kuala Lumpur, on Mount Kinabalu or on the Moon. Do not confuse mass with weight, which is the gravitational force on the object and can change with location.
Inertia
Inertia is the natural tendency of an object to resist any change in its state of motion. An object at rest tends to stay at rest, and an object moving in a straight line tends to keep moving in the same way, unless a force acts on it. This idea is described by Newton's first law of motion.
Key idea
The larger the mass of an object, the greater its inertia. A heavy object is harder to start moving, harder to stop and harder to turn than a light one.
Everyday examples
- When a bus brakes suddenly, passengers lurch forward because their bodies tend to keep moving.
- When a bus starts off, passengers are pushed backward because their bodies tend to stay at rest.
- Tapping the bottom of a chilli-sauce bottle makes the sauce move down, using the inertia of the sauce.
- A fully loaded lorry has much more inertia than a bicycle, so it needs a longer distance to stop.
Example
Wearing a seat belt in a car is important because of inertia. If the car stops suddenly, your body tends to continue moving forward; the seat belt provides the force needed to hold you back safely.
Remember
Mass measures how much matter there is; inertia is how strongly that matter resists a change in motion. More mass means more inertia, but the mass value itself stays the same everywhere.