Newton's first law
Inertia is the natural tendency of an object to stay at rest, or to keep moving at constant velocity in a straight line, unless a resultant force acts on it. This is described by Newton's first law of motion. Inertia is not a force; it is a property of matter.
Key idea
The mass of an object is the measure of its inertia: the greater the mass, the greater the inertia. A loaded lorry is much harder to start moving or to stop than an empty one.
Everyday examples
- When a car stops suddenly, passengers lurch forward because their bodies tend to keep moving.
- Chilli sauce leaves a bottle when you shake it down and stop sharply — the sauce continues moving.
- A coin on a card flicked off a glass drops straight into the glass because it tends to stay at rest.
Worked example
Trolley A has mass 2 kg and trolley B has mass 6 kg. Which needs a larger force to give the same acceleration of 3 m s⁻²?
Using F = ma: for A, F = 2 × 3 = 6 N; for B, F = 6 × 3 = 18 N. Trolley B needs a larger force because it has greater mass, hence greater inertia.
Reducing negative effects
Safety belts, air bags and headrests reduce injuries caused by inertia during sudden stops or collisions.
Remember
- Inertia is a tendency, not a force.
- Greater mass means greater inertia.
- Newton's first law is the law of inertia.