The tendency of a body to resist any change in its motion (speed or direction) – in other words, to resist any change in its acceleration – is called its ‘inertia’. Mass can be thought of as a measure of a body’s inertia.
Inertia means ‘reluctance to change’. Inertia reduces a rate of change but cannot stop it. Inertia can take many forms, e.g.
- Electromagnets have electrical inertia: they resist changes of current through their coils
- Flutes and organ pipes have acoustic inertia: their vibrations take time to diminish after the forces causing them stop
You might help students develop a feeling for inertia by asking them:
- Can you stop a moving railway carriage that is smoothly running along a line, having just been shunted? Yes, but can you stop it easily, or at once?
- What keeps a spaceship going once it is far out in space, well away from the gravitational pull of the Earth and Sun, and the rocket motors are turned off? Is there anything to stop it moving?
In both cases, inertia keeps the object moving. A force is needed to change its velocity, but even the smallest resultant (net) force will do so.
Masses of objects can be compared in principle by seeing how their velocity changes compare, in response to the same force or in the same interaction.