Section 1 of 6What inertia means
Resisting changes in motion
- Resisting changes in motion Inertia is the tendency of an object to stay at rest or to carry on moving at constant velocity unless a resultant force acts on it.
- It applies to both stationary and moving objects.
Newton's first law
- Newton's first law If the resultant force on an object is zero, a stationary object stays at rest and a moving object keeps the same speed in a straight line.
- A change in speed or direction needs a non-zero resultant force.
Inertial mass
- Inertial mass Inertial mass measures how difficult it is to change an object's velocity.
- It is defined as the ratio of force to acceleration: m = F ÷ a.
Section 2 of 6Everyday examples
Inertia in action
Cars and seat belts
- Cars and seat belts When a car stops suddenly, passengers keep moving forwards at the old speed.
- A seat belt supplies the force that slows them down.
Tablecloth trick
- Tablecloth trick A cloth pulled quickly from under crockery leaves it in place,
- because the brief friction force is too small to change the motion of the heavy objects.
Space travel
- Space travel A spacecraft far from planets and with its engines off keeps moving at constant velocity, as there is almost no resultant force.
Pushing a shopping trolley
- Pushing a shopping trolley A full trolley is harder to start and harder to stop than an empty one because it has greater inertia.
Section 3 of 6Linking mass and acceleration
1
Newton's second law states F = m × a, so a = F ÷ m.
2
For the same resultant force, a larger mass gives a smaller acceleration.
3
Example: a 1500 kg car and a 750 kg car are each pushed by 3000 N. The first accelerates at 2 m/s2, the second at 4 m/s2.
4
Mass is therefore a measure of an object's inertia, not only of how much matter it has.
Section 4 of 6Explaining motion in terms of forces
| Situation | Resultant force | Motion |
|---|---|---|
| Car at steady 30 m/s on a level road | Zero (driving force = resistive force) | Constant velocity |
| Driver presses accelerator harder | Forward and non-zero | Speeds up |
| Brakes applied | Backward and non-zero | Slows down |
| Satellite in circular orbit | Non-zero, towards the centre | Direction keeps changing |
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Inertia | The tendency of a body to remain at rest or in uniform motion unless acted on by a resultant force. |
| Inertial mass | The ratio of resultant force to the acceleration it produces, m = F ÷ a. |
| Resultant force | The overall force obtained when all the forces on an object are added, taking direction into account. |
| Constant velocity | Moving at the same speed in the same direction. |
| Newton's first law | An object stays at rest or moves at steady velocity if the resultant force on it is zero. |
Section 6 of 6Exam technique
1
Believing that a force is needed to keep an object moving. A force is needed only to change its motion.
2
Thinking inertia is a force. It is a property of an object, not something that pushes.
3
Saying that passengers are 'thrown forwards' by a force in a crash. They simply carry on at their original velocity.
4
Confusing mass with weight. Mass (kg) measures inertia; weight (N) is the force of gravity on that mass.