Section 1 of 4What Inertia Means
Inertia is the tendency of an object to remain at rest or to keep moving in a straight line at constant speed, unless a resultant force acts on it.
Definition
- Definition Inertia describes how an object resists any change to its velocity, whether that change is starting, stopping or turning.
Newton's first law
- Newton's first law An object stays at rest or in uniform motion unless a resultant force acts on it.
Mass
- Mass The greater the mass of an object, the greater its inertia, so a larger force is needed to change its velocity by the same amount.
Not a force
- Not a force Inertia is a property of matter rather than a force, so it does not push or pull an object.
Section 2 of 4Everyday Examples
Inertia explains many common observations about moving objects.
1
A passenger in a car that brakes suddenly keeps moving forward, which is why seatbelts are needed.
2
A tablecloth pulled quickly from under dishes leaves them nearly in place, because they have inertia and little time to be moved.
3
A ball rolling across a smooth floor keeps going, slowing gradually only because friction and air resistance act on it.
4
A heavy lorry is harder to start and harder to stop than a small car, because its larger mass gives it more inertia.
5
Water leaves a spin dryer through its holes, because the water tends to keep moving in a straight line.
| Property | Low inertia | High inertia |
|---|---|---|
| Mass | Small | Large |
| Effect of the same force | Large change in velocity | Small change in velocity |
| Example | Table tennis ball | Loaded lorry |
Section 3 of 4Inertia and Safety
Inertia and Safety
Seatbelts
- Seatbelts Seatbelts supply the force that stops a passenger moving forward with the car during sudden braking.
Crash helmets
- Crash helmets Padded helmets increase the time over which the head slows down, which reduces the force of an impact.
Vehicle design
- Vehicle design Heavier vehicles need more force to speed up or slow down, so drivers must allow greater distances.
Section 4 of 4Exam Checklist & Tips
5.7.5 – Key Points for the Exam
- Inertia The tendency of an object to remain at rest or at constant velocity unless acted on by a resultant force.
- Resultant force The single force that results from combining all the forces acting on an object.
- Mass A measure of the amount of matter in an object, which also determines its inertia.
- Uniform motion Motion at a constant speed in a straight line.
- Friction A force that opposes motion between surfaces in contact, which slows moving objects.
Common Mistakes
- Describing inertia as a force that keeps objects moving;
- a moving object needs no force to keep its constant velocity.
- Saying that objects stop naturally because they lose inertia;
- they stop because a resultant force, such as friction, acts on them.
- Confusing inertia with weight; weight depends on gravity, whereas inertia depends only on mass.
- Forgetting that the same force produces a smaller acceleration on a larger mass.