1.4.1Forces and Their Effects
Braking Force — Oxford AQA International GCSE Combined Science - Physics Revision Notes & Topical Questions
5 min read 6 parts Lesson 15 of 16
Part 1 of 4Travelling at a steady speed
Balanced forces
- Balanced forces When a car moves in a straight line at constant speed, the driving force from the engine equals the total resistive force from friction and air resistance.
- The forces are balanced, so the resultant force is zero.
Newton's first law
- Newton's first law With no resultant force, the car's velocity does not change.
- It keeps moving at the same speed in the same direction.
- A force is only needed to change the motion, not to maintain it.
What braking changes
- What braking changes Applying the brakes adds a large resistive force that is not balanced by the driving force.
- A resultant force now acts against the direction of travel, so the car decelerates.
Part 2 of 4How the brakes slow a car
1
The driver presses the brake pedal, which pushes the brake pads against a disc (or drum) that turns with the wheel.
2
Friction between the pads and the disc produces the braking force, which acts against the rotation of the wheel.
3
The braking force does work on the brakes, because it acts over the distance the surfaces slide past each other. Work done = energy transferred.
4
Energy is transferred mechanically from the kinetic store of the car to the thermal store of the brakes, so the car slows down and the brakes get hot.
5
The car stops once all of its kinetic energy has been transferred away.
Part 3 of 4Energy changes when braking
1Moving car
- Moving car The car has energy in its kinetic store, which depends on its mass and speed.
2Friction does work
- Friction does work The brake pads rub on the disc and the braking force acts over a distance.
3Kinetic store falls
- Kinetic store falls The car's speed decreases, so its kinetic energy decreases.
4Thermal store rises
- Thermal store rises Brake pads and discs warm up, and some energy is later dissipated to the surroundings by heating.
Part 4 of 4Speed, force and deceleration
| Situation | Braking force needed | Consequence |
|---|---|---|
| Low speed, stop in a given distance | Smaller | Gentle deceleration and little heating |
| High speed, stop in the same distance | Much larger | Large deceleration and more energy to transfer |
| Very large braking force for a long time | Very large | Pads can overheat and discs can warp, so braking becomes less effective |
| Constant speed | None needed | Driving force equals resistive forces |
Key terms
| Term | Meaning |
|---|---|
| Braking force | The frictional force between the brake pads and the wheel discs that acts against the motion of the vehicle. |
| Resultant force | The single force that has the same effect as all the forces acting on an object combined. |
| Deceleration | A decrease in speed, which is a negative acceleration. |
| Kinetic store | The store of energy that an object has because it is moving. |
| Thermal store | The store of energy that depends on the temperature of an object; the hotter the object, the more energy it holds. |
Avoid these mistakes
1
Explaining the heating of the brakes using friction between the tyres and the road. The brakes heat up because the braking force acts between the pads and the disc.
2
Saying energy is 'used up' or 'lost' when a car stops. It is transferred from the kinetic store to the thermal store.
3
Thinking a moving car needs a resultant force to keep going. At constant speed the forces are balanced.
4
Forgetting that a bigger speed means a bigger braking force is needed to stop in the same distance, because Newton's second law links force to acceleration.
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