Forces - 5.6.3.4 Factors Affecting Braking Distance 2 (GCSE Physics AQA)

Factors Affecting Braking Distance 2

How Brakes Work

In order to stop a vehicle from moving, a force must be applied to the brakes. This force is created by the driver pressing down on the brakes. As we have previously discussed, forces can result in work being done. This means that when the driver presses on the brakes, they are doing work. This has two effects:

• Reduces kinetic energy of tyres – as work is being done, the kinetic energy of the tyres will be reduced. As the kinetic energy decreases, the tyres will move less quickly. The kinetic energy from the tyres will be transferred into other forms of energy in the brakes.
• Increases temperature of brakes – some of the kinetic energy from the tyres will be transferred into thermal energy in the brakes. As the thermal energy of the brakes increases further, the brakes will get hotter. This means that the temperature of the brakes has increased.

Braking at High Speeds

Difficulty of Breaking at High Speed

• Speed makes braking difficult. When travelling at high speeds, it is difficult to brake over a small distance. We know that the higher the speed, the longer it takes for a vehicle to brake.
• Higher speeds require larger braking forces. When travelling at high speeds, vehicles will require a larger braking force to slow them down. Large forces will result in more work being done, so slowing down the car more quickly

Large Decelerations

• Large braking forces lead to large decelerations. When a large braking force is used, the vehicle will slow down very rapidly. ’Slowing down’ is known as deceleration, and large braking forces will lead to large decelerations.
• Large decelerations affect the brakes. When a vehicle has a large deceleration, there is a significant effect on the brakes. In order to slow down the vehicle, a very large braking force is being applied, which will lead to lots of work being done. In turn, this work done will lead to lots of thermal energy in the brakes, potentially leading to them overheating.

Forces and Deceleration

In AQA exams, you may be asked about the forces involved in a vehicle’s deceleration.

During deceleration, we know that the resistive forces will be greater than the driving force of the vehicle.

Therefore there is a resultant force going backwards. The car is still moving forwards, but is decelerating (remember resultant forces cause a change in speed).

You may be asked to estimate the size of these forces in an AQA exam, such as the following example.

Question: A car travelling at average speed has to brake suddenly to avoid a child running into the road 15 metres away. Estimate the braking force involved in the car’s deceleration.

1. We know the average speed.
We know that the average speed of a car is about 30 m/s, from our table earlier on in this chapter.

Speed of car = 30 m/s

2. We are assuming uniform deceleration.
Given this scenario, we need to assume that the deceleration is uniform to perform any kind of calculation.

3. We need to find the deceleration.
Now that we have a speed for the car, we can use our equation v² – u² = 2as. We need to rearrange this to find the deceleration.

v² – u² = 2as.
(v² – u²) / 2s = a

4. Substitute in the numbers.

(v² – u²)/ 2s = a
(30² – 0²) / 2 x 15 = a
900 / 30 = 30
a = 30 m/s²

5. Estimate the mass of the car.
Now that we have the deceleration of the car, we need to estimate the mass of the car.

Mass of a car = ~ 1200 kg

6. Use F = ma.
We have a mass and a deceleration, so now we can substitute in the numbers.

F = ma
F = 1200 x 30
Force = 36 000 Newtons