Stopping distance Revision Notes for AQA GCSE Physics Combined Science

Stopping distance

What is stopping distance?

Stopping distance is how far a car travels from when danger first appears until it completely stops. This is made up of two parts that happen one after the other.

When a driver sees danger ahead, the car doesn't stop immediately. First, the driver needs time to react and press the brakes. During this reaction time, the car keeps moving forwards. Then, once the brakes are applied, it takes more distance for the car to actually come to a complete stop.

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Understanding stopping distance is crucial for safe driving. Many drivers underestimate how far their car will travel before coming to a complete stop, especially at higher speeds.

The two parts of stopping distance

Thinking distance

This is how far the car travels while the driver reacts to the danger. During this time:

The driver sees the hazard
The brain processes what's happening
The driver moves their foot to the brake pedal
The car continues at the same speed

Braking distance

This is how far the car travels while slowing down after the brakes have been applied. During this time:

The brakes are working to slow the car
Friction between tyres and road does the stopping
The car gradually reduces speed to zero

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Total stopping distance = thinking distance + braking distance

This simple formula is fundamental to understanding road safety and why maintaining safe following distances is so important.

How speed affects stopping distance

Speed has a huge impact on how far it takes to stop:

Thinking distance and speed

Thinking distance increases directly with speed
If speed doubles from 20 mph to 40 mph, thinking distance also doubles

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Worked Example: Thinking Distance

At 20 mph: thinking distance = 6m At 40 mph: thinking distance = 12m (doubled) At 60 mph: thinking distance = 18m (tripled)

Braking distance and speed

Braking distance increases much more dramatically with speed
It's proportional to speed squared $(v^2)$
If speed doubles, braking distance increases by four times

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Worked Example: Braking Distance

At 20 mph: braking distance = 6m At 40 mph: braking distance = 24m (4× increase) At 60 mph: braking distance = 54m (9× increase)

The relationship follows: $\text{braking distance} \propto v^2$

This means total stopping distance increases very quickly as speed goes up.

Factors that increase thinking distance

Several things can make reaction time longer:

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Human Factors Affecting Reaction Time:

Tiredness - slower to notice and respond to danger
Distractions - using phone, loud music, passengers talking
Alcohol or drugs - impaired judgement and slower reflexes
Age - older drivers may have slower reaction times

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Even small distractions can add crucial seconds to reaction time. At 60 mph, an extra second of reaction time means an additional 27 metres of thinking distance.

Factors that increase braking distance

Road and car conditions affect how well brakes work:

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Vehicle and Environmental Factors:

Wet or icy roads - less grip between tyres and surface
Worn tyres - reduced friction with the road
Worn brakes - less effective at slowing the car
Heavy loads - more mass means more energy to lose
Steep downhill slopes - gravity works against the brakes

The science behind braking

When brakes are applied, the car's kinetic energy (movement energy) must be transferred away. This energy becomes heat through friction between the brake pads and wheels.

The faster a car is moving, the more kinetic energy it has. Since kinetic energy depends on speed squared, this explains why braking distance increases so rapidly with speed.

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The Physics Behind It:

Kinetic energy is given by: $KE = \frac{1}{2}mv^2$

Where:

$m$ = mass of the car
$v$ = velocity (speed) of the car

This $v^2$ relationship explains why doubling speed quadruples the braking distance.

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Very hard braking can cause brakes to overheat, making them less effective or even causing loss of control. This is why maintaining safe speeds and following distances is essential.

Remember!

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Key Points to Remember:

Stopping distance = thinking distance + braking distance
Speed doubles, thinking distance doubles, braking distance quadruples
Wet conditions and worn equipment increase stopping distance
Distractions and tiredness increase reaction time
Higher speeds make accidents much more likely and severe

Stopping distance (AQA GCSE Physics Combined Science): Revision Notes