Stopping distance (AQA GCSE Physics): Revision Notes
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 really important for road safety because it helps us understand how much space drivers need to stop safely.
When a driver sees danger ahead, the car doesn't stop immediately. The car keeps moving while the driver reacts, then continues moving while the brakes slow it down. The total distance covered during this whole process is the stopping distance.
Understanding stopping distance is crucial for safe driving. It explains why drivers need to maintain safe following distances and why speed limits exist, especially in areas where stopping quickly might be necessary.
The two parts of stopping distance
Stopping distance has two parts that add together:
Stopping distance = Thinking distance + Braking distance
Thinking distance
This is how far the car travels while the driver notices the danger and reacts by pressing the brakes. During this reaction time, the car is still moving at the same speed.
Braking distance
This is how far the car travels while it's actually slowing down after the brakes have been applied. The car gradually gets slower until it stops completely.
What affects thinking distance?
Several things can make thinking distance longer:
- Speed - The faster you're going, the further you travel during reaction time
- Being tired - Tired drivers react more slowly
- Distractions - Using phones or talking can slow reaction times
- Alcohol or drugs - These substances slow down reaction times significantly
Key fact: When speed doubles, thinking distance doubles too. So if thinking distance is 6m at 20mph, it becomes 12m at 40mph.
What affects braking distance?
Braking distance depends on several factors:
- Speed - This has the biggest effect on braking distance
- Road conditions - Wet or icy roads reduce grip between tyres and road
- Tyre condition - Worn tyres don't grip the road as well
- Brake condition - Worn brakes don't work as effectively
- Mass of the vehicle - Heavier vehicles take longer to stop
Key fact: When speed doubles, braking distance increases by four times! So if braking distance is 6m at 20mph, it becomes 24m at 40mph.
Speed and stopping distance relationships
Understanding how speed affects stopping distance is crucial:
-
Thinking distance is directly proportional to speed
- Double the speed = double the thinking distance
-
Braking distance is proportional to the square of the speed
- Double the speed = four times the braking distance
This means that small increases in speed cause big increases in overall stopping distance, especially because of the braking distance effect.
Worked Example: Comparing Stopping Distances
At 30 mph:
- Thinking distance = 9m
- Braking distance = 14m
- Total stopping distance = 23m
At 60 mph (double the speed):
- Thinking distance = 18m (doubled)
- Braking distance = 56m (four times larger)
- Total stopping distance = 74m
Notice how doubling the speed more than triples the total stopping distance!
Why braking distance increases so much
When a vehicle brakes, it needs to get rid of all its kinetic energy. The faster something moves, the more kinetic energy it has. In fact, kinetic energy increases with the square of speed, which explains why braking distances increase so dramatically.
The relationship between kinetic energy and speed can be expressed as:
Where is mass and is velocity. Since energy is proportional to , this explains why braking distance (which depends on removing this energy) increases with the square of speed.
Key Points to Remember:
- Stopping distance = thinking distance + braking distance
- When speed doubles, thinking distance doubles but braking distance increases by four times
- Many factors affect stopping distances - speed is the most important one
- Wet roads, worn tyres, and tired drivers all increase stopping distances
- Higher speeds make accidents much more serious because stopping distances become very long