Stopping Distances (AQA GCSE Physics): Revision Notes
Reaction time
What is reaction time?
Reaction time is the gap between when something happens (a stimulus) and when you respond to it. It shows how fast your brain can process information and tell your body what to do.
Real-world example: When you see a red traffic light, your reaction time is how long it takes from seeing the light to pressing the brake pedal. This demonstrates how reaction time affects everyday activities like driving.
Typical human reaction times
Most people take between 0.2 seconds and 0.9 seconds to react to something. That's quite quick, but it's still long enough to matter in situations like driving.
Some people can train to have faster reaction times:
- Professional athletes like cricketers and racing drivers
- Fighter pilots
Even highly trained individuals like professional athletes and fighter pilots can't get much faster than 0.2 seconds - this represents the physical limit of human neural processing speed.
Factors that make reaction time slower
Several things can make your reaction time longer:
- Tiredness - when you're sleepy, your brain works more slowly
- Alcohol and drugs - these affect how your brain processes information
- Distractions - thinking about other things means you're not focused
- Age - reaction times generally get slower as people get older
This is especially important for drivers, who need to react quickly to traffic lights, other cars, or hazards on the road. Impaired reaction times can be the difference between a safe stop and a dangerous collision.
Measuring reaction time with the ruler drop test
You can measure reaction time using a simple experiment with a ruler:
Method:
- One person holds a ruler vertically at the top
- Another person places their finger and thumb either side of the ruler (without touching it)
- The ruler is dropped without warning
- The person catches the ruler as quickly as possible
- Measure how far the ruler fell before being caught
Calculation:
The distance the ruler falls can be used to work out reaction time using this equation:
Where gravitational field strength is 10 N/kg on Earth.
Worked Example: Calculating Reaction Time
If a ruler falls 5 cm (0.05 m) before being caught:
Step 1: Identify the values
- Distance fallen = 0.05 m
- Gravitational field strength = 10 N/kg
Step 2: Apply the formula
Step 3: Calculate
Reaction time and driving
When driving, reaction time affects thinking distance - how far your car travels while you're deciding what to do.
The relationship is given by:
Worked Example: Calculating Thinking Distance
If you're driving at 30 km/h (8.3 m/s) and your reaction time is 0.25s:
Step 1: Apply the formula Thinking distance = speed × reaction time
Step 2: Substitute the values Thinking distance = 8.3 × 0.25 = 2.1 metres
The faster you drive, the further you travel during your reaction time, which is why speed limits matter for safety. At higher speeds, even a normal reaction time can result in significantly longer thinking distances.
Summary
Key Points to Remember:
- Reaction time is the gap between seeing something and responding to it
- Typical reaction times are 0.2 to 0.9 seconds for most people
- Factors like tiredness, alcohol, and distractions make reaction times longer
- The ruler drop test can measure reaction time using the formula
- In driving, reaction time affects thinking distance - how far you travel before you start braking