Reaction time (AQA GCSE Physics Combined Science): Revision Notes
Reaction time
What is reaction time?
Reaction time is the time 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.
For example, when you see a red traffic light, your reaction time is how long it takes from seeing the light until you actually press the brake pedal.
Typical reaction times
Most people have a reaction time between 0.2 seconds and 0.9 seconds. This might seem very quick, but it's actually quite important, especially when driving.
Some people can train to get faster reaction times. Athletes like sprinters and cricket players often work to improve their reactions through practice.
Factors that slow down reaction time
Several things can make your reaction time slower:
Three major factors that dangerously increase reaction time:
- Tiredness - when you're sleepy, your brain works more slowly
- Alcohol and drugs - these affect how your brain processes information
- Distractions - if you're thinking about other things, you react more slowly
This is why these factors are so dangerous when driving. Even a small increase in reaction time can mean the difference between stopping safely and having an accident.
Reaction time and driving
When driving, reaction time affects your thinking distance. This is how far your car travels while your brain processes a hazard and decides to brake.
If you're driving at 30 km/h and something unexpected happens, your car keeps moving forwards during your reaction time. The longer your reaction time, the further you travel before you start braking.
Safety Impact: Longer reaction times mean greater thinking distances and higher accident risk - this is why factors like tiredness and alcohol are so dangerous for drivers.
Measuring reaction time
You can measure reaction time using a simple ruler drop test. Here's how it works:
Practical Method: The Ruler Drop Test
Step 1: One person holds a ruler vertically
Step 2: Another person places their fingers near the bottom of the ruler (about 5cm gap)
Step 3: The first person drops the ruler without warning
Step 4: The second person tries to catch it as quickly as possible
Step 5: You measure how far the ruler fell before being caught
Key principle: The further the ruler falls, the slower the reaction time was.
Calculating reaction time from ruler drops
You can work out the exact reaction time using this equation:
(Note: We use 10 because that's the gravitational field strength on Earth in m/s²)
Worked Example: Calculating Reaction Time
If a ruler falls 0.16m before being caught:
Step 1: Apply the formula
Step 2: Calculate inside the square root
Step 3: Find the final answer
Getting reliable results
Improving Accuracy of Results
To get accurate measurements, you should:
- Repeat the test several times
- Calculate the average (mean) distance fallen
- Use this average in your calculation
This helps account for any mistakes or random variations in the results.
Key Points to Remember:
- Reaction time is the gap between seeing something happen and responding to it
- Typical reaction times are between 0.2s and 0.9s for most people
- Three main factors slow reaction time: tiredness, alcohol/drugs, and distractions
- Ruler drop test can measure reaction time - the further it falls, the slower your reactions
- In driving, longer reaction times mean greater thinking distances and higher accident risk