5.6.3Forces
Factors Affecting Thinking Distance and Reaction Time — AQA GCSE Combined Science Trilogy 8464 - Physics Revision Notes & Topical Questions
5 min read 7 parts Lesson 39 of 44
Section 1 of 7Reaction time basics
What it is
- What it is Reaction time is the time between a stimulus, such as a hazard appearing, and the person's response, such as pressing the brake.
- A typical value for a healthy adult is between 0.2 s and 0.9 s.
Link to thinking distance
- Link to thinking distance Thinking distance = speed × reaction time.
- At a given speed, a longer reaction time means a longer thinking distance, so the total stopping distance rises.
Speed matters too
- Speed matters too At a fixed reaction time, doubling the speed doubles the thinking distance because the car covers twice as much ground in the same time.
Section 2 of 7Factors that lengthen reaction time
Slower response
Tiredness
- Tiredness A tired driver's brain processes information more slowly and attention may lapse.
Alcohol
- Alcohol Alcohol is a depressant that slows the nervous system and impairs judgement.
Drugs
- Drugs Many illegal drugs and some medicines cause drowsiness or confusion and lengthen the response.
Distractions
- Distractions Using a mobile phone, talking to passengers or loud music divides attention so a hazard is noticed later.
Section 3 of 7Measuring reaction time: ruler drop test
1
Ask a partner to hold a 30 cm ruler vertically with the zero mark level with your open thumb and finger. Your hand must be resting on a table edge.
2
Without warning, the partner lets go. Catch the ruler as quickly as you can.
3
Read the distance on the ruler at the top of your finger, in centimetres.
4
Convert the distance to a time using a calibration table or the equation s = ½ × g × t2, so t = √(2s ÷ g).
5
Repeat several times, then calculate the mean and compare conditions such as a distraction or a different hand.
Section 4 of 7Sample results
| Distance ruler fell | Time calculated | Meaning |
|---|---|---|
| 5 cm | about 0.10 s | Very fast reaction |
| 10 cm | about 0.14 s | Fast reaction |
| 20 cm | about 0.20 s | Typical reaction |
| 30 cm | about 0.25 s | Slower reaction |
Section 5 of 7Improving the investigation
1Keep variables fixed
- Keep variables fixed Use the same ruler, same partner dropping and the same starting position each time.
2Use many repeats
- Use many repeats Repeat at least five times and discard anomalies to reduce random error.
3Use computers
- Use computers A computer reaction test or light gates gives times with higher resolution than reading a ruler.
4Avoid anticipation
- Avoid anticipation The dropper should vary the delay so the catcher cannot guess the release moment.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Reaction time | The time taken to respond to a stimulus. |
| Thinking distance | The distance a vehicle travels during the driver's reaction time. |
| Stimulus | A change in the surroundings that causes a response. |
| Depressant | A substance that slows down the activity of the nervous system. |
| Distraction | Something that draws the driver's attention away from the road. |
Section 7 of 7Exam technique
1
Writing that tiredness or alcohol increases braking distance as a direct effect. They change reaction time and so thinking distance.
2
Saying thinking distance depends on the road surface. It does not; that factor affects braking distance.
3
Reading the ruler from the wrong place. Measure from the zero mark to the top of the fingers, not to the bottom.
4
Forgetting to repeat the ruler drop and relying on one result, which may be a lucky guess.
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