Reaction Time

Definition of Reaction Time

Reaction time is the time taken for a person to respond to a stimulus. It is measured in seconds (s) and represents the delay between noticing something and beginning to react to it. Reaction time varies between individuals and can be affected by many factors such as alertness, age, and distractions.

For example, if a ruler falls 20 cm (0.20 m) before being caught, the reaction time can be calculated using the formula $t = \sqrt{\frac{2d}{g}}$ as $t = \sqrt{\frac{2 \times 0.20}{9.8}} \approx 0.202$ seconds.

Factors Affecting Reaction Time

• Fatigue and tiredness: When a person is tired, their brain processes information more slowly, increasing reaction time.
• Alcohol and drugs: These substances impair brain function and slow down the nervous system, leading to longer reaction times.
• Distractions: Anything that takes attention away from the stimulus (e.g., using a mobile phone, loud noises) can delay reaction time.
• Age: Reaction times tend to be faster in young adults and slower in children and older people due to differences in nervous system efficiency.

Measuring Reaction Time

Reaction time can be measured using simple tests such as the ruler drop test or computer-based tests.

Ruler Drop Test

In this test, a ruler is held vertically above a person's open hand without touching it. The ruler is dropped without warning, and the person must catch it as quickly as possible. The distance the ruler falls before being caught is recorded.

The reaction time is calculated using the formula derived from the equations of motion:

$$t = \sqrt{\frac{2d}{g}}$$ where $t$ is the reaction time in seconds, $d$ is the distance fallen in metres, and $g = 9.8 \, \text{m/s}^2$ is the acceleration due to gravity.

Multiple trials should be done to get an average reaction time for accuracy.

For example, if the ruler falls 20 cm (0.20 m) before being caught:

$$t = \sqrt{\frac{2 \times 0.20}{9.8}} = \sqrt{0.0408} \approx 0.202 \text{ seconds}$$

Computer-Based Tests

These tests display a stimulus on a screen and measure the time taken for the participant to press a key or click a mouse in response. They are more precise and can record reaction times to the nearest millisecond.

Reaction Time and Stopping Distance

Reaction time is important in driving because it affects the thinking distance—the distance a vehicle travels during the driver's reaction time before the brakes are applied. The longer the reaction time, the longer the thinking distance.

Since the thinking distance depends on the speed of the vehicle, higher speeds increase the stopping distance. This is why driving too fast reduces the time available to react safely.

Understanding reaction time helps improve road safety by emphasising the dangers of tiredness, alcohol, and distractions while driving.

For example, if a driver’s reaction time is 0.8 seconds and they are travelling at 20 m/s (72 km/h), the thinking distance is:

$$\text{Thinking distance} = \text{speed} \times \text{reaction time} = 20 \times 0.8 = 16 \text{ metres}$$