Reaction time
Definition
Reaction time
The time between a stimulus happening and a person making a response to it.
Definition
Stimulus
A change in the surroundings that a person can detect, such as a light, a sound or a movement.
- A stimulus is a change that a person detects, such as a traffic light turning red, a ball being thrown or a ruler starting to fall.
- The response is the action the person makes after detecting it, such as pressing the brake pedal or closing their fingers.
- Reaction time is the time between the stimulus happening and the response being made.
- During this time a sense organ detects the stimulus, electrical signals travel along nerves to the brain, the brain decides what to do and signals travel back to the muscles.
- Each of these stages takes time, so nobody can react instantly.
- A typical reaction time in a simple test, such as catching a dropped ruler or pressing a button when a light comes on, is about 0.20.20.2 to 0.3 s0.3\ \text{s}0.3 s.
- A driver who is not expecting a hazard usually takes longer, about 0.7 s0.7\ \text{s}0.7 s, and anything from 0.50.50.5 to 0.9 s0.9\ \text{s}0.9 s is common.
- Reaction time varies from person to person and from one attempt to the next, so a single measurement is never enough to describe it.
- A person's reaction time is made longer by tiredness, alcohol, some drugs and distractions such as using a phone.
The ruler drop test
Definition
Free fall
The motion of an object when the only force acting on it is its weight.
- In the ruler drop test, one person holds a metre rule vertically while the other waits with their thumb and first finger open on either side of the zero mark.
- The ruler is released without warning, and the catcher closes their fingers as soon as they see it move.
- The distance the ruler falls before it is caught is read from the scale at the top of the catcher's thumb.
- After release the ruler is in free fall, so it accelerates at about 10 m/s210\ \text{m/s}^210 m/s2 and falls further in each successive moment.
- A longer reaction time gives the ruler more time to fall and speed up, so a larger distance means a longer reaction time.
- The distance is not directly proportional to the reaction time, because the ruler speeds up as it falls, so doubling the reaction time makes the ruler fall about four times as far.
- The distance can be turned into a reaction time with a conversion chart or by calculation.
- For the calculation, v2−u2=2asv^2 - u^2 = 2asv2−u2=2as gives the speed of the ruler when it is caught, using u=0u = 0u=0 because it starts from rest.
- The reaction time then comes from rearranging a=v−uta = \dfrac{v-u}{t}a=tv−u to give t=v−uat = \dfrac{v-u}{a}t=av−u.
Example
Converting a drop distance into a time
- A ruler falls 0.20 m0.20\ \text{m}0.20 m before it is caught, and its acceleration is 10 m/s210\ \text{m/s}^210 m/s2.
- Find the speed of the ruler when it is caught, using u=0u = 0u=0:
- Use the acceleration equation to find the time taken to reach this speed:
- The reaction time is 0.20 s0.20\ \text{s}0.20 s, which is a typical value for this test.
Electronic methods
- A computer reaction timer starts a clock when a shape or colour appears on the screen and stops it when the person presses a key or clicks the mouse.
- The program displays the reaction time directly, often in milliseconds, so 0.25 s0.25\ \text{s}0.25 s appears as 250 ms250\ \text{ms}250 ms.
- A lamp or buzzer connected to a data logger works in the same way, switching on at a random moment and starting a timer that stops when the person presses a switch.
- Electronic methods measure the time directly to the nearest millisecond, so they avoid converting a distance and avoid the error in reading a ruler scale.
- Changing the stimulus to a sound, or the response to pressing a foot pedal, lets the test model a driver more closely.
- A random delay before the stimulus is essential in every method, because a person who can predict the stimulus may respond before it happens.
Practical
Measuring reaction time with a falling ruler
- Aim: to measure a person's reaction time with a falling ruler and compare their reaction time with and without a distraction.
- Apparatus: a wooden or plastic metre rule with a clear centimetre scale, a table, a chair, a calculator or conversion chart and a results table.
- Method, setting up:
- The catcher sits with their forearm resting flat on the table so that their hand reaches just past the edge.
- The catcher uses the same hand, normally their writing hand, for every attempt.
- The tester holds the top of the ruler so that it hangs vertically, with the 0 cm0\ \text{cm}0 cm mark level with the top of the catcher's thumb.
- The catcher holds their thumb and first finger about 2 cm2\ \text{cm}2 cm apart on either side of the ruler, without touching it.
- Method, taking readings:
- The catcher watches the zero mark on the ruler and not the tester's hand.
- The tester waits a random time of between 111 and 5 s5\ \text{s}5 s and then lets go of the ruler without any warning.
- The catcher closes their thumb and finger on the ruler as quickly as possible.
- Read the distance fallen at the top of the thumb with your eye level with it, to avoid a parallax error, and record it to the nearest 0.5 cm0.5\ \text{cm}0.5 cm.
- Take at least five readings, then repeat the whole set while the catcher holds a conversation, to test the effect of a distraction.
- Variables: the independent variable is whether the catcher is distracted, the dependent variable is the distance fallen, which gives the reaction time, and the control variables are the ruler, the catcher, the hand used, the starting gap between finger and thumb and the starting position of the zero mark.
- Results: record each distance in a table, convert each one into a reaction time, leave out any anomalous result and calculate the mean reaction time for each condition.
- Maths: a drop of 0.18 m0.18\ \text{m}0.18 m gives v=2×10×0.18=1.9 m/sv = \sqrt{2 \times 10 \times 0.18} = 1.9\ \text{m/s}v=2×10×0.18=1.9 m/s, and so t=1.910=0.19 st = \dfrac{1.9}{10} = 0.19\ \text{s}t=101.9=0.19 s.
- Watch out: a catcher who watches the tester's hand can predict the drop, a zero mark set above or below the thumb adds the same error to every reading, and lots of practice attempts can shorten the reaction time and make the comparison unfair.
- Safety: keep feet clear of the falling ruler and never hold the ruler above head height.
Common Mistake
- Do not say that a larger drop distance means a faster reaction, because the ruler falls further when the reaction is slower.
- Do not assume that the distance fallen is directly proportional to the reaction time, because the ruler speeds up as it falls.
- Do not describe reaction time as a speed, because it is a time and is measured in seconds.
Exam technique
Describing a reaction time method
- Name the stimulus and the response in your method, such as the ruler being released and the catcher closing their fingers.
- Say that the ruler is released at a random moment without warning, because this detail is often missed.
- Say that the measurement is repeated and a mean is calculated, and explain that this reduces the effect of random variation between attempts.
- When asked to recall typical results, give a value with its unit, such as 0.2 s0.2\ \text{s}0.2 s for a ruler drop or about 0.7 s0.7\ \text{s}0.7 s for a driver who is not expecting a hazard.
Self review
- Define reaction time.
- State a typical reaction time measured with a ruler drop test.
- Explain why a longer reaction time makes the ruler fall further.
- Explain why the ruler must be released without warning.
- Calculate the reaction time of a person who catches a ruler after it has fallen 0.45 m0.45\ \text{m}0.45 m.
