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Forces and Motion: 5.9 Human response times - simple reaction time experiments in the school laboratory - computer screen test and falling ruler

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INDEX of physics notes: reaction times, stopping distances of road vehicles, Newton's 2nd Law, KE calculations


5.9 Human response times - simple reaction time experiments in the school laboratory

Can you devise a simple experiment to measure somebody's reaction time?

YES, but, can be accompanied by some moderately complicated calculations!

Your reaction time to a situation may be typically 0.2 to 0.8 seconds when fully alert. However your reaction time can be affected by  tiredness, feeling unwell, drugs, alcohol, in other words anything that affects the speed of your brain function.

See An introduction to the nervous system including the reflex arc (GCSE level biology notes)

You can conduct quite simple experiments to test your reaction time to a particular situation. However, since the reaction time is too short, a stopwatch is no good, but there are ways of measuring your reaction time indirectly by making other measurements from which you can calculate your reaction time.

(a) Computer screen test - where you respond as quickly as possible to something appearing on the screen.

In this situation, the computer software generates something up on the screen and automatically times your response by monitoring your contact with the keyboard or by clicking the mouse.

I've quickly written an extremely simple computer programme to test your response to a X appearing on the screen.

Response time test: It probably only works on Microsoft platforms, and maybe not all of them?

Your anti-virus protection might query it, because it is a .exe file, but its written with compiled BBC BASIC and should not pose any threat. Unfortunately I never learned to write in a multi-platform professional computer programming language, but I'm not exactly short of website projects!

 

(b) A simple physical response test - catching the falling ruler drop test

You get someone to hold a ruler vertically, with thumb and first finger, above someone else's hand, who is ready to catch it with their thumb and first finger.

First image on the right. The ruler should be held at the top of the scale and steady hands from both people.

The catching person should have the middle of their thumb and finger adjacent to zero on the cm scale - squat down to make sure you are reading the scale horizontally.

Then, without warning, the person holding the ruler, lets go of it. The second person has to react as fast as possible and catch the dropped ruler between their thumb and first finger.

Second image on the right. The longer the distance, the slower your reaction time!

When caught, you then read how far the ruler as fallen by taking the reading, to the nearest centimetre, from where the middle of their thumb and finger are.

You repeat the experiment a number of times to get an average, but its not a particularly accurate experiment.

You need to have steady hands and not let the ruler wobble about or fall at an angle other than vertical. You should also use the same ruler and the same people dropping the ruler and catching it (fair test criteria), though, obviously, you can compare one person's results with another.

The slower your response time, the further the ruler falls before being caught. You might repeat the experiment by having some background distractions - a group of people talking nearby, or somebody trying to engage you in conversation or music.

Follow-up question on response time

 You can then do some 'nifty' calculations to actually obtain a real response time - so you using indirect data to get the response time.

It involves a two stage calculation (at the highest GCSE level !!!).

Suppose the ruler is caught after an average fall of 25 cm.

(i) You use the equation v2 - u2 = 2ad, to calculate the final velocity  (more calculations using this equation)

v = final velocity (m/s), u = initial velocity (m/s) = 0, a = acceleration = 9.8 m/s2 (gravitational field acceleration constant),

and d = distance fallen (m) before catching the ruler.

Since u = 0 and d = 25/100 = 0.25 m

v2 - 0 = 2 x 9.8 x 0.25 = 4.9

v = √4.9 = 2.214 m/s (its not this accurate, but we'll leave the s.f. decision until the end)

(ii) We can now use the acceleration formula to calculate the response time.

a = ∆v / ∆t, where a = acceleration (9.8 m/s2), ∆v = change in velocity (m/s) and ∆t = response time

Therefore : 9.8 = 2.214 / ∆t,    ∆t = 2.214 / 9.8 = 0.23 s (2 s.f.)

So, on average, the response time was about a quarter of a second.

 

INDEX of physics notes on reaction times, stopping distances of road vehicles, Newton's 2nd Law, braking friction force, KE calculations


Keywords, phrases and learning objectives for the physics of road vehicles response time experiments

Be able to describe and explain a simple human response experiment to measure a reaction time e.g. catching the falling object in the school laboratory or a computer screen test.


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