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Are your reaction times fast enough to be a race car driver? On SCOPE's Motor Car Science episode, you saw Briden doing a reaction-time test with Ruby. Here are the instructions for you to try it yourself.
You will need
A 30 centimetre ruler
Paper
Pen or pencil
A friend
What to do
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Hold the ruler above their hand, with the bottom end of the ruler between your friend’s thumb and finger so that the ‘0’ on the ruler is level with the top edge of their thumb and finger. |
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Now drop the ruler without warning your friend. Your friend should try to catch the ruler with their thumb and finger as quickly as possible. |
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After they’ve caught the ruler, take note of where they caught it – write down the measurement point that’s level with the bottom of their thumb (in millimetres). |
1. Ask your friend to hold one hand out in front of them, with their thumb and first finger about one centimetre apart.
2. Hold the ruler above their hand, with the bottom end of the ruler between your friend’s thumb and finger so that the ‘0’ on the ruler is level with the top edge of their thumb and finger.
3. Now drop the ruler without warning your friend. Your friend should try to catch the ruler with their thumb and finger as quickly as possible.
4. After they’ve caught the ruler, take note of where they caught it – write down the measurement point that’s level with the bottom of their thumb (in millimetres).
5. Now repeat the experiment four times, so you have five measurements altogether.
6. To calculate the average reaction speed, add all five measurements together and divide by five.
7. Try the experiment again, with your friend using their other hand. Does it make a difference?
8. You could also try this experiment on different people: boys vs girls, old people vs young people, computer game champs vs sporting stars. What conclusions can you reach?
What’s happening?
A person’s reaction time is influenced by the time taken for three different actions to occur.
1. Information passes from the eyes to the brain
2. The brain processes the information: first "the ruler is falling"; then "catch it!"
3. The brain sends a signal to the fingers to catch the ruler.
A hold-up in any part of this process can affect someone’s reaction times. You can train your body to improve your reaction time; that’s why you get better at computer games or sports if you practice.
The number you write down in this experiment is actually a distance. But because the ruler is falling due to gravity, it’s quite easy to convert the distance to a time using this formula:
t = √(2 x d ÷ 9.8)
Where
t = time (in seconds)
d = distance that the ruler fell (in metres – divide your results by 1000)
x = multiply
÷ = divide
√ = square root
On the Motor Car Science SCOPE episode, which explores the science of motor cars, Ruby gets an average result of 189.8 mm (0.1898 metres).
Using the formula, Ruby’s average reaction time is:
t = √(2 x 0.1898 ÷ 9.8)
= √(0.3796 ÷ 9.8)
= √(0.0387)
= 0.1968 seconds
So Ruby catches the ruler (on average) in less than 0.2 seconds! That’s pretty fast!
Applications
Quick reaction times are important in sports (particularly when stopping penalty goals) and when driving. Alcohol and other drugs can affect your reaction time, which is why there are laws about their use when driving. Your reaction times will also slow down if you are tired or if you are distracted (for example by talking on a mobile phone when driving).
The Reaction Time formula:
The formula above was derived from the common displacement/acceleration equation:
d = v0t + ½at2
where:
v0 = initial velocity (m/s)
t = time (s)
a = acceleration (m/s2)
In our activity, the ruler was not moving before it was dropped, so v0 is 0. And because the ruler is being dropped, the acceleration is due to gravity, i.e. 9.8 m/s2 for most places on Earth.
So the formula becomes:
d = ½ x 9.8 x t2
If we multiply both sides by 2, we get:
2 x d = 9.8 x t2
Then we can divide both sides by 9.8 to get:
2 x d ÷ 9.8 = t2
And finally, we take the square root of both sides to find t:
t = √(2 x d ÷ 9.8)
More information
Try these online reaction time tests: The Online Reaction Time Test and How fast are your reactions?
Find more information about reaction times and what can affect them: A Literature Review on Reaction Time
Here’s more information about the equation for displacement and acceleration.
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