How To Calculate Your Reaction Times With A Ruler Content from the guide to life, the universe and everything

How To Calculate Your Reaction Times With A Ruler

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A ruler.

We have all heard people talking about reaction times, usually in connection with how good a driver they are or how quickly a fighter pilot has to think. Computer programs exist that can time how long it takes you to hit a key when an image appears on a screen. But there is a simpler way to calculate your reaction times using only a ruler.

In this experiment we are going to discover how quickly you can catch a ruler when your friend drops it. By measuring how far it falls, we can calculate your reaction time. You will need:

  • A ruler with a centimetre scale
  • A friend
  • A pencil and piece of paper, or a calculator1

Ask your friend to hold the ruler straight up and down, suspended from one end by their fingers. Line up the two fingers you are going to use to catch the ruler with the zero on the cm scale. Keep your fingers as close to the ruler as possible without touching it. Your friend should now drop the ruler. As soon as you see the ruler move, grab it.

Take a note of the point on the ruler where you caught it. The h2g2 experimenter caught the ruler at the 14cm mark, so we can work out their reaction time.

When things fall close to the Earth's surface they accelerate at a constant rate of approximately 9.8 metres per second per second. That means that at the end of each second, the falling thing is moving 9.8 metres per second faster than it was at the end of the previous second. Wind and air resistance can affect this, but since we are using a ruler, which is fairly aerodynamic, and conducting the experiment indoors, these factors do not really make a difference to the result.

The formula we need is: reaction time = square root(distance fallen in centimetres / 490)

We know that the ruler fell 14cm before it was caught. So we divide 14 by 490 and take the square root. This gives a result of 0.169. The reaction time was 0.169 seconds, or 169 thousandths of a second.

That is pretty amazing when you consider everything that happened. First, your eye had to see the ruler was moving and send a signal to the brain. Then the brain had to send a signal all the way to the muscles in your fingers. Then the muscles had to contract and pull your fingers closed. This was all done in less than 17 hundredths of a second.

How fast can you react?

1This is recommended, as you will be calculating square roots later.

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