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• Reaction time challenge

Learning objectives

After completing the practical you should be able to:

• measure a person’s reaction time
• estimate the uncertainty in a measurement.

When you are driving, your stopping distance is affected by your reaction time. Using a mobile phone is a distraction when driving. How does it affect your reaction time?

• Make sure the ruler will not land on anyone’s foot. Avoid rulers with sharp edges – wooden rulers are best. Take particular care when carrying or dropping metre rulers.

Equipment and materials
• Metre ruler
• Table converting fall distance to reaction time
• Mobile phone

1. Hold the ruler near the 100 cm mark and let it hang straight down so that when your partner is sitting with their hand held out, most of the ruler is above their hand.
2. Your partner places a thumb and forefinger about 1 cm from each side of the ruler, without touching it. Note what marking (in cm) is just above their forefinger on the ruler.
3. Without warning you drop the ruler. Your partner grabs it between finger and thumb.
4. Check the marking (in cm) just above where their forefinger grabbed the ruler.
5. Calculate the distance fallen by the ruler (the difference between the two markings).
6. Repeat the above procedure five times and record the results in a table.
7. Repeat another five times, but during these tests get your partner to hold the mobile phone with their free hand and tell an imaginary caller about what they did last weekend, or about a TV programme they watched, or a book they read. Record the results in the table.
8. Use the table which converts fall distance to reaction time to find the reaction times for each ruler catch. For an extra challenge, you could calculate the reaction times instead – you will find how to do this in the Extension section.

Before you start to take measurements, write down a prediction: what effect do you think the distraction will have on the reaction times?

Use this table to record your results.

| |Fall distance in cm |Mean reaction time in |
| | |ms |
| |1st drop |
|Fall time with parachute in s |1.75 |

a. Suggest why it may be more difficult for these students to estimate the uncertainty in their time measurements.

[pic] (1 mark)

b. The students do not have time to make any more measurements. Suggest an estimate for the uncertainty in the fall times. Justify your answer.


[pic] (2 marks)

9. Suggest one reason why a scientist might prefer to measure times using light gates or other automatic timing devices instead of a stopwatch.

[pic] (1 mark)

Experiment extension
Step 8 of the Method invited you to calculate the reaction times for each ruler catch.

If you are calculating reaction times yourself, the instructions below show how to do this.

The ruler falls with an acceleration of about 10 m/s2, because of Earth’s gravity. To calculate a reaction time (t) from a fall distance (d), use this formula:

[pic], where t is in seconds and d is in metres.

Here is an example calculation, for a fall distance of 16 cm (that is, 0.16 m):

[pic] 0.18 s, or 180 ms (to 2 significant figures).[pic]