P5 Reaction Times (page 215)

Question 1

Everyone’s reaction time is different, but what is a typical reaction time?

Answer 1

between 0.2 and 0.9 s.

Question 2

How can peoples reaction times be affected?

Answer 2

by tiredness, drugs, or alcohol.

Distractions can also affect your ability to react.

Question 3

How can you measure ‘Reaction Times?

Answer 3

with the ‘Ruler Drop Test’

(you can do simple experiments to investigate your reaction time, but as reaction times are so short, you haven’t got a chance of measuring one with a stopwatch).

Question 4

Apart from the ‘Ruler Drop Test’ what other ways can you measure reaction times?

Answer 4

by using a computer-based test (e.g. clicking a mouse when the screen changes colour).

Question 5

Explain the ‘Ruler Drop Test’ experiment process?

Answer 5

1) Sit with your arm resting on the edge of a table (this should stop you moving your arm up or down during the test).

Get someone else to hold a fuler so it hangs between your thumb and forefinger, lined up with zero. (you may need a third person to be at eye level with the ruler to check it’s lined up).

2) Without giving any warning, the person holding the ruler should drop it. Close your thumb and finger to try to catch the ruler as quickly as possible.

3) The measurement on the ruler at the point where it is caught is how far the ruler dropped in the time it takes you to react.

4) The longer the distance, the longer the reaction time.

5) You can calculate how long the ruler falls for (the reaction time) because acceleration due to gravity is constant (roughly 9.8 m/s²)

If you catch the ruler at 20 cm. From p.208 you know: v² - u² = 2as.

U = 0, a = 9.8 m/s² and s = 0.2 m so: v = √2x9.8 x 0.2 + 0 = 1.97…m/s

V is equal to the change in velocity of the ruler.

You also know a = Δv ÷ t so t = Δv ÷ a = 1.97… ÷ 9.8 = 0.202… s = 0.2 s (to 1 s.f.)

This gives your reaction time.

Question 6

It is hard to do the ‘ruler drop test’ accurately, so what should you do to get the best results?

Answer 6

you should do a lot of repeats and calculate an average reaction time. The results will be better if the ruler falls straight down - you might want to add a blob of modelling clay to the bottom to stop it from waving about.

Question 7

How do you make the ruler drop test, a fair test?

Answer 7

use the same ruler for each repeat and have the same person dropping it.

Question 8

You could try investigating some factors affectig reaction time, give some examples?

Answer 8

you could introduct distractions by having some music playing or by having someone talk to you while the test takes place (see p.214 for more on the factors affecting reaction time).

Remember to still do lots of repeats and calculate the mean reaction time with distractions, which you can compare to the mean reaction time without distractions.

Question 9

Mark’s reaction time is tested using the ruler drop test. He is tested in the early afternoon and at night. In the afternoon, he catches the ruler after it has fallen a distance of 16.2 cm. At night, he catches the ruler after it has fallen 18.5 cm

a) calculate Mark’s reaction time in the afternoon. Give your answer to 2 significan figures. (5 marks)

b) Explain why Mark’s thinking distance might be longer when driving in the evening (2 marks)

Answer 9

a) v² - u² = 2as
v² 2 x 9.8 0.162 + 0 [1 mark] = 3.1752 m²/s²
v² = √3.1752 = 1.781…m/s [1 mark]
a = Δv ÷ t so
t = Δv ÷ a [1 mark]
= 1.781… ÷ 9.8 [1 mark] = 0.181… s
= 0.18 s (to 2 s.f.) [1 mark]

b) His reaction time is longer in the evening [1 mark] so whilst driving, he may take longer to react to a hazard, meaning his thinking distance would be longer [1 mark]