Egypt Primary 4 Science, Week 5: How fast can you react?

Reacting quickly

Week 5 · Axis 1, Unit 1 Living Systems, Concept 1.2 How Do Senses Work? (B) · 3 lessons

How fast can you react?

Discovery Education Science P4: Term 1, Unit 1, pp. 47–57

GoalMeasure reaction time with a falling stick, find the average of three tries and compare sight with hearing.

New words

reaction time · how long it takes to react to somethingaverage · add the results and divide by how many there arefair test · a test where only one thing is changed

Explanation

Reaction time is the time between a signal and your response. We can measure it with a stick. A grown-up supervises. Sit at a table with your arm resting on the edge. A partner holds a metre stick at the top so that the zero end hangs between your open thumb and finger, not touching. The partner lets go without warning, and you catch it. The number where your finger stops is how far it fell, and a longer fall means a slower reaction. The chart turns distance into time. Do three tries and find the average: add the three times and divide by three. Then change one thing, the signal: close your eyes and catch when your partner says “now”. Keep everything else the same. Compare the averages. Never throw the stick, and keep it away from faces.

Distance caught (cm)	5	10	15	20	25	30
Reaction time (s)	0.10	0.14	0.17	0.20	0.23	0.25

Worked examples

Mariam catches the stick with her eyes open. Her three catches are 10 cm, 15 cm and 20 cm. From the chart the times are 0.14 s, 0.17 s and 0.20 s. Average: (0.14 + 0.17 + 0.20) ÷ 3 = 0.51 ÷ 3 = 0.17 s.

With her eyes closed Mariam catches at 15 cm, 20 cm and 25 cm. The times are 0.17 s, 0.20 s and 0.23 s. Average: 0.60 ÷ 3 = 0.20 s.

Signal	Average time
sight	0.17 s
hearing	0.20 s

Mariam’s average with sight is 0.17 s and with hearing 0.20 s. For her, reacting to what she sees was a little quicker. Her result is only one result: another child may get a different one. We compare the senses, not the children.

Class activity

Reaction-time investigation. With a grown-up supervising, do the stick test with a partner: three tries with sight, three with hearing. Record the distances, change them to times with the chart, then find each average. Predict first: which signal will give the shorter time? Swap places so both of you have a turn. Write: what you changed, what you kept the same, which average was shorter.

Practice

Seif catches the stick at 20 cm and Nour at 10 cm. Who reacted faster?

Read the chart: what time matches a catch at 25 cm?

Three catches give 0.14 s, 0.17 s and 0.17 s. Find the average.

Why must everything stay the same except the signal?