Year 7 Science · Unit 4 · Lesson 6
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Learning Goals
True or false? Fix the false ones
Circle T or F. If a statement is false, correct it on the line.
1. Controlled variables are kept the same so that the test is fair.
Correct it:
2. The dependent variable is the one thing you deliberately change.
Correct it:
3. In a results table, the unit should be written next to every number.
Correct it:
4. When you measure something over time, a line graph is usually the best way to show the pattern.
Correct it:
5. Measuring a plant once, on one day, is enough to show how it grows over time.
Correct it:
6. In the seedling investigation, the number of hours of sunlight each plant gets per day is the independent variable.
Correct it:
Name the variables
Fill in the table: what is changed on purpose, what is measured, and one thing that must be kept the same.
| Question | Independent | Dependent | One controlled |
|---|---|---|---|
| 1. Does the colour of a cup affect how warm the water in it gets in the sun? | |||
| 2. Does the amount of fertiliser affect how many tomatoes a plant grows? | |||
| 3. Does the number of holes in the bottom of a paper cup affect how long it takes to empty? |
Order the steps: the stretching rubber band investigation
Number the stages from 1 (first) to 6 (last).
| Step | Stage |
|---|---|
| Describe the trend and write a conclusion that answers the question, using numbers from the data. | |
| Ask one clear question: does the mass hung on a rubber band affect how far it stretches? | |
| Hang 50, 100, 150 and 200 g from the band, three times each, and measure the stretch with a ruler in millimetres. | |
| Decide the variables: change the mass, measure the stretch, and keep the same rubber band and the same clamp stand. | |
| Plot the mass along the bottom axis and the stretch up the side, then draw a trend line. | |
| Record every result in a table, with the units in the column headings. |
Read the data: the stretching rubber band
Scenario
A class hung masses from one rubber band held by a clamp stand, and measured how much longer the band became each time. Table 1 shows their results.
| Mass hung (g) | Trial 1 | Trial 2 | Trial 3 | Average |
|---|---|---|---|---|
| 50 | 20 | 22 | 21 | 21 |
| 100 | 41 | 43 | 42 | 42 |
| 150 | 62 | 65 | 62 | 63 |
| 200 | 83 | 86 | 86 |
Source: sample class results.
1. Calculate the average stretch for the 200 g mass. Show your working.
Answer mm
2. Why is the average of three trials more useful than a single trial?
3. Describe the trend in Table 1, then write a conclusion that answers the question. Use two numbers from the table.
4. Priya’s group ran the same test. For the 200 g mass only, they used a thicker rubber band from another packet, and a classmate held it by hand instead of using the clamp stand. It stretched 30 mm, so Priya concluded: “A heavier mass does not always stretch a rubber band more.” Evaluate Priya’s conclusion, and explain how the group should fix their test.
True or false
1. True. 2. False: that is the independent variable; the dependent variable is what you measure. 3. False: the unit goes once, in the column heading. 4. True. 5. False: you need a series of observations over time, e.g. every few days. 6. True. 6 correct = 3; 4 or 5 = 2; 2 or 3 = 1. [3 marks]
Name the variables
1 mark per row, all three cells. 1: cup colour; water temperature (°C) after a set time; e.g. volume of water, time in the sun. 2: amount of fertiliser; number of tomatoes; e.g. plant type, water, pot size. 3: number of holes; time to empty (s); e.g. hole size, volume of water. [3 marks]
Order the steps: the stretching rubber band investigation
Reading down the table as printed, the correct step numbers are 6, 1, 3, 2, 5, 4. In order: ask the question; decide the variables; hang the masses and measure; record in a table; plot the graph; trend and conclusion. All correct = 2; four in the right place = 1. [2 marks]
Read the data: the stretching rubber band
1. (83 + 86 + 86) ÷ 3 = 255 ÷ 3 (1) = 85 mm (1). [2 marks]
2. One odd result or measuring slip has less effect on the average (1), so the results are more reliable and anomalies can be spotted (1). [2 marks]
3. Increasing: as the mass increases, the stretch increases (1). Conclusion that answers the question (1) with data, e.g. the average stretch rose from 21 mm with 50 g to 85 mm with 200 g, so a heavier mass stretches the band further (1). [3 marks]
Question 4
Marking guide: 1 the conclusion cannot be trusted; 1 both changes named: a thicker band, held by hand not the clamp stand; 1 more than one variable changed, so the thicker band, not the mass, could explain the small stretch; 1 a fair test changes only the independent variable; 1 fix: same band and clamp stand, three trials per mass; 1 the class data (85 mm with 200 g) suggest a larger stretch. [6 marks]
Model answer: Priya’s conclusion cannot be trusted. For 200 g the group changed the band and the support as well as the mass, so the thicker band could explain the 30 mm. A fair test changes only one thing: use the same band on the same clamp stand, three trials per mass. The class’s fair test gave 85 mm with 200 g, so a heavier mass does stretch the band further.
Common error: blaming “bad luck”. The problem is that controlled variables were changed.