Year 7 Science · Unit 3 · Lesson 9
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Learning Goals
Order the steps: weighing a bag of rice
Number the steps from 1 (first) to 5 (last).
| Step number | What you do |
|---|---|
| Move your head so your eye is level with the pointer, then read the scale. | |
| With nothing on the hook, check the pointer is at 0 N; turn the zeroing screw if it is not. | |
| Write the reading, with its unit, in the results table. | |
| Wait until the bag stops swinging and the pointer is still. | |
| Hang the bag of rice on the hook, holding the scale upright. |
1. Why should you wait for the bag to stop swinging before you take the reading?
2. A rockmelon pulls the pointer down to 7.35 N. How many grams is that? Divide by 9.8 N/kg to get kilograms, then convert.
Answer g
3. The rockmelon is swapped for a pumpkin, which stretches the same spring three times as far. The spring is still below its elastic limit. Use Hooke's Law to work out the new reading.
Answer N
Plot the data
Scenario: testing a spring for a homemade scale
A class tests a new spring. They pull it with forces from 0 to 5 N and measure its extension each time. The spring is only safe to use up to 6 N.
| Force (N) | 0 | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|---|
| Extension (mm) | 0 | 14 | 29 | 43 | 58 | 72 |
Source: sample class results.
4. Graph Table 1 on the grid: force on the horizontal axis, extension on the vertical axis, both with units. Then rule a straight trend line through the points.
5. Use your line to find (a) the extension for a 2.5 N force and (b) the force that stretches the spring 50 mm.
6. Ben extends the trend line and predicts that a 12 N force will stretch the spring about 173 mm. Explain why his prediction may be wrong.
7. Mei builds a spring scale from a spring, a pointer and a strip of card. Describe how she should calibrate it using 100 g masses, and explain why her markings should end up evenly spaced. Her spring is safe up to 6 N, like the one in Table 1. Say what would happen if she hung a 5 kg bag on it.
Order the steps
Reading down the table as printed, the correct step numbers are 4, 1, 5, 3, 2. In order: zero the scale with nothing on the hook; hang the bag with the scale upright; wait for it to stop swinging; read at eye level; record the reading with its unit.
Question 1
As the bag swings, the pull on the spring keeps changing and the pointer moves (1). A moving pointer gives an unreliable reading (1). [2 marks]
Question 2
7.35 ÷ 9.8 = 0.75 kg (1) = 750 g (1). [2 marks]
Question 3
Extension is proportional to force, so three times the stretch means three times the force (1): 3 × 7.35 = 22.05 N (1). [2 marks]
Question 4
Axes labelled Force (N) and Extension (mm) (1); six points plotted correctly (1); straight best-fit line through the origin (1). [3 marks]
Question 5
(a) about 36 mm (accept 35 to 37) (1). (b) about 3.5 N (accept 3.4 to 3.6) (1). [2 marks]
Question 6
12 N is far beyond the tested range and the 6 N safe limit (1). Past its elastic limit the stretch is no longer proportional and the spring stays stretched (1). [2 marks]
Question 7
Marking guide: 1 mark 0 N with nothing on; 1 hang 100 g and mark 1 N (100 g weighs about 1 N); 1 add 100 g at a time, marking 2 N, 3 N and so on; 1 Hooke's Law: equal steps in force give equal stretches, so even marks; 1 5 kg weighs about 49 N, far above 6 N; 1 the spring passes its elastic limit, stays stretched and reads wrongly from then on. [6 marks]
Model answer: Mei marks 0 N with nothing on. She hangs 100 g (about 1 N) and marks 1 N, then adds 100 g at a time for 2 N, 3 N and so on. Stretch is proportional to force, so the marks are evenly spaced. A 5 kg bag (about 49 N) would stretch the spring past its elastic limit and ruin the scale.
Common error: extending a line of best fit far past the tested range.