Year 7 Science · Unit 3 · Lesson 9
Challenge Worksheet
Learning Goals
1. What is the elastic limit of a spring? Why does every spring scale carry a maximum force rating?
Match each term to its meaning
Write the letter of the correct meaning next to each term. Each letter is used once.
| Term | Letter | Meaning |
|---|---|---|
| newton (the unit) | A. A wrong reading caused by looking at the pointer from an angle | |
| Balance | B. Checking an instrument against known values so that it reads correctly | |
| Calibration | C. The unit of force | |
| Parallax error | D. Setting the pointer to 0 N with nothing on the hook | |
| Zeroing | E. An instrument that measures mass by comparing it with known masses |
Find the mistakes
A student wrote this method
“To weigh my school bag, I grabbed a spring scale rated to 20 N. It showed about 0.5 N with nothing on it, but that was close enough. I hooked the bag on and read the scale straight away, while the bag was still swinging. The bag was heavy, so I held the scale at a slant and looked down at the pointer from above. The pointer went right to the end of the scale, so I wrote down 20 N.”
2. Find four different errors in this method. For each one, quote the error and say how to fix it.
Read the data
Scenario: testing three faulty scales
A lab technician hangs known masses from three old spring scales, P, Q and R. The second column shows what a perfect scale would read. Each old scale has a different fault.
| Mass (g) | Correct reading (N) | Scale P (N) | Scale Q (N) | Scale R (N) |
|---|---|---|---|---|
| 100 | 1.0 | 1.5 | 1.1 | 1.2 |
| 200 | 2.0 | 2.5 | 2.2 | 1.8 |
| 300 | 2.9 | 3.4 | 3.2 | 3.0 |
| 400 | 3.9 | 4.4 | 4.3 | 3.7 |
Source: made-up results for this worksheet.
3. Match each scale to its most likely fault: not zeroed; spring stretched past its elastic limit; read carelessly from different angles. Use the pattern of errors in Table 1 as evidence for each.
4. How could the technician confirm scale P's fault in two seconds, and how should she correct the readings it has already given?
5. Liam wants to weigh his 3 kg cat with the class's 10 N spring scale. He says: “Hooke's Law says the stretch always keeps step with the force, so the scale will still work. I'll just measure how far past the end of the scale the pointer goes.” Evaluate Liam's plan.
Question 1
The largest force a spring can take and still spring back to its original length (1). Beyond it the spring is permanently stretched and every reading is wrong, so the rating warns you not to overload it (1). [2 marks]
Match each term to its meaning
newton (the unit) → C; Balance → E; Calibration → B; Parallax error → A; Zeroing → D.
Find the mistakes: Question 2
Any four, 1 each (error and fix): “0.5 N with nothing on it”: zero it first; “still swinging”: wait until the pointer is still; “at a slant”: hold the scale upright; “from above”: read with your eye level with the pointer (parallax); “right to the end”: the bag is over the 20 N rating, so use a scale with a higher rating. [4 marks]
Question 3
P: not zeroed, always 0.5 N too high, a fixed amount (1). Q: past its elastic limit, the error grows with the load, 0.1 to 0.4 N (1). R: read from different angles, errors go up and down with no pattern (1). [3 marks]
Question 4
Take the mass off and check the pointer reads 0.5 N with nothing on it (1). Subtract 0.5 N from every reading it gave (1). [2 marks]
Question 5
Marking guide: 1 weight = 3 × 9.8 = 29.4 N; 1 about three times the 10 N rating; 1 Hooke's Law only holds up to the elastic limit; 1 beyond it the stretch no longer keeps step with the force, so the extra stretch gives a wrong value; 1 the spring is permanently stretched, ruining the scale for everyone; 1 better: a scale rated above 30 N, or a balance (3 kg). [6 marks]
Model answer: The plan will not work. The cat weighs 3 × 9.8 = 29.4 N, about three times the rating. Hooke's Law only holds up to the elastic limit; past it the stretch stops keeping step with the force, so the reading is wrong, and the spring stays stretched. Liam should use a scale rated above 30 N, or a balance.
Common error: treating Hooke's Law as true for any force.