Measuring Forces — Spring Scales
In 1660, Robert Hooke stretched hundreds of springs and noticed that doubling the stretch always doubled the pull-back force — a pattern now built into every spring scale on Earth.
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Q1 · Hold a book in your hand with your arm straight. As you add more books, what happens to the force on your arm? How could you measure that force?
Q2 · Two scales: one shows kilograms, one shows Newtons. What different things are they measuring?
● Know
- The Newton (N) is the SI unit of force
- A spring scale (Newton meter) measures force in Newtons
- Hooke's Law qualitatively: spring extension is proportional to applied force
● Understand
- How a spring scale works (extension → force reading)
- Why a spring scale would read differently on the Moon (but a balance would not)
- What calibration means and why it matters
● Can do
- Use a spring scale correctly: zero, hang, read at eye level
- Identify and avoid parallax error
- Distinguish between measuring force (N) and measuring mass (kg)
A spring scale measures in . When you hang an object from the hook, the spring and the pointer moves to show the .
Every spring has a personality. Stretch it a little — it pulls back a little. Stretch it a lot — it pulls back a lot. Robert Hooke noticed this in 1660, and now every spring scale in every science lab owes him a debt.
Hooke's Law (qualitative): The extension of a spring is proportional to the applied force — up to the elastic limit.
- Apply 1 N → spring stretches 1 cm.
- Apply 2 N → spring stretches 2 cm.
- Apply 4 N → spring stretches 4 cm.
- Pattern holds until the elastic limit — above this, the spring is permanently deformed.
A spring scale (Newton meter) works on exactly this principle:
- An object hangs from the hook, pulling the spring down with its weight.
- The greater the force, the more the spring stretches.
- A pointer attached to the spring moves along a scale marked in Newtons.
- You read the force directly in Newtons.
Using a spring scale seems simple — but there are five important steps that prevent measurement errors:
- Zero the scale. Before attaching anything, check that the pointer reads exactly 0 N. If not, adjust using the zeroing screw. This is essential — a scale not zeroed will give readings that are too high or too low by a fixed amount.
- Hang the object vertically. Hold the spring scale upright and attach the object at the hook. If the scale is tilted, gravity doesn't act fully along the spring axis and you get a wrong reading.
- Wait for the scale to settle. Let it stop swinging before reading. A moving pointer gives an unreliable reading.
- Read from eye level (avoid parallax error). Position your eye directly in front of the pointer — at the same height as the pointer. Looking from above makes the reading appear lower; looking from below makes it appear higher.
- Don't exceed the maximum force. Every spring scale has a maximum force rating (e.g. 10 N). Beyond this, the spring is permanently stretched and the calibration is ruined.
Spring scale vs balance scale:
- Spring scale: Measures the gravitational force (weight) in Newtons. On the Moon, where gravity is weaker, it would give a lower reading — the weight is less.
- Balance scale: Compares the object's mass against known masses. Mass doesn't change with gravity — so a balance gives the same reading on the Moon as on Earth.
Calibration means making sure an instrument reads correctly. For a spring scale:
- Hang a known mass (e.g. 100 g, which has a weight of about 1 N on Earth).
- Mark the pointer position as "1 N".
- Repeat with 200 g (2 N), 300 g (3 N), and so on.
- Connect the marks — this is the calibrated scale.
If the spring is old or damaged, the calibration may be wrong — this is why you always zero the scale first.
Parallax error in detail: The pointer sits slightly in front of the scale markings. If you look from above, the pointer appears to be opposite a higher number. If you look from below, it appears lower. The fix: align your eye with the pointer at exactly the same height, looking straight across.
Australian applications of force measurement:
- Fishing scales: Anglers use spring scales (in N or kg equivalent) to weigh their catch after fishing on the Great Barrier Reef or in Australian rivers.
- Luggage scales at airports: Sydney, Melbourne and Brisbane Airport staff use scales that measure in kg — but they measure the force of gravity on the bag and convert.
- Sports science: AFL and NRL teams use force plates and sensors (measuring in Newtons) to assess kick force, jump height and impact in training.
Wrong: "A spring scale measures mass in kilograms." No — a spring scale measures force (weight) in Newtons. A balance scale measures mass in kilograms by comparing with known masses.
Right: Spring scale → Newtons (force). Balance scale → kilograms (mass). Different instruments measuring different quantities.
Wrong: "You can zero the scale after you attach the object." Zeroing must happen BEFORE attaching the object — zeroing after would just set the zero to include the object's weight, and all readings would be wrong.
Right: Zero the scale first (nothing attached), then attach the object and read the force.
Wrong: "Hooke's Law means springs always behave the same no matter how far you stretch them." Hooke's Law only works up to the elastic limit. Stretch too far and the spring is permanently deformed — the proportional relationship breaks down.
Right: Hooke's Law is proportional up to the elastic limit only. Beyond the elastic limit, the spring is permanently stretched.
A spring scale reads 9.8 N when a 1 kg mass hangs from it on Earth. Predict what the scale will read when the same mass is hung from it on the Moon (g = 1.6 N/kg). Will a balance scale measuring the same object on the Moon give a different reading?
How close was your prediction?
The hook at the start of this lesson revealed something interesting: take a spring scale to the Moon and it gives a different reading, but a balance scale gives exactly the same reading. Now you understand why!
Explain why the spring scale changes on the Moon but the balance doesn't — it comes down to what each instrument is actually measuring. Use the words mass, force, Newton and gravity at least once each.
Q1. Describe how you would use a spring scale to measure the weight of your science textbook. Include at least 3 specific steps and one source of error to watch for. (3 marks)
Q2. A student measures the weight of a 5 kg dumbbell on Earth (spring scale reads 49 N) and then on the Moon. (a) Would the spring scale reading change on the Moon? (b) Would a balance scale reading change on the Moon? Explain both. (3 marks)
Q3. A student applies the following forces to a spring and records the extension: 2 N → 1 cm; 4 N → 2 cm; 6 N → 3 cm; 8 N → 4 cm; 10 N → 5 cm; 12 N → 5 cm. (a) Describe the pattern for the first 5 readings. (b) What happened at 12 N? (c) What is this called? (4 marks)
Answers
▾MCQ 1
C — A spring scale measures force in Newtons. It works because spring extension is proportional to force (Hooke's Law). A balance scale measures mass in kilograms.
MCQ 2
C — Hooke's Law: extension is proportional to force (up to the elastic limit). Double the force → double the extension. This is why spring scales have evenly spaced markings.
MCQ 3
B — Parallax error happens because the pointer is in front of the scale markings. Read from directly in front with your eye at the same level as the pointer — this eliminates the angle.
MCQ 4
C — The Moon's gravity is about 1/6 of Earth's. On Earth a 10 kg object weighs 98 N (≈ 100 N); on the Moon it weighs about 16 N. The spring scale measures force (weight), which depends on gravity.
MCQ 5
B — Zeroing means setting the pointer to 0 N with nothing attached. This ensures subsequent readings reflect only the force from the object being measured, not any pre-existing offset.
Short Answer 1
Model answer: Step 1: Zero the spring scale before use — check the pointer reads 0 N with nothing attached (1 mark). Step 2: Hang the textbook from the hook and hold the scale vertically (1 mark). Step 3: Wait for the scale to stop moving, then read the force in Newtons from eye level to avoid parallax error (1 mark). Parallax error is the key source of error to watch for.
Short Answer 2
Model answer: (a) Yes, the spring scale reading would be lower on the Moon (1 mark). The spring scale measures weight (gravitational force), which is smaller on the Moon because the Moon's gravity is weaker (1 mark). (b) No, the balance scale reading would NOT change on the Moon (1 mark). A balance compares the object's mass against known masses — mass doesn't change with gravity, so both sides of the balance would be equally affected and the reading stays the same.
Short Answer 3
Model answer: (a) For the first 5 readings: every time the force increased by 2 N, the extension increased by 1 cm — extension is proportional to force (Hooke's Law) (1 mark). (b) At 12 N, the extension stayed at 5 cm instead of increasing to 6 cm — the spring stopped stretching further (1 mark). (c) This is called reaching the elastic limit (1 mark). Beyond the elastic limit, the spring is permanently deformed and no longer obeys Hooke's Law (1 mark).