Science · Force and motion

Force and the spring balance

Pushing a door, kicking a ball, stretching a spring: all of these are forces. Find out what a force is, why its unit is the newton, and how a spring balance (dynamometer) measures force by how far a spring stretches.

15 min
1 simulation
Mini quiz

Updated: 15 October 2026

Preview

The question

When you carry a bag of apples home from the market, you feel it pulling on your arm. But how big is that pull? Could you put a number on the feeling in your arm? The greengrocer's scales tell you the mass of the apples, but to measure the force the bag puts on your arm you need a different instrument.

Try it yourself

No account needed: change the values and check your answer. Some simulations ask for a prediction before they start; it doesn't matter whether it's right or wrong.

Number line

Read the spring balance

Move the marker in steps of 0.5 to the correct value and submit your answer.

Predict first

If you hang a heavier bag on the spring balance, what do you think the spring will do?

It doesn’t matter whether you’re right; the simulation opens once you choose.

Lab

Spring balance

Open full page in the Lab →

Fig. 1Spring balance

Drag a mass onto the hook, or use the buttons

0.00 N · 0.0 cm

Keyboard: ↓ hangs the next mass, ↑ takes the last one off, 1–3 pick the spring, Esc takes them all off.

Parts and inner mechanism

Apart0%

With the balance apart, hover over a part, tap it or pick it from the list; the arrow keys move through the list.

Pick a part to see what it does and why a real spring balance is made that way.

Masses

Spring

Your own values

Mass

1–1,000 g; a comma works as a decimal point too.

Spring constant

0.1–5 cm per N (k = 0.2–10 N/cm).

Balance reads

0.00 N

Mass: m = 0 g = 0 kg
Weight: F = m · g = 0 × 9.8 = 0.00 N
Stretch: x = 0.00 N × 2 cm/N = 0.00 cm
Stretch ÷ force: 2 cm/N

It stays the same whatever you hang: the stretch is directly proportional to the force. That is why the scale can turn the stretch straight into newtons.

Force–stretch graph

The dots are the loads you measured. They all lie on one straight line.

Change the spring: the ruler shows a different stretch, but the scale is drawn for that spring, so the force you read stays the same.

Learn

Progress is kept on this page only; no points are given.

1Predict
2Try
3Explain

With 200 g on the soft spring, the balance reads 1.96 N. If you swap in the stiff spring and hang the same 200 g, what will it read?

Weight is worked out as F = m · g (g = 9.8 N/kg, as in the lesson). While the spring stays within its elastic limit, it stretches by x = F ÷ k: the soft spring 2 cm per newton, the medium one 1 cm, the stiff one 0.5 cm. The weight of the hook and pull rod has been zeroed with the adjusting screw, so only the hung mass is measured. The pointer swings like a real spring and mass and settles with light damping; the damping is only for display. You can also type your own mass (1–1000 g) and spring constant (0.1–5 cm per N, that is k = 0.2–10 N/cm); the scale ends at 15 cm, so beyond that the pointer rests on the stop and the reading is off the scale.

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Frequently asked

5 questions

What is a force?

A force is a push or a pull. It can change an object's speed, its direction of motion or its shape. Its unit is the newton, written N.

How does a spring balance work?

A spring balance (newton meter) has a spring inside. The bigger the force, the more the spring stretches; as long as the spring stays within its elastic limit, the stretch is proportional to the force. The scale shows this stretch in newtons.

What is the difference between mass and weight?

Mass is the amount of matter in an object, is measured in kilograms and is the same everywhere. Weight is the pull of gravity on the object and is measured in newtons. On Earth a 1 kg object weighs about 9.8 N; on the Moon it weighs about one sixth of that.

What happens if I hang a very heavy load on a spring balance?

Every spring balance has a maximum it can measure. Go past it and the spring can stretch beyond its elastic limit and stay longer for good; then it no longer returns to its starting length and its readings are wrong.

Do objects have to touch to exert a force?

No. Gravity, the force between magnets and the electric force that lets a rubbed balloon pull on your hair all act without touching.