13.1.4Forces and Matter

Core Practical: Investigating Force and Extension — Edexcel GCSE Combined Science 1SC0 - Physics Revision Notes & Topical Questions

5 min read 9 parts Lesson 5 of 5

📚 Edexcel GCSE Combined Science 1SC0 - Physics Hub / Topical Revision

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Part 1 of 7Aim and idea

The aim is to find how the extension of a spring changes as the force stretching it increases, and to find whether the spring obeys Hooke's law. The extension is the increase in length compared with the natural (unstretched) length:

extension = stretched length − original length

This is a core practical, so you should be able to describe it from memory, name the variables, draw the graph and explain what its shape shows. Examiners also like to see an explanation of why repeating readings improves reliability.

Part 2 of 7Apparatus

spring, clamp stand, boss and clamp,

set of slotted masses (for example 100 g) and a hanger,

metre rule (or a ruler with mm divisions) held vertically beside the spring,

a pointer or a small marker taped to the bottom of the spring for reading the scale,

a set square to check the rule is vertical.

The slotted masses should be added gradually and the spring allowed to settle each time, because a spring may oscillate for a moment after a mass is added.

Apparatus

A pointer made from a bent paper clip or a strip of card taped to the bottom coil gives a clear reading.

The ruler should be as close as possible to the spring and clamped vertically.

Using a mirror behind the scale reduces parallax: the reading is taken when the pointer and its reflection line up.

Part 3 of 7Method

Hang the spring from the clamp, with the metre rule fixed vertically next to it, and read the original length with the pointer.

Add a 100 g mass; the weight added is W = mg = 0.100 × 9.8 = 0.98 N.

Wait until the spring is still, and read the new length at eye level to avoid parallax error.

Add further masses one at a time, up to about 6, recording the length each time.

Remove the masses one at a time and read again, to check whether the spring returns to its original length.

Calculate force and extension for each load.

Because the masses are in grams, convert carefully: 100 g = 0.100 kg and the force is 0.100 × 9.8 = 0.98 N.

Method

Many schools use g = 10 N/kg so a 100 g mass gives about 1 N, but follow the value given in the question.

A results table should have columns for mass, force, length, and extension, each with units in the heading and consistent decimal places.

Part 4 of 7Results and graph

Plot force (y-axis, N) against extension (x-axis, m or cm) and draw the best-fit line.

1Straight line through the origin

For a spring that obeys Hooke's law the graph is a straight line through the origin, and the gradient gives the spring constant k = F/x.

2Limit of proportionality

If the points curve at higher loads the spring has passed its limit of proportionality, and may be permanently stretched beyond the elastic limit.

Part 5 of 7Safety, accuracy and improvement

Wear eye protection: a spring may snap and fly off.

Place a box or cushion below to catch dropping masses and keep feet clear.

Take readings at eye level and use a pointer to reduce parallax.

Repeat and average, and do not overload the spring.

The independent variable is force (mass added); the dependent variable is extension; control variables are the same spring and temperature.

Possible sources of error include reading the pointer at different angles, a ruler that is not vertical, a spring that has already been stretched permanently, and masses that are not exactly the stated size.

Safety, accuracy and improvement

To improve reliability, repeat the whole set and average the extensions, or use smaller steps in the load.

If one point lies off the line, check it by remeasuring rather than just deleting it.

Part 6 of 7Using the results

The gradient of the force–extension graph is k. A steep line means a stiff spring. Readings should be in metres for k to be in N/m. Comparing loading and unloading shows whether the behaviour is elastic. See the Hooke's law lesson for the equations.

Part 7 of 7Worked examples

A spring has original length 12.0 cm. With a 2.0 N load its length is 16.0 cm. Find the extension and the spring constant.
Why is it better to plot a graph than to use one result to find k?
The graph of force against extension is a straight line to 6 N, then curves. A 4 N load gives 0.08 m extension. What is the extension at 6 N?

Key terms

Term	Meaning
Extension	Increase in length from the natural length.
Limit of proportionality	Point beyond which force and extension are no longer proportional.
Elastic	Returns to original shape when force is removed.
Parallax error	Error from viewing the scale at an angle.
Spring constant	Force needed per unit extension, N/m.

Avoid these mistakes

Measuring the length of the spring instead of the extension.

Reading from the top of the mass hanger instead of a fixed pointer.

Using masses as the force without converting to newtons.

Reading the ruler at an angle (parallax).

Forcing a straight line through points that clearly curve.

Last updated: 20 October 2026