1.1.5Forces and Their Effects

Investigating Force and Extension — Oxford AQA International GCSE Combined Science - Physics Revision Notes & Topical Questions

5 min read 7 parts Lesson 6 of 16

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Part 1 of 5Aim and variables

Item	Detail
Aim	To investigate the relationship between the force applied to a spring and its extension
Independent variable	Force, F, produced by the weight of the hanging masses (N)
Dependent variable	Extension of the spring, e (cm or m)
Control variables	The same spring, so the spring constant stays constant; the same ruler and starting point; same temperature
Resolution of ruler	1 mm

Support

Support A clamp and stand hold the spring vertically and give an upward force on it.

Loads

Loads Five 100 g masses and a mass hanger provide the downward force, added one at a time.

Measuring

Measuring A ruler measures length, and a pointer attached to the bottom of the spring makes it easier to read the position accurately.

Hang the spring from the clamp and place a ruler beside it; line up the pointer with a mark on the ruler and record the original length with no load.

Add the 100 g mass hanger and wait for the spring to stop moving.

Record the mass in kg and read the new position from the ruler.

Add another 100 g mass, wait until stationary, and record the new position.

Continue until all the masses have been added, then remove them.

Repeat the whole process two more times and calculate an average position for each load.

1Force

Force The force is the weight of the masses: W = m × g, with g = 9.8 N/kg. For 0.1 kg the force is 0.98 N.

2Extension

Extension Extension = final length − original length, so a spring that grows from 10.0 cm to 12.5 cm has e = 2.5 cm.

3Graph

4Conclusion

Conclusion Where the graph is a straight line through the origin, force is directly proportional to extension, so F = k × e.

Evaluating the investigation

Systematic error

Systematic error Read the ruler at eye level to avoid parallax error, and make sure it is vertical.

Random error

Random error Wait for the spring to stop bouncing, measure from the same point at the bottom of the spring each time,
- and use a pointer to improve precision.

Limit of proportionality

Limit of proportionality Do not overload the spring;
- beyond the limit of proportionality the graph curves and the spring may not return to its original length.

Safety

Safety Wear eye protection in case the spring snaps, and keep feet clear of falling masses.

Term	Meaning
Extension	The increase in length of a spring compared with its original length.
Spring constant	A measure of the stiffness of a spring, in N/m, from F = k × e.
Limit of proportionality	The point beyond which the extension is no longer directly proportional to the force.
Parallax error	A reading error caused by viewing a scale from the wrong angle.
Resolution	The smallest change in a quantity that a measuring instrument can detect.

Plotting the total length of the spring instead of the extension, so the line does not pass through the origin.

Using the mass in grams as the force instead of converting the mass to kg and calculating the weight.

Saying the spring obeys proportionality at all loads; a line that curves shows it has passed the limit of proportionality.

Taking readings while the spring is still bouncing.

Describing the relationship as 'directly proportional' for a straight line that does not go through the origin.

Last updated: 20 October 2026