Core Practical: Investigating Force and Extension — Forces, Movement & Changing Shape | IGCSE Physics

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 14% of your exam marks.

stable

High

Stable14%

F = ma, resultant forces and Hooke's Law calculations are high-frequency multi-mark questions.

Measure the extension of a spring (or a rubber band, or a length of metal wire) as the stretching force on it is increased, and decide whether the two are directly proportional

Independent variable: stretching force, F (N), set by hanging known masses
Dependent variable: extension, e (cm or mm), read off a fixed ruler
Control variables: the same spring (or rubber band, or wire) used throughout; the same starting attachment point; the same ruler with the same zero alignment

Equipment	Purpose	Resolution
Retort stand, clamp and boss, G-clamp	Holds the spring vertically and prevents the stand from toppling as masses are added	—
Spring (or rubber band, or thin metal wire)	The object whose extension is being measured	—
100 g slotted masses with a 100 g hanger	Provides the known stretching forces in 1 N steps (since W = 0.1 × 10 = 1 N per mass)	—
Vertical ruler clamped beside the spring	Measures the position of a pointer attached to the bottom of the spring	1 mm
Small horizontal pointer (or piece of tape acting as a fiducial marker)	Lets you read the bottom of the spring against the ruler accurately	—
For a metal wire: a bench pulley and a wooden block clamped over the wire	Allows the wire to be stretched horizontally with the masses hanging vertically off the bench edge	—

Hang the spring (or rubber band, or metal wire) from the clamp without any mass attached. Read and record its original length L₀ using the pointer and the clamped ruler
Place the 100 g mass hanger on the spring. Give the spring a couple of seconds to come to rest, then read the new pointer position L₁ and record it together with the mass
Add one further 100 g mass to the hanger. Allow it to come to rest, then read and record the new length
Continue adding masses in 100 g steps until all five are loaded
Carefully remove the masses one at a time and check that the spring returns to its original length. If it does not, it has been stretched beyond its limit of proportionality and the run must be repeated with a fresh spring
Repeat the whole sequence three times and take the mean of the three readings at each load
For the metal wire variant: clamp the wire horizontally across the bench, run it over a bench pulley, attach the hanger to the dangling end, and stick a small piece of tape on the wire as a fiducial marker for the ruler

Convert each loaded mass m (in kg) to a stretching force F with F = m × g, taking g = 10 N/kg (so each 100 g step adds 1.0 N)
Calculate the extension e at every load:

e = mean loaded length − original length

Plot force (y-axis) against extension (x-axis), or extension against force, but force-vs-extension is the conventional Hooke's law plot
A straight line through the origin shows that force and extension are directly proportional, i.e. the material is obeying Hooke's law throughout the tested range
A region where the line curves away from a straight line shows the material has passed its limit of proportionality (see section 9)

Systematic error, parallax. Always read the pointer against the ruler with your eye exactly level with the pointer
Random error, equipment movement. A wobble of the stand between readings shifts the apparent zero; the G-clamp removes this
Random error, slow settling. Let the spring fully stop oscillating before taking each reading; a length read mid-bounce will be off by the amplitude of the wobble
Safety: wear safety goggles in case the spring or wire snaps and whips back, keep feet clear of the hanging masses, and place a soft mat or tray beneath the masses to catch them if they drop