1.5.1 Change of Shape

Stretching Materials

• When forces are applied to materials, the size and shape of the material can change
• The method below describes a typical procedure for carrying out an investigation into the properties of a material

An experiment to measure the extension of a spring

• Set up the apparatus as shown in the diagram
• A single mass (0.1 kg, 100g) is attached to the spring, with a pointer attached to the bottom, and the position of the spring is measured against the ruler
• The mass (in kg) and position (in cm) are recorded in a table
• A further mass is added and the new position measured
• The above process continues until a total of 7 masses have been added
• The masses are then removed and the entire process repeated again, until it has been carried out a total of three times, and averages can then be taken

• Once measurements have been taken:
• The force on the spring can be found by multiplying the mass on the spring (in kg) by 10 N/kg (the gravitational field strength)
• The extension of the spring can be found by subtracting the original position of the spring from each of the subsequent positions
• Finally, a graph of extension (on the y-axis) against force (on the x-axis) should be plotted

A graph of force against extension for a metal spring

Extended Only

Hooke's Law

• Hooke’s law states that:
• The extension of a spring is proportional to the applied force

(where k is the spring constant, which represents how stiff a spring is)

• Many other materials (such as metal wires) also obey Hooke’s law
• Hooke’s law is associated with the initial linear (straight) part of a force-extension graph
• Objects that obey Hooke’s law will return to their original length after being stretched
• If an object continues to be stretched it can be taken past the limit of proportionality (sometimes called the elastic limit). At this point the object will no longer obey Hooke’s law and will not return to its original length

The spring on the right has been stretched beyond the limit of proportionality

Exam Tip

A relationship is said to be proportional if the graph is a straight line going through the origin.

If a graph is a straight line but does not go through the origin the relationship is said to be linear.

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
Close

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.