Stretching and compressing materials
Working with a range of materials provides opportunity for observation and discussion of the effects of forces.
Apparatus and materials
For each pair of students
Rubbber bands, or length of elastic cord
Steel springs which can be compressed or extended
Latex foam block or cylinder
Health & Safety and Technical notes
Eye protection should be worn.
The latex foam could be of the type used for lagging piping, taped to hold the seam closed if necessary. Draw a square onto it to show its changes in shape when deforming forces are applied, for example by bending or twisting it.
a Pull the string from one end only. Then pull the string from both ends at the same time. Compare the very different behaviours.
b Repeat this with the rubber band or elastic cord, and with the spring. Again, compare what happens.
c Exert pairs of forces on the foam cylinder. Notice the different changes in shape shown by the square.
1 It is incorrect to say that force produces motion. Motion is possible without force, as on a slippery surface or in Outer Space where there is no resistance to motion.
Force produces change in motion (acceleration), unless it is balanced by other forces. So when the string is pulled from one end only it accelerates. When it is pulled from both ends at once, the forces may balance, so that there is no acceleration. Instead the string is in tension.
2 The rubber band, the elastic cord and the steel spring change their shape when subject to tension. The changes can include extension, compression, twisting and bending. Larger forces are needed to deform materials by larger amounts. If the force is big enough, the material may become permanently deformed, or snap.
3 Vivid effects: stretch strips of clear adhesive tape, stick them in criss-cross patterns onto an overhead projector transparency, and view them through crossed polarising filters.
4 Compare elastic behaviour, as shown by the cord and the springs, with plastic behaviour, in which the change in shape remains after the force is removed. A rubber ball could be compared with a ball of Plasticine, for example. A gas-filled balloon can be squashed and released.
5 Connect two similar elastic bands or springs in parallel. Students will find that they need twice the force to extend the pair compared with a single spring, for the same extension.
6 Two similar elastic bands, or springs, connected in series will extend by twice the amount of a single spring for the same force.
7 A comparison between series and parallel elastic bands and series and parallel electric circuits could be made.
Parallel Circuit: currents add at junction.
Parallel Elastic bands: tensions add at junction.
Series Circuit: current the same all the way through.
Series Elastic bands: tension the same all the way through.
This experiment was safety-checked in February 2005