Stretchy sweets

Demonstration

By stretching confectionery laces, students learn that extension is not always proportional to load. They also gain experience in adopting consistent procedures to make and record measurements.

Apparatus and Materials

For each student group

• Strawberry, apple or cola laces (preferably not sugar-coated)
• Retort stand
• Clamp
• Felt-tip pen (dark colour)
• Metre rule
• Clotted massess, 100 g set
• Slottee masses, 10 g set

Health & Safety and Technical Notes

Make sure that students do not eat the laces since eating anything in a laboratory is hazardous.

Read our standard health & safety guidance

If the laces are sugar-coated, take care to avoid getting sugar into other equipment (open the packet over a sink). Wash off the sugar under a cold tap and allow the laces to dry before use.

Procedure

1. Tie one end of a lace around the clamp and the other to a mass-holder. Make two marks on the lace a measured distance apart (approx 0.5 m).
2. Add masses singly or a few at a time. Observe how the lace behaves over a short period after the load is increased.
3. Observe how the lace behaves if the load is removed. For each load, record the distance between the two marks.
4. Continue until the lace breaks.
5. Plot a graph to show how extension varies with load.

Teaching Notes

• This activity can be used for a variety of purposes, depending on the ability, age and experience of the students.
• For some students, it will be a useful exercise in making measurements and displaying them graphically.
• For others, it will provide an example of a material whose load-extension graph is not a straight line (it does not obey Hooke's law) and which exhibits creep (gradual deformation under a steady load). They can be asked to discuss when they should record the extension for a given load (immediately? or after the sample has stopped stretching?). There is no right answer, but students should be consistent and state clearly what strategy they have adopted.
• You might want to discuss the role of tests such as these in the food industry. Measurements can be directly related to how a confectionery product feels when eaten, and samples are tested before a batch of products leave the factory to ensure they are of suitable quality.
• How Science Works extension: If students have obtained a graph from one lace, they may assume that this will describe the behaviour of all laces. A nice extension is to ask them to investigate the variation in stretchiness (or spring constant) within a packet of fruit laces. Terms such as variation and range could be introduced and used, if appropriate.
• Students could carry out a similar process as seen in the experiment Investigating simple steel springs and possibly go on to compare the variation in springs behaviour with the variation in confectionery laces.
• This experiment comes from Salters Horners Adanved Physics©, University of York Science Education Group.

This experiment was safety-checked in January 2007