Squashing sweets

In this introduction to Hooke’s law students squash marshmallows to find that force is proportional to compression.

Preparation

Test your marshmallows to ensure they give approximately linear results. If not, reduce the number of masses you provide.

Equipment

Each group of students will need:

• A marshmallow (giant ones work best - vegan options are available and may be more appropriate for your class)
• Two identical drinks cups that are wider than the marshmallow (ideally transparent)
• Six 100 g masses
• Ruler

Procedure

Ask students to:

1. Place the marshmallow inside a cup and then place the second cup inside the first so that it rests on the marshmallow.
2. Measure the distance between the rims of the two cups to find the initial height of the marshmallow in millimetres.
3. Add a mass to the second cup and then measure the new distance between the rims of the cups.
4. Repeat to collect data for different heights (mm) and masses added (g).
5. Calculate the applied force in newtons for each value of mass added (100 g is equivalent to 1 N)
6. Calculate the compression (ie change in height) for each value of height (compression = initial height – height).
7. Plot a graph of force against compression and determine its gradient

Teaching notes

An object obeys Hooke’s law if the compression (or extension) is proportional to the applied force. The constant of proportionality is equal to the gradient of their graph and is about 0.5 N/mm for a giant marshmallow.

Marshmallows deform plastically. They won’t return to their original height after being compressed. If students want to repeat the investigation, provide fresh ones.

Learning outcome

Students can plot a graph of force against compression and determine the constant of proportionality from the gradient.