# Stretched elastic band: an elastic store of energy

##### Class practical

Comparing the energy supplied from a stretched elastic band with the kinetic energy gained by a dynamics trolley.

Dynamics trolley

Runway

Ticker-tape

Ticker-timer

Elastic band

Forcemeter

#### Health & Safety and Technical notes

Long runways or heavy shorter ones should be handled by two persons. Ensure that a buffer is tied across the bottom of the runway, to prevent the trolley falling into anyone.

The runway must be adapted, as described in the experiment Energy carried by a moving trolley so that there is a catapult at one end. This is made with an elastic band stretched between dowel rods.

#### Procedure

a The runway should be compensated for friction. (See the experiment Compensating for friction.)

Set up the catapult towards the upper end of the runway. Do this by stretching a 10 cm elastic band taut between two dowel rods. This should be at a height that will engage the vertical rod on the trolley firmly.

b Pull the trolley back so that the rubber band stretches. Measure the distance that the trolley has been pulled back from its initial point in increments of 1.0 cm. Energy will be stored in the catapult. Release the trolley and obtain a ticker-tape record of its constant velocity. Calculate its kinetic energy = 1/2 mv2.

c Repeat the experiment, obtaining ticker-tape records for the trolley after it has been pulled back through various known distances. See also the experiment Using the ticker-timer to measure time.

d Pull the trolley back from the initial mark with the forcemeter. Note the forcemeter reading at 1 cm intervals, for a range of distances covering the distances used to obtain the ticker-tape records.

e Plot a graph of ‘force’ against ‘distance pulled back’ as shown in the diagram.

#### Teaching notes

1 This experiment is not easy. Students should not expect more than general agreement between their values of the elastic (strain) energy lost and kinetic energy gained.

2 Stopwatches and metre rules, or light gates, could be used instead of tape and timers.

3 On the graph of force against distance pulled back, a small part of the distance pulled back is selected and vertical lines drawn up to meet the curve. The area of that small section gives the force x distance pulled back. This is the energy transferred from the chemical store (fuel + oxygen) in muscles to the elastic energy in the elastic band, in stretching the band that small distance.

Count the squares to work out the total area under the graph from the beginning of the loading process, when the forcemeter begins to extend the rubber band, to the total distance pulled back. This gives the total energy transferred to the rubber band for that particular distance pulled back. When the elastic band is extended by the trolley pushing into it and then the trolley is released, this strain energy in the elastic band will be transferred to kinetic energy of the trolley.

4 If a spring were used instead of a rubber band, the shape of the graph would be a straight line. Instead of working out the area under the graph by counting squares, it could be calculated as
1/2 x force x distance pulled back
because the area under the graph would be a triangle.

5 Compare the values for the elastic energy and kinetic energy. Discuss why they might not be equal (e.g. energy used to warm the rubber band).

This experiment was safety-checked in November 2005