Stretched elastic band: energy stored elastically
Practical Activity for 14-16
Comparing the energy stored in a stretched elastic band with the energy stored kinetically in a dynamics trolley.
Apparatus and Materials
- Dynamics trolley
- Elastic band
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:
...so that there is a catapult at one end. This is made with an elastic band stretched between dowel rods.
- The runway should be compensated for friction. See the experiment:
- 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.
- 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 elastically in the catapult. Release the trolley and obtain a ticker-tape record of its constant velocity. Calculate the energy stored kinetically = 1/2 mv2.
- Repeat the experiment, obtaining ticker-tape records for the trolley after it has been pulled back through various known distances. See also the experiment:
- 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.
- Plot a graph of ‘force’ against ‘distance pulled back’ as shown in the diagram.
- This experiment is not easy. Students should not expect more than general agreement between the energy stored elastically and energy stored kinetically.
- Stopwatches and metre rules, or light gates, could be used instead of tape and timers.
- On the graph of
forceagainst 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
forcex distance pulled back. This is the energy stored chemically (fuel + oxygen) in muscles that is transferred to 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 energy stored elastically in 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 energy stored elastically in the elastic band will be stored kinetically in the trolley.
- 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
forcex distance pulled back because the area under the graph would be a triangle.
- Compare the values for the energy stored elastically and energy stored kinetically. Discuss why they might not be equal (e.g. energy is stored thermally as the rubber band heats up).
This experiment was safety-tested in November 2005