Elastic Energy
In this laboratory you will investigate the elastic force and potential energy of a stretched rubber
band. This activity is to be completed in class. You need not write up a formal report but should
make a conscientious effort to carry the lab to completion and answer all the questions.
Procedure:
1) Obtain a rubber band and appropriate apparatus from the front of the room. Set up a rubber
band holder similar to the one in the front of class. Place a rubber band over the prongs and tape
it in a fixed position. Take note of where the band is on the prong in case the tape accidentally
becomes dislodged. It is important to keep the band at a fixed location.
2) Position a ruler adjacent to the band so that you are able to easily take measurements of how
far it has stretched. Locate the point where the rubber band is just taught but has no force exerted
on it. Choose this point to be your origin when measuring how far it has stretched downward.
3) Using the appropriate spring scales at the front of the room take various readings of the force
required to stretch the rubber band a given distance downward. Record your data in a table
listing force F and displacement x. Take 10 separate measurements and take data up to a force
large enough to stretch the rubber band to a maximum displacement of 10cm.
Analysis:
1) On graph paper plot the force versus displacement x. Attempt to choose scales on the axes
that will allow for a large graph. If you do otherwise I might have you do it again! To what
extent does the rubber band obey Hooke’s Law? What is the effective “spring constant” for
displacements x < 5cm?
2) Consider the following problem: How far must the rubber band be stretched in order to project
a 50g mass 2.5m upward into the air? Carry out the required analysis and predict the distance the
rubber band must be stretched to project the 50g mass 2.5m upward. In the course of your
calculation you should make reference to your F versus x plot. Do not use Hooke’s Law unless
your graph is a perfectly straight line.
3) Now test your prediction by placing the 50g mass on the band and shooting it upward.
Perform at lest 5 trials and average your results and your uncertainty. Please be careful, do not
situate your apparatus beneath a light in the event your calculations are incorrect!
4) Compare your prediction with your experimental findings and quote your experimental error.
Did your observations continually fall short or overshoot your prediction? Can you explain any
observed trend in terms of the conservation of energy?
5) If time permits perform the following measurement: Consult your table of F versus x data and
take the data in reverse. That is, stretch the rubber band to its maximum displacement and
sequentially determine the force required to bring it back to the displacements appearing in your
table. Record any differing data on your graph using a different symbol (). Are there any
differences? If so what does this imply about the conservative nature of the rubber band force?
What net work is required to stretch the band and bring is back to zero displacement? Where
does this lost energy go?