Work and Potential Energy Lab
Suppose that you hang a mass from a spring. If you raise the mass,
you do (positive) work on it, while gravity is doing negative work -
we say that the work that you do is saved as gravitational potential
When you let go of the mass, it falls. As it falls, its
gravitational potential energy decreases, and its elastic potential
energy increases (because the spring is stretched).
How does the increase in the elastic potential energy compare to
the decrease in the gravitational potential energy?
0.5 and 1.0 kg. masses
- Be sure to keep your feet out of the area in
which the mass will fall if it comes off the spring, or the spring
- Be sure to clamp the ring stand to the lab table, or weight it
with several books so that the mass does not pull it off the
- You need to hang enough mass to the end of the spring to get a
measurable stretch, but too much force will permanently
damage the spring. (An engineer would say that it has
exceeded its "elastic limit"). "You break it, you
- Assemble the spring and mass apparatus as shown in the
- First, in order to determine the work done in stretching the
spring, you need to determine the spring constant, k. Remember
Hooke's Law (F = kx)? You can determine k by measuring the stretch
of the spring for a known force. Of course, you will want to try a
few different forces as a check on the precision of the
calculation. Record your measurements in a data table.
- Now, attach a 1.0 kg. mass to the spring. Hold the mass 20 cm
or so below the normal unstretched length (that is the length when
no mass is hanging from it) of the spring. Let go of the mass, and
note the position of maximum stretch of the spring. It helps to
mark the 3 positions with clothespins on the ring stand or a meter
stick. Repeat until you are confident you have located the
maximum-stretch position. Record your measurements in a data
- Repeat for a few other starting heights.
- Replace the 1.0 kg. mass with the 0.5 kg. mass, and
- Knowing the spring constant, k, you can calculate the work
done in stretching the spring from the starting position to the
position of maximum stretch in each trial. (This is the increase
in the elastic potential energy.) Be sure to show a sample
- Knowing the mass and the distance that the mass falls you can
calculate the work done by gravity as the mass falls in each
trial. (This is the decrease in the gravitational potential
energy.) Show a sample calculation.
- How does the work done in stretching the spring compare to the
work done by gravity as the mass falls? Why is this so?
adapted from Haber-Schaim, et. al. Laboratory guide for PSSC
Physics Fourth Edition, D.C. Heath and Company, Experiment 32,
Changes in Potential Energy
last update January 16, 2001, by JL