Rotational Kinetic Energy of a Soup
When an object falls from rest, its potential
energy is converted into kinetic energy. Initially, it contains only
potential energy, PE. As it falls, this stored energy is converted
into kinetic energy, KE. If there are no frictional forces acting on
the object, the total energy (potential plus kinetic) will remain
constant. In this experiment, we will see if this works for a falling
An interesting situation arises when an object
unwinds down a string rather than simply falling. Not only is the
object moving in a straight path toward the ground, but it is also
rotating. Now the initial potential energy is converted into
both translational and rotational kinetic energy. In this experiment,
we will determine what fraction of the kinetic energy goes into
rotating a soup can as it unwinds down a string.
- Measure the changes in potential and kinetic
energy of a soup can as it falls and as it unwinds down a
- Observe how the total energy of the falling
soup can changes.
- Determine the fraction of kinetic energy that
goes into rotating the soup can as it unwinds down the string.
Computer running logger pro
Excel or Graphical Analysis
empty soup can
nylon cord or string
Vernier motion detector
- What kind of energy does the soup can before
it is dropped?
- What kind of energy does the soup can have
while it is falling?
- What kind of energy does the soup can have
while it is rolling down the string?
- Which soup can will be moving faster, the one
in free fall or the one rolling down the string?
- Do you think frictional forces will be
significant for either the falling soup can or the unwinding one?
Why or why not?
- Measure and record the mass of the soup
- Hang the soup can from the ring stand using
the nylon string. Make sure the can will be able to fall about 1.5
m before hitting the ground.
- Using a small piece of string, tie the soup
can in place at the top of the ring stand. This string will hold
the can in place and can be cut when you are ready to record some
- Set up the motion detector and position it on
the floor directly beneath the soup can. Place a wire basket over
the motion detector to protect it.
- Set up Logger Pro for the motion detector.
- Set up Logger Pro for data collection. Take
data for about 10 seconds, sampling 50 points/second.
- Set up four new data columns on Logger Pro:
velocity, potential energy, kinetic energy, and total energy. You
can do this by choosing DATA --> NEW --> COLUMN -->
FORMULA and typing in an equation for each quantity.
- Set up a graph of energy vs. time. Plot time
on the x-axis. Plot PE, KE, and total energy on the
- When you are ready to take data, hit collect
and wait for the motion detector to start. Without getting your
hands in the way, quickly clip the supporting string on the can
and let it fall.
- When the motion detector has stopped running,
zoom up on the portion of your graph that represents the time the
can was falling. If this was a smooth run, save it. If not, repeat
it until you have a smooth run.
- Now roll the can up the string (like a yo-yo)
and tie it in place. Repeat steps 9 and 10.
- Examine the graph for the free-falling soup
can (Zoom in on the relevant section). What happens to the
potential energy as the can is falling? What happens to the
kinetic energy as the can is falling? What happens to the total
energy as the can is falling?
- Examine the graph for the rolling soup can
(Zoom in on the relative section). What happens to the potential,
kinetic, and total energy as the can is unwinding?
- How can you explain the differences in total
energy for the two soup cans?
- To analyze the energy of the rotating soup
can, you will need to look at the data taken while the can was
- Look at the graph for the unwinding can.
Highlight the section of the graph where the can is falling.
Copy this data and paste it into a spread sheet such as Excel
or Graphical Analysis.
- The graph for the unwinding soup can shows
a loss of total energy. In a new column, calculate the amount
of total energy that seems to have been "lost" at each time
step. (This energy hasn't really been lost-it just went into
rolling the can!) This "lost" energy is actually the rotational
kinetic energy of the can.
- The total kinetic energy of the soup can is
the sum of its translational and rotational kinetic energy. In
a new column, calculate this total kinetic energy.
- In a new column, calculate the fraction of
kinetic energy that is rotational energy. Take an average of
all the values in this column.
- What conclusions can you make about the energy
of an object that rotates as it falls compared to an object that
- How does the rotational kinetic energy depend
of the shape of the object? Try this experiment again with a solid
cylinder, a solid sphere, or a hollow sphere.
- Use a force meter to study the tension in the
string of the unwinding soup can.
Check out some sample data!