Data Sheet

WHAT’S HAPPENING
The breakdancer model and the ice skater spin faster when
their arms are close to their bodies.
This can be explained by the conservation of angular
momentum. As described in the previous experiment,
angular momentum is the product of how fast something
is spinning and how its mass is distributed around its axis
of rotation. The measure of how an object’s mass is
distributed around its axis is called its moment of inertia.
Since angular momentum is conserved, when the
moment of inertia is changed — for example, by moving
an object’s mass in toward its axis of rotation — the
other factor in calculating angular momentum must
change too: the speed of rotation. So, in order to keep
angular momentum constant, if the moment of inertia
changes, the speed of rotation must change too!
You can try this out yourself if you have a rotating
desk chair. Sit in the chair with your arms out straight
to the sides. Have a friend or family member give you
a push to start you turning. Immediately pull your
arms in close to your body. You will speed up! Put
your arms out again, and you will slow down.
HERE’S HOW
1
to
8
Assemble the
model.
9
Place the arms of the
breakdancer stretched
out as far away from
the center of its body
as possible.
10
Hold the model
upside down by the
top surface of the
gyroscope. Insert one
of the rip cords into the
slot in the side of the
gyroscope. Pull the
rip cord and place the
breakdancer down on
the tabletop standing
on its head.
11
Repeat the previous
step, but this time
move the arms of
the breakdancer as
close as possible to
the center of its body.
What differences do
you observe in the way
the model moves?
6
87
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Gyroscopes & Flywheels
EXPERIMENT 18