Global Positioning System Reference
In-Depth Information
Spinning Tops and Angular Momentum
The gyroscope phenomenon, familiar to children and scientists, applies to a
fast-spinning mass. The mass is usually a disk weighted more heavily on
the outer rim than it is near the center. The mass must have an axially sym-
metric shape, meaning that its appearance is unchanged if you rotate it on
its axis. Such an object, when set spinning rapidly, is resistant to changes in
its orientation. That is, a gyroscope will resist forces that try to change the
direction of the spin axis. We see this in the resistance of a spinning gyro-
scope to falling over, whereas the same gyro does fall over when it is not
spinning. The strange and enchanting aspect of the gyroscopic phenomenon
is this: a gyro subjected to a force will move, not in the direction of the applied
force, but in a direction that is perpendicular to it. This is very odd and coun-
terintuitive behavior.
The figure shows a familiar example of a gyroscope: a toy top spinning on
a table. The quality of this top that is responsible for the odd behavior is
angular momentum
. Unfamiliar? Think of ordinary (linear) momentum—the
resistance offered by a moving body to changes of speed or direction. It is
easier to catch a baseball than a cannonball moving at the same speed be-
cause the cannonball has more momentum and will resist being stopped
more strongly. Angular momentum is the rotational equivalent: an object
A toy top. The top's direction of spin is determined by the right-hand rule: fingers curl
around the rotation axis in the same way as the spin, and the extended thumb gives
the spin direction. In this case the spin—and the angular momentum vector of the
top—is directed upward, as indicated by the arrow. A fast-spinning top will resist
forces that try to change this direction.
