Gyroscopes
As seen in figure (a), the forces on a spinning gyroscope are its weight and the supporting force from the stand. These forces create a horizontal torque on the gyroscope, which create a change in angular momentum ΔL that is also horizontal. In figure (b), ΔL and L add to produce a new angular momentum with the same magnitude, but different direction, so that the gyroscope precesses in the direction shown instead of falling over.
Source
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"OpenStax College, College Physics. November 17, 2012."
http://cnx.org/content/m42184/latest/?collection=col11406/1.7
OpenStax CNX
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