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If a force is applied perpendicular to the surface of a disc it topples sideways. Also, a stationary disc topples more easily than a rolling one. The explanations I've seen for this state that since the rolling disc has angular momentum it requires more torque to topple. But how can a force perpendicular to the surface of the disc produce torque?

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    $\begingroup$ The magic word you seek might be "gyroscope" $\endgroup$
    – user854
    Nov 22, 2014 at 20:16

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That's not the definition of torque -- in fact rolling or not, the perpendicular force causes torque about the point of contact with the ground.

The reason the rolling disc does interesting things is that it has a bunch of angular momentum which leads to precession of the axes. It's a brain-mangler to work out (tho' there are plenty of intro Mechanics books which will show how to start the problem). Basically the precession keeps the disc from "flopping" directly onto its face, so it keeps rolling along as the angular momentum conservation acts against the current falling direction of the top of the disk.

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Yes. Precession is the answer but it seems to be a sbject which is poorly understood and insufficiently researced.

It becomes particularly interesting when two gyroscopes are mechanically-coupled: e.g. a bicycle - and closed feeedback-loops come into play.

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