Author Topic: the Q-factor  (Read 6452 times)

Offline rabbit

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the Q-factor
« on: September 28, 2005, 06:24:27 PM »
A dimensionless - and somewhat obscure - property of a pendulum "system" is what is called the "Q- Factor".

"Q" is defined as the ratio of the restoring force to the resisting force.

The "restoring force" is the force that makes the pendulum "want to go to the bottom". It is essentially due to gravity, and is proportional to the distance of the center-of-gravity from the pivot (the length), the mass, and to the amplitude of the swing.  (A pendulum suspension spring also contributes to the restoring force, as well as some escape forces, but don't worry about those, yet...)

The "resisting force" is basically the frictional resistance of the pivot and the air drag on the pendulum.  (Inertia and escape impulse are also involved, but don't worry about those, yet, either...)

Put very simply, "the higher the Q, the better".

The incremental energy imparted to the pendulum system by the escape mechanism must add enough energy to "trip the escape" (done by the inertia of the pendulum), as well as overcome the difference in the resisting force and the restoring force.

This is why a heavy bob "works better" than a light one.  A longer pendulum is "easier to control" than a short one.  An aerodynamic bob is better than "that crazy thing" i just built.  A suspension spring works better than any knife-edge support i can build. Etc.

So... coming soon, what makes compound pendulums so tricky.
- rabbit