Approximate Motion of Compound Pendulum

Theorem

Let $\PP$ be a compound pendulum free to swing about a pivot $A$.

Let $\PP$ be pulled to one side by a small angle $\alpha$ (less than about $10 \degrees$ or $15 \degrees$) from the vertical and then released.

Let $T$ be the period of $\PP$, that is, the time through which $\PP$ takes to travel from one end of its path to the other, and back again.


Then:

\(\ds T\) \(\approx\) \(\ds 2 \pi \sqrt {\dfrac {k^2 + h^2} {g h} }\)
\(\ds \) \(\approx\) \(\ds 2 \pi \sqrt {\dfrac I {M g h} }\)

where:

$g$ is the Acceleration Due to Gravity
$h$ is the distance from the center of mass to $A$
$k$ is the radius of gyration of $\PP$
$I$ is the moment of inertia of $\PP$ about $A$
$M$ is the mass of $\PP$.


Proof


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Sources

  • 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): pendulum
  • 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): pendulum