What variables affect how fast a pendulum swings?

Answer 1

The simple answer (what most high school teachers, for example, would say)

is that the period (length of time for a swing) only depends on the length of the

pendulum. This is a pretty good approximation for a well-made pendulum.

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When you sit down to work out the period of a pendulum on paper, you draw a mass,

hanging in gravity, from the end of a string that has no weight, with no air around it.

When you turn the crank, you discover that the period of the pendulum ... the time

it takes for one complete back-and-forth swing ... depends only on the length of

the string and the local acceleration of gravity, and that the pendulum never stops.

When you build the real thing, you discover that your original analysis is a little bit 'off'.

Your physical pendulum always stops after a while, and while it's still going, the

period is slightly different from what you calculated. So you begin to do research

experiments to figure out why.

Eventually, you figure out that the weight of the string makes the effective length

of the pendulum different from the actual length of the string, and that the pendulum

loses energy and stops because it has to plow through air.

What you do to reduce these influences:

-- You use the lightest, strongest string you can find, and the heaviest mass that

the string can hold, so that the mass at the end is huge compared to the mass of

the string.

-- You operate the whole pendulum in an evacuated tube ... with all the air pumped out.

When you do that, you have a pendulum that's good enough, and close enough

to the theoretical calculation, that you can use it to measure the acceleration of

gravity in different places.