Playing with Pendulums

The name Galileo is on a lot of peoples' lips these days. Turns out he first turned his telescope skyward exactly 400 years ago, and thus begat a revolution in humanity's view of the cosmos, and of its place in that cosmos.

As someone who occasionally turns his own telescope skyward, I'll be blogging about Galileo's telescopic discoveries, many of which you can repeat yourself with all but the very worst telescopes. But today I want to ruminate briefly about some of the work that the famed mathematician (yes, that was his job description) isn't quite so well known for: his experiments with different ways to measure the passage of time.

When we roll Hot Wheels cars or golf balls down ramps in today's physics classes, we mimic experiments that Galileo did in order to understand the nature of motion. But typically we "time" our rolling toys with digital stopwatches: electronic devices that can reliably measure the passage of time with hundredths-of-a-second accuracy.

Galileo didn't have this luxury. So he measured the passage of time with typical inventiveness, using a water clock (in which water drips at a regular rate), a pendulum (in which a massive "bob" swings at the end of a string, rope, or wire), or even his pulse.

Want to follow in Galileo's footsteps? Try this experiment. Tie a half-dozen paper clips together at the end of a foot-long piece of string. Find a way to hang the string so that it can swing freely. (I taped mine to the edge of my kitchen table.) Pull the paper clips a few inches to the side, then let go. Watch them swing back and forth. You'll notice that they go a little less far with each swing, and eventually they come to a stop. But, amazingly, they take the same amount of time to swing once from one side to another - even though they are losing energy to the unavoidable forces of friction and air resistance.

Armed with a stopwatch, you can try to figure out what determines a pendulum's period - the time its "bob" (in this case, the bunch of paper clips) takes to swing once out and back again. Does the number of paper clips affect the period? The length of the string? The number of inches you pull the paper clips to the side before letting go? Once you've explored these factors - ideally, by varying just one of them at a time! - can you make a string-and-paper-clip pendulum swing back and forth ten times per second? Five times? Once? Do so, and you can deservedly call yourself an experimental physicist.

Copyright 2009 Joshua Roth.