## Tuesday, May 24, 2016

### Playground Physics

Playgrounds are a great place to learn some basic physics. Or at least they were 50 years ago, before the safety mavens moved in and rebuilt everything in plastic and foam rubber. Take a look at this old seesaw:

The physics lesson is right in the center: the seesaw rested on an open pivot, so that you could just lift it up and move it, making one side shorter than the other. This was a perfect illustration of a lever -- a lighter kid on the longer end could easily balance against a heavier kid on the shorter end. At least until one of them jumped off, sending the other crashing to the ground. But it was the movable pivot that doomed this particular type of seesaw. You can see how easy it would be to get your fingers crushed in the center. I never actually saw this happen, but we all lived in dread of the possibility.

An even more interesting physics demonstration was the old-fashioned merry-go-round:

The game here was to push the merry-go-round as fast as possible, until centrifugal force almost flung everyone off. Disclaimer: we teach all of our college freshmen that there's really no such thing as centrifugal force. The force actually points inward, and it's called "centripetal force." But if our students go on to a more advanced class, they learn that there really is such a thing as centrifugal force, but it's a "fictitious force" --  a force that only appears in an accelerating frame of reference. So when you're flung off the merry-go-round and scrape your legs while sliding across the concrete, you can comfort yourself with the fact that it was only a fictitious force that threw you across the playground.

Another fictitious force that's illustrated by the merry-go-round is the Coriolis force. If you and a friend are spinning on opposite ends of the merry-go-round, and you toss a tennis ball straight across to your friend, it will to curve away. (Yes, I really did this when I was a kid). Take a look at this very nice merry-go-round video from MIT. (Those guys don't look like they're having much fun). This is a "fictitious force" because the ball actually moves in a straight line -- it only appears to curve from the point of view of someone spinning around the merry-go-round.

The Coriolis force on the rotating earth is the reason that weather patterns show massive rotation. Low pressure systems (in the northern hemisphere) spin counterclockwise, and high pressure systems spin clockwise. So if you get struck by a hurricane, just console yourself with the fact that it was produced by a fictitious force.