Water surface tension

If you look closely at a puddle during a rainstorm, you see a weird thing: little balls of water skating around as though the are being rejected by the receiving pool. They disappear quickly, so it’s hard to keep your eye on them. What’s going on? What would it look like if you had faster eyes?

Via Steve Crandall I came across this lovely slow motion video of bouncing water drops. For an instant, before the intervening air is squeezed out, the taut surface of the puddle is essentially a trampoline for descending droplets. I’ve always wondered about the physics of this. Watch!

Train wrecks and momentum in the movies

When it comes to big things, it’s hard to get momentum right. We don’t have much experience with truly massive things moving quickly and unpredictably. Airplanes, trucks, and trains in normal circumstances aren’t surprising. But here’s a video of a train wreck during a tornado (nobody gets hurt). Watch through to the end to see Old Man Momentum get his due.

In old movies, they had to substitute small models for the real thing, so the physics were all wrong. Watch this video of the steamboat in the African Queen going over a waterfall. It’s clear the boat in the waterfall is about two feet long. With computer simulations and graphics, they can do much better these days. Here’s a clip from The Incredibles that involves a van speeding through traffic. Since they aren’t using a mechanical model, they can get the physics right, or close enough for a cartoon fantasy world.