You idly try throwing stones in different ways: a high arching trajectory makes a bigger plop, but still a ring. A hard low throw of a flat stone gives a skip, and a ring at each impact point. Your physics teacher walks over and says, "Didja notice that the ring always expands the same, irregardless (you always hated that non-word) of how the stone moved?" S'truth, but so what? "Well, it's not the same for things thrown from other things. If you're moving on skates and you throw a ball forward, the ball moves faster than if you threw it the same way while standing still. Your speed adds to the ball speed. Contrariwise, if if throw the ball backwards over your shoulder while moving forward, the ball moves slower because the speeds subtract."

"But if you make a wave with a stone, it doesn't matter how fast the stone is going, the speed of the wave is the same in all directions, so it stays a circle. The speed of the wave only depends on the media, as we scientists say. The wave isn't a thing thrown off by an object, but rather a disturbance in the Force, er, the water."

Of course, if you look a little closer, it isn't quite that simple, as usual.

In particular, suppose we make many plops from a moving object. The skipping stone is sort of like that, but the interesting stuff requires a little more control of what's happening. Luckily, you have a friend who keeps a falcon, and you remember those nature shows where the osprey or whatever skims over the lake and grabs a fish. All you need to do is have the bird fly at different speeds over the water and periodically dip its feet! (Don't try this at home...this is a specially trained virtual falcon) If your Browser supports Java, you can click and drag (that is, hold down your mouse button while moving the mouse) and the falcon will follow your mouse, dipping his toes every so often and starting a ring. Otherwise he'll hover at the last spot. You can make some cool patterns, but for our purposes the most interesting is when the falcon follows the fish swimming back and forth (don't worry, he won't strike...no virtual fish were harmed in the making of this Web page).

Fish Speed-o-Mat Slower Wave Speed Faster

If you try this, you'll see a nested set of rings systematically squeezed to one side: the waves will be closed spaced on the left and wide spaced on the right, or vice versa, depending on which way you're moving. Now try speeding up the fish (everything's trained here) by clicking on the Fish Speed-o-Mat and try again. You can make the fish and bird move at just the wave speed, or faster so they outrun their waves.

Go ahead and Catch the next wave!

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