Recent content by Doc Al

Such experiments are typically done in a vacuum.

I think @Baluncore has nailed it. When breaking a board, you need to deliver enough energy to exceed its tensile strength limit and create a crack that spreads rapidly. In 1979, there was an article in Scientific American: The Physics of Karate by Feld, McNair, and Wilk. You might find it...

Realize that the field from the surface charge points in different directions above and below the surface. Those fields might have the same magnitude, but not the same direction. Don't forget that the field is a vector!

Yes. If there were no surface charge, just a surface in space, you'd expect the field above and below the surface to be the same -- thus the difference would be zero. But if there's a surface charge the field above and below would have to be different -- due to the field of the surface charge...

Realize that there is a non-zero surface charge contained within the pillbox.

On that note, it's time to close this thread.

This seems reasonable:

I'm in Monmouth County. Help me! :oldsurprised:

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I think you're mixing things up. Imagine two people on the moon, one on each side. You can send signals to each however you want, but those signals move at the speed of light. Those two people cannot use your signal to transfer information to each other. (No matter how you move the laser...

You're talking about communicating between Earth and Moon, not from one side of the moon to the other. Since you're using a light beam, all signals will be at the speed of light. (Of course! :smile:)

That moving laser dot cannot transfer information across the moon! Similarly, the flashing lights (in the example from @PeroK) carry no information as they "move" along.

Plenty of "things" (that aren't really things) can go faster than light. There's the classic of shining a laser pointer at the moon. You can sweep the pointer across in a fraction of a second, but the light hitting the moon's surface will appear to move faster than light speed. And look up...