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Old 04-03-2007, 09:43 PM   #16
joepole
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>If you mark 2 points on a balloon with a marker and begin blowing it up, these two points would most certainly move away from each other. They would not, however, move away from each other near as fast as the expansion speed of the balloon itself.

For specific points it depends on where exactly they are in relation to each other. if they are on opposite sides of the balloon they would move apart at twice the rate of the expanding radius. If they were right next to each other they would barely move apart.

Let's move it to two dimensions to make it simpler here:

Assuming the angle between points a and b is 2Θ (you'll see why i chose 2Θ instead of Θ in a second) then I get the straight (not circumferential) distance between them is 2r(cosΘ), so as Θ approaches 90 from 0 (and thus the angle between the points approaches 180 from 0) you'll see that ab approaches 2r from 0

I have absolutely no idea how to post an integral here, but it's a fairly straight-forward first order one that shows for the entire circumference the mean distance is linear with r with a coefficient of 2π.
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