# What is Cd^2

#### ddr

this is about SR in practice...

imagine a Cd with inner radius r, outter radius R and middle radius d. the Cd starts to spin from 0 to some V spatial speed with respect to the radius d. as the speed increases all the perimeters shrink, but the outter one shrinks far beyound the inner because it has greater spatial velocity cause it corresponds to bigger radius. so when the speed of d-circle becomes some V (critical) the Cd will (in a SR kinda sense) turn into square ie. the outter and the inner parameters will equalize.
am I right?

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#### FZ+

No. The radius does not shrink as it lies on a direction perpendicular to the direction of motion. But relative to us at the centre of the CD which the cd is moving relative to, we will have length contraction, which leads us to a value for circumference that is shorter than that obtained if we were moving with the CD. Therefore, the moving measurement would give values of pi that are not 3.14159...

#### ddr

I didn't meant that the radiuses are shrinking but the perimeters do cause the displacement is parallel with them.Ok?

I wonder what would a speeding fly look like in a relative sense?

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