Why is the area of a cross section not pi * r^2 for a rod?

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SUMMARY

The area of a cross section for a rod is calculated using the formula πD²/4, where D represents the diameter of the rod. This formula arises because the area is derived from the diameter rather than the radius. While the radius (r) is half the diameter, the standard formula for area uses the diameter for practical measurement purposes. Understanding this distinction is crucial for accurate calculations in mechanical engineering.

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ktpr2
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I'm going through my intro to mechanical engineering material and there was one thing I was never clear on:

If you have a cross section of a rod, why isn't its' area at a cross section [tex]pi * r^2[/tex] as a opposed to [tex]piD^2/4[/tex], where D is the diameter?
 
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Becasue people are actually measuring the diameter...?

Daniel.
 
whoops. That makes sense :D
 

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