## Calculus For Breakfast

Robert Austin, of the RobertLovesPi blog, got to thinking about one of those interesting mathematics problems. It starts with the equations that describe the volume and the surface area of a sphere.

If the sphere has radius r, then the surface area of the sphere is 4πr^{2}. And the volume is (4/3)πr^{3}. What’s interesting about this is that there’s a relationship between these two expressions. The first is the derivative of the second. The derivative is one of the earliest things one learns in calculus. It describes how much a quantity changes with a tiny change in something it depends on.

And this got him to thinking about the surface area of a cube. Call the length of a cube’s side s. Its surface is six squares, each of them with a side of length s. So the surface area of each of the six squares is s^{2}, which is obvious when you remember we call raising something to the second power “squaring”. Its total surface area then is 6s^{2}. But its volume is is s^{3}. This is why we even call raising something to the third power “cubing”. And the derivative of s^{3} is 3s^{2}. (If you don’t know calculus, but you suspect you see a pattern here, you’re learning calculus. If you’re not sure about the pattern, let me tell you that the derivative of s^{4} would be 4s^{3}, and the derivative of (1/3)s^{2} would be (2/3)s.)

There’s an obvious flaw there, and Austin’s aware of it. But it got him pondering different ways to characterize how big a cube is. He can find one that makes the relationship between volume and surface area work out like he expects. But the question remains, why that? And what about other shapes?

I think that’s an interesting discussion to have, and mean to think about it some more myself. And I wanted to point people who’d be interested over there to join in.

## RobertLovesPi 8:05 pm

onSaturday, 12 March, 2016 Permalink |Much thanks for the “signal boost” — I appreciate it!

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## Joseph Nebus 3:34 am

onMonday, 14 March, 2016 Permalink |Quite happy to be of service. Thank you.

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## How Much March 2016 Liked My Mathematics Blog | nebusresearch 3:01 pm

onSaturday, 2 April, 2016 Permalink |[…] Calculus For Breakfast, pointing over to Robert Austin’s blog and a question about convenient ways to measure how big things are. […]

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