## CDs and Cavalieri’s Principle

I finally got around to shedding my library of CD cases (I know, I’m quite behind), but it got me thinking about **Cavalieri’s Principle**.

Cavalieri’s Principle essentially states that if two **prisms **have the property that all corresponding **cross sections** have the same **area**, then those prisms have the same **volume.**

For instance, here we have two stacks of CD cases. Every cross-section of each “prism” here is a single CD case. Since corresponding cross-sections always have equal area, **Cavalieri’s Principle** tells us that these prisms have equal **volume**, even though one of the stacks is **oblique.**

That the stacks have equal volume is made clearer with a simple **transformation** of the stack on the right.

Here’s another demonstration of Cavalieri’s Principle using the CDs themselves.

*Click here to see more in Geometry.*

Would this also apply to cylinders?

Yes. Cavalieri’s Principle doesn’t make any [unusual] demands of the cross-sectional

shape; it’s all about thearea.For a visual demonstration with cylinders, check back next week!

It doesn’t have to be slant either. It could be crooked but as long as the cross section stays the same, it has the same volume. The easiest way to do this at home is with a stack of coins.

Or a stack of CD’s that you pulled out of their boxes. 🙂

You can also think of Cavalieri’s principle in two dimensions by comparing a rectangle and a parallelogram with equal bases and heights. Andy’s point holds here, too: you could also throw in a shape whose with the same base, but sides are [equally] curvy.