Math & Physics Problems Wikia
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Problem

The animation below shows a central cross-section of a sphere of radius through which a centrally placed cylinder of radius has been cored out (drilled out and the material removed). The remaining shape is called a napkin ring. Determine the volume of the napkin ring.


Cut Napkin ring problem Animation created with openscad.gif

Solution

Consider the diagram of the cross-section of the napkin ring. Let the radius of the sphere be . Let the radius of the cylindrical hole be , and half the height of the cylindrical hole be .

Napkin ring cross-section2.jpg

To compute the volume of the napkin ring, observe that its volume is equal to:

.


The volume of the sphere and the cylinder are well known:

Note that .


Use integration to compute the volume of a spherical cap.


Hence, the volume of the napkin ring is:

Since , we get

The volume of the napkin ring expressed in terms of the height of the cylindrical hole () is:


Note that this volume is independent of the radius of the sphere, who would have guessed! This looks unbelievable at first because it means that if you core out any sphere of any size so that the remaining rings have the same height, those rings will also have the same volume!

Napkin ring-animation.gif

Historical Remark

The napkin ring problem dates back to Edo Japan. Seki Kowa (1642 - 1708), the leading Japanese mathematician at the time was the first person to have solved this problem using a form of integral calculus called Enri. Seki Kowa called the shape an “arc ring”.

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