Math & Physics Problems Wikia
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Problem

Figure 1 shows an animated 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.

Figure 1. Animation of the napkin ring


Solution

Figure 2. Cross-section of the napkin ring

Method 1: Washer method

Consider the diagram of the cross-section of the napkin ring (Figure 2). Let the radius of the sphere be . Let the radius of the cylindrical hole be , and half the height of the cylindrical hole be . Note that .

The volume of the napkin ring is equal to the volume of the sphere minus the volume of the cylinder and two spherical caps.

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

Use integration to compute the volume of a spherical cap:

Hence, the volume of the napkin ring is

Since , we get

Expressing the volume of the napkin ring in terms of the height of the cylindrical hole yields:

Method 1: Shell method

The shell method is much faster for this problem. By revolving the function around the y-axis, and applying the shell method formula, we get the upper half of the napkin ring. Doubling this gives the total volume:


Since , we get

Expressing the volume of the napkin ring in terms of the height of the cylindrical hole yields:

Who would have guessed that this volume is independent of the radius of the sphere! This means that if you core out any sphere of any size so that the remaining napkin rings have the same height, those napkin rings will also have the same volume! This looks unbelievable at first, but is in fact true!

Figure 3. Vertical "body scans" of two napkin rings with equal height


Historical Remark

Figure 4. Seki Kowa (1642 - 1708 AD)

The napkin ring problem dates back to Edo Japan. Seki Kowa (1642 - 1708 AD), 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 kokan (弧環, lit. "arc ring").

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