A hole through a ball
A cylindrical hole is drilled straight through the centre of a solid ball, and what remains is a ring whose height (the length of the hole) is 6 cm. The radius of the ball is not given.
What is the volume of the remaining ring, in cubic centimetres?
Fractions, decimals with at least 3 significant digits, and expressions such as sqrt(2)/2, e^-1 or pi/4.
If the answer does not depend on the radius, try an extreme case.
For a proof, slice the ring horizontally at height . What is the area of each slice?
h
s Shown after a correct answer. Revealing it first marks the problem as seen, not solved.
Idea. Every horizontal slice of the ring is an annulus whose area does not depend on the radius of the ball. So the volume does not either, and the shortcut of a hole of zero width (a ball of diameter 6) gives it.
Step 1: a slice. Let the ball have radius and the hole height , so the hole has radius with . At height , with , the slice is an annulus between radius and , of area with no left.
Step 2: integrate.
The shortcut. As the hole gets thinner, the ball shrinks towards a ball of diameter 6, whose volume is . In general the ring has volume , whether it was cut from an orange or from the Earth.