Question about turning a hemisphere into an equivalent circle?

  • Thread starter goldust
  • Start date
  • #1
87
1
I was thinking about this when I was trying to work out a simpler way of finding the volume of a sphere. Suppose we cover a hemisphere with a piece of pliable thin cover. Stretching the cover flat would make a circle. The circumference of the sphere is 2*pi*r. The distance along the surface of the hemisphere from the top to the bottom is a quarter of the circumference of the sphere, or 0.5*pi*r. Is it correct that the radius of the circle after stretching out the cover is also 0.5*pi*r? Many thanks in advance!
 
Last edited:

Answers and Replies

  • #2
87
1
My bad, I meant turning *the surface of* a hemisphere into an equivalent circle :redface:
 
  • #3
34,688
6,394
I was thinking about this when I was trying to work out a simpler way of finding the volume of a sphere. Suppose we cover a hemisphere with a piece of pliable thin cover. Stretching the cover flat would make a circle. The circumference of the sphere is 2*pi*r. The distance along the surface of the hemisphere from the top to the bottom is a quarter of the circumference of the sphere, or 0.5*pi*r. Is it correct that the radius of the circle after stretching out the cover is also 0.5*pi*r? Many thanks in advance!
My bad, I meant turning *the surface of* a hemisphere into an equivalent circle :redface:
And finding the area of the resulting circle?

This won't work. The surface area of the hemisphere is ##2\pi r^2##. If you take the thin cover off the hemisphere and stretch it out, it's true that you get a circle of radius ##(1/2)\pi r##. The area of that circle is ##\pi (\frac{\pi r}{2})^2 = \frac{\pi ^2}{4} \pi r^2 \approx. 2.47 \pi r^2##. I wrote the area in this form to make it easier to compare with the surface area of the hemisphere.

The area of the flattened hemisphere skin is larger than the surface area of the hemisphere, because in flattening the skin, you would need to stretch it, which adds area.
 
  • #4
87
1
Very interesting. Many thanks! :biggrin:
 

Related Threads on Question about turning a hemisphere into an equivalent circle?

Replies
2
Views
836
Replies
6
Views
745
Replies
3
Views
600
  • Last Post
Replies
2
Views
6K
Replies
2
Views
1K
Replies
11
Views
5K
  • Last Post
Replies
15
Views
10K
  • Last Post
Replies
5
Views
899
  • Last Post
Replies
9
Views
2K
Replies
10
Views
950
Top