Alternative for sphere volume:FAIL

  • Thread starter Sulfur
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  • #1
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Main Question or Discussion Point

My brother thought of an alternative formula for the volume of a sphere:
(1/2)(∏R2)(2∏R).
it didn't work

could any one tell me why?
 

Answers and Replies

  • #2
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So it seems to be (area of circle) * (circumference) * (1/2). It's less a question of why it doesn't work, than why your brother thought it would work. Maybe if you posted the derivation, we could point out the problem with it.
 
  • #3
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the idea is that if a circle was turned around 180 degrees on a line through the middle of it, and if every frame of its rotation was kept there, it would make a sphere. The circumference is the distance the edge of the circle has to travel 360 degrees.
hmm.... maybe the problem is that different parts of the circle have to travel different lengths to go 360 degrees around. :rolleyes:
Do you think if the problem was solved, I could derive another formula that is correct?
http://imageshack.us/a/img43/9046/circle1.png [Broken]
http://imageshack.us/a/img839/5682/circle2.png [Broken]
 
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  • #4
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This is pretty much integral calculus, summing an infinite number of infinitely small areas. You should learn it, it's really useful.
 
  • #5
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OK
could you give me an integral calculus tutorial? :smile:
 
  • #6
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OK
could you give me an integral calculus tutorial? :smile:
To get an idea and basic concepts, try Khan Academy. If you wan to learn the "standard" way, any university calc textbook will do. I'm not sure about online ones, but try Paul's Online Maths Notes
http://tutorial.math.lamar.edu/sitemap.aspx

PS Presuming you've learnt differentiation already.
 
  • #7
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i haven't :(
 
  • #8
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i haven't :(
Then you should, so you can learn understanding instead of just memorising integrals.
 
  • #9
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Note that if you slowly rotate a circle (in the manner of your pictures), the parts near the edge move farther than the parts near the middle. So every little rotation sweeps out volumes that are greater the farther away from the axis they are. For situations like that, you need calculus (or a smart Greek).

edit: I now noticed that you caught your mistake. well done.
 
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