Alternative for sphere volume:FAIL

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An alternative formula for the volume of a sphere proposed in the discussion was found to be incorrect. The formula attempted to combine the area of a circle with its circumference but failed due to the varying distances traveled by different parts of the circle during rotation. Integral calculus is suggested as the appropriate method to derive the correct volume formula, as it accounts for these variations. Participants recommend resources like Khan Academy and university textbooks for learning calculus concepts. The discussion concludes with acknowledgment of the initial mistake in the proposed formula.
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My brother thought of an alternative formula for the volume of a sphere:
(1/2)(∏R2)(2∏R).
it didn't work

could anyone tell me why?
 
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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.
 
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
http://imageshack.us/a/img839/5682/circle2.png
 
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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.
 
OK
could you give me an integral calculus tutorial? :smile:
 
Sulfur said:
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 learned differentiation already.
 
i haven't :(
 
Sulfur said:
i haven't :(

Then you should, so you can learn understanding instead of just memorising integrals.
 
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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