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Forums > Technical Discussion > Problems with your Mathematics

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jamie


member
Location: Hillsborough, San Francisco, C...

Total posts: 21
Posted:I have a few problems with the mathmatics section on this site, I think your values for 'rotational force' are a little too high.The page states that the 'rotational force' (which I will assume to mean centripetal acceleration) for a poi on a string with length .8 meters swinging at 2 revolutions per sec is 15 times the force of gravity and at 3 revolutions per sec is 50 times the force of gravity(assuming no friction and gravity=9.8m/s^2).Math for 2rev/sec:v=2(pi)(.8m)(2rev/sec)=10.05m/sa=(v^2)/r= (101.06 m^2/s^2)/.08m= 126.33m/s^2Force at bottom(2rev/sec):((126.33m/s^2)+9.8m/s^2)/9.8m/s^2= 13.9 times gravityForce at top(2rev/sec):((126.33m/s^2)-9.8m/s^2)/9.8m/s^2=11.9 times gravityMath for 3rev/sec:v=2(pi)(.8m)(3rev/sec)=15.075m/sa=(v^2)/r= (227.26 m^2/s^2)/.08m= 284.07m/s^2Force at bottom(3rev/sec):((284.07m/s^2)+9.8m/s^2)/9.8m/s^2= 30.0 times gravityForce at top(3rev/sec):((284.07m/s^2)-9.8m/s^2)/9.8m/s^2= 28.0 times gravityFor 3rev/sec neither values are anywhere near 50 times.Conclusion:I'm not sure where your formulas went wrong, but in your calculations you switched radii from (.6m) to (.8m) and also you labled v^2 as having units of (m/s^2) which is the derivative of velocity not the square. Also note: Blue glowstix + Progressive Trance = Ecstacy

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Cantus
SILVER Member since Jul 2001

Cantus

Tantamount to fatuity
Location: Down the road

Total posts: 15965
Posted:I'd been thinking along these lines for a while but was too shy to speak out for fear of stigmatising myself.Thank you for being strong for both of us.------------------C@ntusWe are old, we are young, we are in this together, vagabonds and children, we're prisoners forever, with pulses raging and eyes full of wonder, Kicking out behind us again.

"I'll carry this....It's harder to spill a hat" - Chellybean
"...like a rabbit caught in a lighthouse?" - Chellybean

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Malcolm
SAPPHIRE Member since Nov 2003

Malcolm

HOP admin
Location: HOP

Total posts: 998
Posted:Jamie,You are right
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I am not teaching maths here, but hopefully, through my very basic formula, people will see that there are lots of additional forces once you start swinging stuff and will consider that when designing and making gear.Thanks for checking Malcolm


"May your balls always burn"

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NYC


NYC

NYC
Location: NYC, NY, USA

Total posts: 9232
Posted:We chatted about this before... You are right though, good math.I had some typically long winded and useless post that gave the rundown for most of the torque you'd get. Keep in mind that NONE of the math takes into computations of non-cicular stresses such as wraps and even changing direction. Be sure to leave lots of room for margin in the stress tolerances of your poi.

Well, shall we go?
Yes, let's go.
[They do not move.]

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Cantus
SILVER Member since Jul 2001

Cantus

Tantamount to fatuity
Location: Down the road

Total posts: 15965
Posted:and so all the inherent (even if unintentional) humour in the original post is lost as the grown ups take to the floor......

"I'll carry this....It's harder to spill a hat" - Chellybean
"...like a rabbit caught in a lighthouse?" - Chellybean

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Jangla


member
Location: Oxford, UK

Total posts: 155
Posted:**applause** good math indeed but you forgot to derive your second equation to the 2nd level:Blue glowstix + Progressive Trance = Ecstacytherefore;Blue glowstix + Inner Purple glowstix + Progressive Trance = PURE Ecstacy------------------"The tincture of night began to suffuse the soup of the afternoon. He considered the sentence, and found it good. He liked 'tincture' particularly. Tincture. Tincture. It was a distinguished word, and pleasantly countered by the flatness of 'soup'. The soup of the afternoon. Yes. In which may be found the croutons of teatime."

---------------------With a bit of luck, his life was ruined; always thinking that just behind some narrow door, in his favourite bars, men in red woolen suits are getting incredible kicks from things he'll never know.

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