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Power prep Circular Rim

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by parallel_chase » Tue Oct 14, 2008 11:57 am
A circular rim 28 inches in diameter rotates the same number of inches per second as a circular rim 35 inches in diameter. If the smaller rim makes X revolutions per second, how many revolutions per minute does the larger rim makes in terms of X ?

48pi/X

75X

48X

24X

X/75


OA 48X
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by gmat009 » Tue Oct 14, 2008 12:03 pm
X(28)=N(35)
N=28/35(X) rev/sec
N=48X

Hope that Helps......
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by parallel_chase » Tue Oct 14, 2008 12:19 pm
gmat009 wrote:X(28)=N(35)
N=28/35(X) rev/sec
N=48X

Hope that Helps......
Can you pls explain why it is that

rate * 2pi*r

and not

2*pi*r / rate

Usually this what the standard formula for RPM is, pls correct if i am wrong.

Thanks

PS: Form next time check the option, "Disable Similes in this post" while posting. Thanks.
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by gmat009 » Tue Oct 14, 2008 12:26 pm
parallel_chase wrote:
gmat009 wrote:X(28)=N(35)
N=28/35(X) rev/sec
N=48X

Hope that Helps......
Can you pls explain why it is that

rate * 2pi*r

and not

2*pi*r / rate

Usually this what the standard formula for RPM is, pls correct if i am wrong.

Thanks

PS: Form next time check the option, "Disable Similes in this post" while posting. Thanks.
Standard formula for RPM is 2pi*R.
lets assume rim with 28 inches diameter took N1 no of revolutions.
Therefore 1 revolution = 2pi*R
N1 revolutions = N1*2pi*R
Same goes for rim with 35 inches diameter.
Now in question N1=X
So eqn becomes (x)(28pi)=(N2)(35pi)

Let me know if you still have problem
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by raunekk » Wed Oct 15, 2008 2:35 am
for 28 inches dia,

circumfrence = 2 pai r = 28 pai

inches traveled / sec = x * 28 pai
inches travel / min = 60 * x * 28 pai

which is equal to inches traveled by 35 inches dia,

let inches traveled by 35 is y

thus 35 y = 60 * x * 28 pai

therefore,
y = 48 x

hence C
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Re: Power prep Circular Rim

by sudhir3127 » Wed Oct 15, 2008 4:13 am
parallel_chase wrote:A circular rim 28 inches in diameter rotates the same number of inches per second as a circular rim 35 inches in diameter. If the smaller rim makes X revolutions per second, how many revolutions per minute does the larger rim makes in terms of X ?

48pi/X

75X

48X

24X

X/75


OA 48X
I go with C as well

ur right on the formula of 2*pi*r

pi *28x=pi*35y
y=4x/5 rev per sec

hence its 48X
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by 4meonly » Thu Oct 16, 2008 12:41 am
Guys, I was really stuk when you got 0.48 from 28/35. This was very interesting.

y the revolutions per second for the 35 one rime
28pi*x=35pi*y
28x=35y
y=28/35x

Now per minutes:
60y=60*(28/35)x
60y=48x
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