finding time in circular motion


by tyro008
Tags: circular, motion, time
tyro008
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#1
Nov1-08, 04:06 PM
P: 20
1. The problem statement, all variables and given/known data
The tips of the blades in a food blender are moving with a speed of 15 m/s in a circle that has a radius of 0.052m. How much time does it take for the blades to make on revolution?


2. Relevant equations
T=(2XpiXr)/T
T: period of time it takes to complete one cycle

3. The attempt at a solution
I tried using the equation under "relevant equations" and got 0.02seconds as an answer. This is wrong but I don't know what other equation to use..
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djeitnstine
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#2
Nov1-08, 04:13 PM
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Your equation is all wrong. Its T = 2*pi*r / velocity
tyro008
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#3
Nov1-08, 04:19 PM
P: 20
oh whoops. but.. i tried that and it still didn't work..
is there another equation that i should use instead?
i feel that my T represents time, but not the one i'm looking for

djeitnstine
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#4
Nov1-08, 04:26 PM
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finding time in circular motion


Lets break it down. r = 0.052 meters. The circumference is 2 pi 0.052 which is 0.104 pi meters. It rotates at velocity 15 meters every second. So this means that it does 15/0.104 pi revolutions every second. Then just inverse that quantity to find how many seconds is one revolution
tyro008
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#5
Nov1-08, 04:40 PM
P: 20
i keep getting 0.02 seconds, but this isn't the right answer.
is this what you got, too?
Epsillon
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#6
Nov1-08, 05:02 PM
P: 70
V^2/r= 4 pi^2 r/T^2

solve for T

EDIT: Forgot square T
djeitnstine
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#7
Nov1-08, 05:18 PM
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Quote Quote by tyro008 View Post
i keep getting 0.02 seconds, but this isn't the right answer.
is this what you got, too?
If there is another answer other than 0.02 seconds then that answer has to be wrong. Only thing else I can think of is if you got 15 m/s wrong and it was supposed to be 15 radians per second.
djeitnstine
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#8
Nov1-08, 05:22 PM
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Quote Quote by Epsillon View Post
V^2/r= 4 pi^2 r/T

solve for T
Epsillon what is this equation?? this basically says v =[tex]\frac{2\pi}{ \sqrt{T}}[/tex]
Epsillon
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#9
Nov1-08, 06:03 PM
P: 70
Oh sorry I forgot to square the T (silly mistake)

its 4pi^2r/T^2
djeitnstine
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#10
Nov1-08, 06:06 PM
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Quote Quote by Epsillon View Post
Oh sorry I forgot to square the T (silly mistake)

its 4pi^2r/T^2
Ok it still gives v = [tex]\frac{2 \pi}{T}[/tex] which gives him the "wrong" answer. Obviously which ever book or person giving the answer is wrong.
tyro008
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#11
Nov2-08, 02:42 PM
P: 20
haha, thanks anyways you guys.
you know, i think you guys are right. i'll talk to my teacher about changing the answer :)


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