Finding time in circular motion

[tyro008]

Homework Statement

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?

Homework Equations

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

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..

 

[djeitnstine]

Your equation is all wrong. Its T = 2*pi*r / velocity

 

[tyro008]

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]

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]

i keep getting 0.02 seconds, but this isn't the right answer.
is this what you got, too?

 

[Epsillon]

V^2/r= 4 pi^2 r/T^2

solve for T

EDIT: Forgot square T

 

[djeitnstine]

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]

V^2/r= 4 pi^2 r/T

solve for T

Epsillon what is this equation?? this basically says v =\frac{2\pi}{ \sqrt{T}}

 

[Epsillon]

Oh sorry I forgot to square the T (silly mistake)

its 4pi^2r/T^2

 

[djeitnstine]

Oh sorry I forgot to square the T (silly mistake)

its 4pi^2r/T^2

Ok it still gives v = \frac{2 \pi}{T} which gives him the "wrong" answer. Obviously which ever book or person giving the answer is wrong.

 

[tyro008]

haha, thanks anyways you guys.
you know, i think you guys are right. i'll talk to my teacher about changing the answer :)