# Homework Help: Finding time in circular motion

1. Nov 1, 2008

### tyro008

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..

2. Nov 1, 2008

### djeitnstine

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

3. Nov 1, 2008

### 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

4. Nov 1, 2008

### 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

5. Nov 1, 2008

### tyro008

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

6. Nov 1, 2008

### Epsillon

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

solve for T

EDIT: Forgot square T

Last edited: Nov 1, 2008
7. Nov 1, 2008

### djeitnstine

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.

8. Nov 1, 2008

### djeitnstine

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

9. Nov 1, 2008

### Epsillon

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

its 4pi^2r/T^2

10. Nov 1, 2008

### djeitnstine

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.

11. Nov 2, 2008

### tyro008

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