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Homework Help: Finding time in circular motion

  1. Nov 1, 2008 #1
    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. jcsd
  3. Nov 1, 2008 #2

    djeitnstine

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    Your equation is all wrong. Its T = 2*pi*r / velocity
     
  4. Nov 1, 2008 #3
    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
     
  5. Nov 1, 2008 #4

    djeitnstine

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    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
     
  6. Nov 1, 2008 #5
    i keep getting 0.02 seconds, but this isn't the right answer.
    is this what you got, too?
     
  7. Nov 1, 2008 #6
    V^2/r= 4 pi^2 r/T^2

    solve for T

    EDIT: Forgot square T
     
    Last edited: Nov 1, 2008
  8. Nov 1, 2008 #7

    djeitnstine

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    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.
     
  9. Nov 1, 2008 #8

    djeitnstine

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    Epsillon what is this equation?? this basically says v =[tex]\frac{2\pi}{ \sqrt{T}}[/tex]
     
  10. Nov 1, 2008 #9
    Oh sorry I forgot to square the T (silly mistake)

    its 4pi^2r/T^2
     
  11. Nov 1, 2008 #10

    djeitnstine

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    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.
     
  12. Nov 2, 2008 #11
    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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