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Flywheel rotation question.

  1. Feb 7, 2017 #1
    1. The problem statement, all variables and given/known data
    A flywheel with a D=1.2m is rotating at an angular speed of 200 rev/min
    (a) Whats the angular speed in rad/s ?
    (b)Whats v=? in the point on the rim ?
    (c)What const. ∝ ( in rev/min^2) will increase its angular speed to 1000 rev/min in 60 sec ?
    (d)How many revolutions does the wheel make during that 60 sec ?

    2. Relevant equations
    Rotational Kinematics Equations


    3. The attempt at a solution
    I found a,b,c correctly (for long tries ) but I stucked at d.

    I guess I ll use here ##Δθ=wt-1/2∝t^2##

    (a) w=20.9 rad/s
    (b)v=12.5
    (c)=800 rev/min^2

    Now for d,

    lets to write all equaiton in rev/min form so
    20.9rad/sec=20.9.2π.60=7879 rev/min
    I dont know what will be the left part cause Δθ in units its normally just rev I guess so
    θ-200rev/min=7879rev/min.1min+1/2 (800rev/min^2).(1 min)^2
    Here I get a huge number...

    Thank you
     
  2. jcsd
  3. Feb 7, 2017 #2

    mfb

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    Staff: Mentor

    You established before that 200 rev/min corresponds to 20.9 rad/s (which is correct). How can it correspond to a larger number of revolutions per minute now?
    You cannot subtract an angle and an angular velocity. That's like adding 3 meters to 1 minute: It does not make sense.

    You can use the formula you posted, but it is probably easier to find the average angular velocity (in rev/min) and to use that.
     
  4. Feb 7, 2017 #3
    ohh I see...so I can convert 200rev/min to rev which its just 200 or ..I dont know ?
     
  5. Feb 7, 2017 #4

    mfb

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    Staff: Mentor

    You cannot "convert an angular velocity to revolutions" in the same way you cannot convert apples to minutes. There is no need for such a conversion. Your formula multiplies the angular velocity by a time. The product of those two is a number of revolutions or an angle, depending on which units you use.
     
  6. Feb 7, 2017 #5
    ok then how can I found inital rev ? If other parts are correct ?
     
  7. Feb 7, 2017 #6
    oh I see...
     
  8. Feb 7, 2017 #7
    Here my new try
    Δθ=20.9 rad/s.60s-1/2.(0.035 rad/sec^2).(60sec)^2
    Δθ=1317rad
     
  9. Feb 7, 2017 #8

    mfb

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    Staff: Mentor

    Your angular acceleration seems to be too low.
     
  10. Feb 7, 2017 #9
    yeah,sorry it would be 1.40 rad/sec ?
     
  11. Feb 7, 2017 #10
    I found yeyyyyy
     
  12. Feb 7, 2017 #11
    Thanks :smile::smile::smile::smile:
     
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