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Need help with rotational problems/angular speed

  1. Mar 11, 2009 #1
    HI, i have three problems that i have tried but failed here they are

    1. The problem statement, all variables and given/known data
    SOLVED 1.) A wheel turns through 4 revolutions while being accelerated from rest at 20rpm/s.
    a)find the final angular speed b) find the time it takes to turn 4 revolutions

    2.) A potter's wheel is a stone disk 85 cm in diameter with a mass of 120 kg. If the potter's foot pushes at the outer edge of the initially stationary wheel with a 80 N force for one-eighth of a revolution, what will be the final speed? Use the work-energy theorem.

    3.)A 320 kg motorcycle includes two wheels, each of which is 52 cm in diameter and has rotational inertia 2.1 kg·m2. The cycle and its 74 kg rider are coasting at 87 km/h on a flat road when they encounter a hill. If the cycle rolls up the hill with no applied power and no significant internal friction, what vertical height will it reach?

    2. Relevant equations

    1.) omega final^2= omega initial squared +2a(angular position)/ omega final=omega initial +at
    2.) I used W=1/2I wf^2 -1/2I wi^2
    3.) tried using the v center of mass= sqrt of (2mgh)/(M+I/(R^2))

    3. The attempt at a solution
    for the first one i plugged in everything but i get the wrong answer i think its because of the revolutions, but i changed that to rad and the 20rpm/s i left it since the answer has to be in rpm
    2.) SInce it told me to use the work energy theorem i tried pluggin in the 1/2mr^2 for I sicne its a disk and 80N for W as well as the 120kg for mass but... i keep getting the wrong answer... did i use the wrong equation or?
    3.) for number im not sure if the equation is the correct one.. i tried plugging in the numbers but then for mass i plugged in 320+74 too have the combined mass of 394 and changed the 87 km/h to 24.2 m/s but the answer was wrong so that didnt work out...so im stumped...

    thank you in advance
    Last edited: Mar 11, 2009
  2. jcsd
  3. Mar 11, 2009 #2


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    Welcome to PF!

    Hi silentsaber! Welcome to PF! :smile:

    (have an omega: ω and an alpha: α and a theta: θ and try the X2 tag just above the Reply box :wink:)
    ωf2 = ωf2 + 2αθ is the right equation …

    and all the given units are in revolutions, so that's ok, no need to change to radians …

    perhaps you didn't convert α from rpm/s into rps/s ? :wink:
  4. Mar 11, 2009 #3
    would teh accleration then be 2.094rps? not sure what to do about the bottom second cuz i tried inputting that and got it wrong as well =/
  5. Mar 11, 2009 #4


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    (you mean 2.094rps2)

    nooo … you've included a 2π … you don't need to :smile:
  6. Mar 11, 2009 #5
    omg THANX!
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