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Homework Help: What is the RATE of Rotation! help

  1. Nov 21, 2006 #1
    What is the RATE of Rotation! help!!!!!

    An automatic tumble dryer has a 0.65m diamter basket that rotates about a horizontal axis. As the basket turns, the clothes fall away from the basket's edge and tubmle over. IF the colthes fall away from the basket at a point 60 degrees from the vertical, what is the rate of rotation in units of revolutions per minute.

    i thought this problem would be extremely simple.

    I was wrong :)

    First i get

    V^2 / rg = tan theta
    so i get

    Vc^2 = rg tan theta
    soif ifind Vc i get 1.785

    Now this wouldn't make snese...
    does someone else have a different Apporpach
  2. jcsd
  3. Nov 21, 2006 #2
    ok i can't get mass from anything. what do i do?

    it has tobe something like 0.65 / x = tan theta or something?
  4. Nov 21, 2006 #3

    At sixty degrees the vertical part of this centripetal force is provided solely by the weight of the clothes ie mg.

    thus mv^2/r*cos60 = mg, am I wrong?

    v= sqrt(r*g/cos60) = 2.53m/s? Then remember you have to make it revs/min

    cannot see why you are using tan
  5. Nov 21, 2006 #4
    the answer is 37revs/minute
    i did get 2.53m/s and then multiplied it by 60/2pi
    wrong wrong :(
  6. Nov 21, 2006 #5
    Yeah I get 74 revs/minute =/

    I did 2.53/r to get it in rads/sec and then /2pi * 60 = 74...

    2 times 37, strange
  7. Nov 21, 2006 #6
    maybe because the problem gave it in diameters and u used that.
    i'm still trying to get it
  8. Nov 21, 2006 #7
    wait i still don'tget how uget 2.53m/s to 74rev/min

    2.53 / (2pi x 60) doesn't work nor (2.53/2pi) x 60
  9. Nov 21, 2006 #8
    no i used radius and I told you 2.53/r to get radians per seconds.

    2.53/2pi*60 doesn't mean anything. What you want is

    2.53/r/2pi*60 to get 74
  10. Nov 21, 2006 #9
    ok so any idea why it is 74 instead of 37?
  11. Nov 21, 2006 #10
    Unfortunately no clue =(
  12. Nov 21, 2006 #11
    can you give me the exact equation that gives 74.
    ive tried so many.. it's so weird.

    so 2.53m/s is in seconds so i multiply it by 60 to make it in minutes.
    so it turns 2.53x60 m/minutes
    now since its in meters i have to divide it by 2pi to make it by revolutions
    this gives me 24... Not 74... :(

    am i looking for r? what did i do wrong?
  13. Nov 21, 2006 #12
    What is 2.53 it is velocity, IE m/s

    why would 2.53/2pi give you revs/s? It does not. However 2.53/r gives you rads/s which divided by 2pi gives you revs/s

    The exact equation as stated before is!

    2.53/r/2pi*60 to get 74
  14. Nov 21, 2006 #13
    (2.53 x 60) / (2 * pi * .65) = 37.1688006

    I think you're forgetting r.
  15. Nov 21, 2006 #14
    But r is .65/2 is it not?
  16. Nov 21, 2006 #15
    "0.65m diamter", you are correct.
    Last edited: Nov 21, 2006
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