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Homework Help: Serious problem with centripetal force

  1. Dec 3, 2013 #1
    1. The problem statement, all variables and given/known data

    The soluble problem We have a disc, horisontal, which is rotating 0.5 revolutions per second. A block is placed on this disc with the mass 0.6 kg and 0.2 meters from the shaft. First, I convert 0.5 rates per second to T = 2 seconds. w = 2∏/2 --> w = ∏ --> m = 0.6 kg --> r = 0.2 meters

    Fc = (0.6)(0.2)(∏)^2 --> Fc = 1.184352528 N --> Fc = 1.2 N

    This is correct according to facit. So easy, but I just wanted to show how different this is in comparism to the coming one.

    The insoluble problem A carousel, where the distance between centre and attachment to chain is 0.8 meters, the chain is 1.0 meter long and weight in the chain 1.0 kg. T = 4.0 s. Now, centripetal forces is asked, and I do the same as above: w = 2∏/4 --> w = ∏/2

    Fc = (1.0)(0.8)(∏/2)^2 --> Fc = 1.97392088 N

    This is according wrong according to:


    Even though the latter is hanging in a chain, the horizontal force should be the same; but resulting different. In Walter Fendt, they have found out that the radius is 1.06 m (0.8 + 0.26). The angle is unknown, but they have through 0.26 m counted this: sin a = 0.26/1.00 --> a = 15.

    2. Relevant equations

    Fc = mrw^2

    w = 2∏/T

    What have I missed?
    Last edited: Dec 3, 2013
  2. jcsd
  3. Dec 3, 2013 #2


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    What does 'rates per second' mean? Is it supposed to be 'radians per second' or 'revolutions per second'?
  4. Dec 3, 2013 #3

    It was supposed to mean "revolutions per seconds". In fact, I have always believed that RPM on the motorcycle and cars meant rate per minute, but I did just learn that it means revolutions per minute. English is my second language.
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