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    hicheong's Avatar
    hicheong Posts: 17, Reputation: 1
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    #1

    Oct 11, 2010, 05:05 PM
    uniform circular motion
    Suppose the moon were held in its orbit not by gravity but by the tension in a massless cable. You are given that the period of the moon's orbit is T = 27.3 days, the mean distance from the earth to the moon is R = 3.85 x 108 m, and the mass of the moon is M = 7.35 x 1022 kg.

    What would the tension in the cable be?

    HELP: Remember that the moon is going around the earth in a circle. Therefore, there is centripetal acceleration. What is this acceleration?
    HELP: Newton's 2nd Law says that if there is an acceleration, then there must be a force. What is the force required to provide the centripetal acceleration?


    So, I think I should be using T=m*acceleration, acceleration=w^2*Radius where w=angular velocity, and w=2pf, but I don't know how to get w.
    ebaines's Avatar
    ebaines Posts: 12,131, Reputation: 1307
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    #2

    Oct 12, 2010, 06:08 AM

    You're on the right track. By "w" I assume you mean rotational velocity, usually shown as omega, ω. You can calculate ω if you know the period, T:

    ω = 2π/T

    To get the units all into proper SI form you will need to convert the period of 27.3 days to seconds, so that you get ω in units of . Then the tension can be calculated as you propose:

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