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Homework Help: What would the tension in the cable be?

  1. Feb 23, 2009 #1
    1. 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?

    2. how do i find the tension when there seems to be missing info. (to me)

    3. First i tried finding velocity to put that into the tension equation. v = 3.85 x 10^8 / 27.3. What i used for an equation for tension. T = 7.35 x 10^22 (1.41 x 10^7 )^2 all over 3.85 x 10^8

    please help, i have no clue what to do.
  2. jcsd
  3. Feb 23, 2009 #2


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    Homework Helper

    Welcome to PF.
    What is that? Radius in m / days?

    Surely you know what the circumference of a circle is.
  4. Feb 23, 2009 #3


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    Let's agree that the tension will be equal to the centripetal force (the force needed to keep the moon in orbit). The centripetal force is given by

    [tex] F = m \frac{v^2}{r} [/tex]

    What's going to make you think a little bit is your units. Note that your distances are in meters and the mass in kilograms. You'll need to convert from days to something else (what do you think is appropriate? ) in order to get to newtons.

    Another pertinent question:

    How far around is the moon's orbit?

    Those comments should get you started...
    Last edited: Feb 23, 2009
  5. Feb 23, 2009 #4
    ive never had physics before, and no one has every helped me with it. I dont know anything about physics
  6. Feb 23, 2009 #5
    moons orbit is 27.3 days. the problem doesnt give much info. im completely lost when it comes to this stuff.
  7. Feb 23, 2009 #6


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    Well, You're going to want to express the final answer in newtons. Those a SI units, meaning they use meters, kilograms and seconds. So to start with you will need to convert 27.3 days into so many seconds. That will tell you how long it takes the moon to go once around. Then you have to figure out how far the moon travels in that time (once around). Since it's assumed to be a circular orbit (to make the problem easier). That should be easy (what's the formula for the circumference of a circle?). Then you can compute the velocity of the moon and after that it's "plug and chug" since I gave you the equation.

    By the way if the only thing you study is stuff you've studied before, you'll never learn anything new!
  8. Feb 23, 2009 #7
    well thanks for the input, but i still do not understand it... obviously i shouldnt be in college.
    theres soo many different formulas for just one question. i dont know where to begin. :\
  9. Feb 24, 2009 #8


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    Physics is something that requires time and hard work --much hard work to understand. Most people your age do not realize that confusion is the first step towards learning.
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