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Homework Help: Calculating radius in circular motion without frequency or force?

  1. Oct 13, 2012 #1
    1. The problem statement, all variables and given/known data

    Scientists want to place a 4400.0 kg satellite in orbit around Mars. They plan to have the satellite orbit at a speed of 2645.0 m/s in a perfectly circular orbit. Here is some information that may help solve this problem:

    mmars = 6.4191 x 1023 kg
    rmars = 3.397 x 106 m
    G = 6.67428 x 10-11 N-m2/kg2

    What radius should the satellite move at in its orbit? (Measured from the center of Mars.)

    2. Relevant equations

    v = 2∏r/T

    3. The attempt at a solution

    This question makes no sense. Is there any way to calculate the period of the motion using a different equation? Otherwise, there are two unknowns, irrespective of which equation is used. Even if I attempt to use two different equations that are solved for T and equate them, there is always an unknown such as frequency or acceleration present along with radius.
  2. jcsd
  3. Oct 13, 2012 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    Welcome to PF,

    If the object is moving in a *circle*, then just as a kinematic requirement, what kind of force must be acting on it? What physical force in this problem is acting as that force (that keeps it moving in a circle)?
  4. Oct 13, 2012 #3
    Ahh, universal law of gravitation. Okay, I think I might have a handle on this. Thank you.
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