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Space physics, is this correct?

  1. Apr 11, 2013 #1
    1. The problem statement, all variables and given/known data
    A satellite in orbit above Earths equator is traveling at an orbital speed of 7.45 km/s.

    a) Determine the altitude of the satellite
    b) Determine the satellites period

    2. Relevant equations
    v = sqrt(Gm/r)

    3. The attempt at a solution
    7.45 * 1000 = 7450 m/s

    I plugged in the values to that equation and got a radius of 7.1 * 10^6 m.

    Now, I'm not sure how to find the altitude, but shouldnt it be the value I found (7.1*10^6m) minus earths radius? This gives me the value of 700 km

    For b, I'm not sure how to find the period, I tried T = 2pir / V where r was the radius I found and got 5988 seconds. Is this correct?
  2. jcsd
  3. Apr 11, 2013 #2


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    All of those are approximately correct. You are doing the right things. I get a distance closer to 800km. If you want a closer check you'll need to show all the numbers you are using.
  4. Apr 11, 2013 #3
    okay, thank you for your help. I am doing things this way:

    7450 = sqrt((6.67*10^-11 * 5.9*10^24/(r)

    r = 700 km
  5. Apr 11, 2013 #4
    EDIT: Didn't see this had already been solved; ignore this post.
  6. Apr 11, 2013 #5


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    Nah, that doesn't give you r=700 km. That gives you the radius of the orbit. You have to subtract the radius of earth to get anything close to 700 km. What did you get for that r, and then what did you use for the radius of earth? As I already said, I think you are doing everything correctly except for maybe rounding off and exact numbers. I really wouldn't worry about it.
    Last edited: Apr 11, 2013
  7. Apr 11, 2013 #6
    Ah yes, I was rounding the mass of the Earth too much. I get about 810 km now after using a different value of the mass. Thank you
  8. Apr 11, 2013 #7


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    Right. You were doing everything correctly. You should pay attention to round-off etc. Very welcome.
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