Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Need help with problem

  1. Apr 6, 2004 #1
    I have been asked to review a junior physicist's calculations regarding an artificial satellite travelling 230 km above the earths surface where acceleration due to gravity is about 9.0m/s2. He calculated a velocity of 30000 m/s. And the radius of the earth was measured at about 6370km. Does his speed seem right? I don't even know if I have enough information to figure this out.
  2. jcsd
  3. Apr 6, 2004 #2

    30,000 m/s is WAY to fast for anything in earth orbit. That's 30km/s, or about 20miles/sec, which is 72,000 miles/hr, gets you around the earth every 20 minutes. Can't happen.
  4. Apr 7, 2004 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    A quick calculation on the back of an envelope (literally) gives g at that distance to be about 9.2 m/s^2 (so 9 m/s^2 is OK if you're not fussy), and this gives a speed of about 7700 m/s for a circular orbit. It is entirely possible my math is off, especially when I'm being distracted by a yummy blueberry muffin.

    Maybe your "junior physicist" was doing a highly eccentric elliptical orbit, which if true, is highly eccentric in itself. :)

  5. Apr 7, 2004 #4


    User Avatar
    Homework Helper

    I calculated ~11 km/s. I used Etotal = 0 => KE = -Ugrav. That's a parabolic orbit, though, isn't it. Dammit! Take one semester off and you pay for it the whole time. I'll get back to this.

    OK, now I remember from the virial (sp?) theorem (or something) that KE = -(1/2)Ugrav for a circular orbit. Then that would lead to the velocity I was smoking for the parabolic orbit divided by a factor of √2. This gives ~11√2 km/s ~ 7.8 km/s. I would like to take this opportunity to agree with ZapperZ.
    Last edited: Apr 7, 2004
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook