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2 gravitation questions:

  1. Oct 31, 2004 #1
    1. How can i prove that Eg = mgh

    2. What is the total energy needed to place a 2.0x10^3-kg satellite into circular Earth orbit at an altitude of 5.0x10^2 km?

    For number 2, the answer is apparently 6.7x10^10J. However, shouldn't Total Energy = 1/2 (Eg)? If that is the case, the answer would be 5.8x10^10J.

    (P.S. in the question after this, i must calculate the additional energy required to allow the object to escape Earth's orbit, and the answer is 5.8x10^10J).
  2. jcsd
  3. Oct 31, 2004 #2


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    1. Remember that the work done by a conservative force such as gravity is defined as the negative of the change in the potential energy. You can easily prove it from the definition of work as force X distance, in the simplest scenario in which you consider a constant gravitational force applied to an object that happens to be moving vertically upward in a straight line.
  4. Oct 31, 2004 #3
    For the energy to put the satellite in orbit, how are calculating it? I get 9.36 X 10^9 J. (I might have made a mistake...) Also, what is "Eg"? I'm not familiar with that term.
  5. Nov 1, 2004 #4


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    Don't forget that most of the energy required to put a satellite into orbit goes into kinetic energy! Merely lifting it 500 km won't get you very far.
  6. Nov 1, 2004 #5
    1. Eg is gravitational potential energy

    2. I learned the following equations:

    Etotal = -(GMm)/r
  7. Nov 1, 2004 #6
    1. Eg is gravitational potential energy

    2. I learned the following equations:

    Etotal = -0.5(GMm)/r
    Ek = -Eg

    Ek is kinetic energy, G is the grav. constant, m is the mass of the satellite, is the mass of the earth and r is the distance of the satellite from the centre of the Earth.
  8. Nov 1, 2004 #7
    OK, I got 6.77 X 10^10 J by adding the KE ((G*m1*m2)/(2*d)) to the PE. I got the PE (9.36 X 10^9 J) by integrating the force of gravity (G*m1*m2/d^2) with respect to the distance from the Earth's surface to the height of the orbit.
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