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

Question on gravitation

  1. Mar 9, 2009 #1
    Hi everyone. I know about the equations of gravitation but something is still bothering me.

    The equation for total energy of a satellite in an orbit is -GMm/2r

    However, when the satellite suddenly lose control and heads to the Earth, why does the total energy in the system change according to r? Excluding air resistance of course... I always thought in a closed system, total energy remains constant? Im quite blur and my teacher just asked us to memorize the eqns.

  2. jcsd
  3. Mar 9, 2009 #2


    User Avatar

    I think the potential energy of the satellite is converted to its kinetic energy when it starts falling towards the earth ... its potential gets lesser and lesser as r decreases i.e. as it falls towards the earth ...

    experts, please correct me if i am wrong, I am not very sure abt my answer :)
  4. Mar 9, 2009 #3
    We are talking about total energy here.

    My best guess (another theory!haha) is that the Earth AND the Object is the system. But it still does not make sense as both objects are moving closer to each other (although distance moved by Earth is negligible), both are losing total energy..... Why is there energy loss? Im still confused
  5. Mar 9, 2009 #4

    Doc Al

    User Avatar

    Staff: Mentor

    That's the total energy (kinetic plus potential) of a satellite in a circular orbit, where r is the radius of that orbit.

    It doesn't.
    Correct--the total energy remains constant.
  6. Mar 9, 2009 #5
    So when it changes orbit, energy levels will follow the eqn -GMm/2r. When r is decreased (go to a lower orbit), -GMm/2r becomes more negative. So there is an energy difference. But where does the difference go?
  7. Mar 9, 2009 #6
    Just another question (unrelated to this qn), when gravitational potential is 0 is gravitational field strength zero. I have a hunch that it isnt and cannot explain why or think up another example to support it. Anyone?
  8. Mar 9, 2009 #7
    If you neglect friction why would the satellite change its orbit?

    What do you mean by "lose control"?
    If the orbit is changed on purpose then there is a force acting on the satellite (produced by the rocket engines).
    Or the satellite may collide with something else - again external force.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook