1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Energy Considerations When A Satellite Changes Orbit

  1. Jan 28, 2013 #1
    1. The problem statement, all variables and given/known data
    A 1 034-kg satellite orbits the Earth at a constant altitude of 102-km.

    (a) How much energy must be added to the system to move the satellite into a circular orbit with altitude 191 km?

    (b) What is the change in the system's kinetic energy?

    (c) What is the change in the system's potential energy?

    2. Relevant equations
    For (a): [itex]\Delta E = \frac{GM_Em}{2}(\frac{1}{r_i}-\frac{1}{r_f}[/itex]

    3. The attempt at a solution

    I have futilely attempted problem many vexing times; I am beginning to regard this as the bane of my existence.

    For (a): [itex]r_i = R_E + 102000[/itex] and [itex]r_f = R_E + 191000[/itex], right?

    [itex]\Delta E = \frac{GM_E \cdot 1034}{2}(\large \frac{1}{R_E + 102000}-\frac{1}{R_E + 191000}) = 890~MJ[/itex]

    For (b) Supposedly there is some relation between gravitational energy and kinetic. I was just going to find the tangential velocity with respect to each orbit, and use those to find the change in kinetic energy. Although, seeing as this direct relationship will provide desired accuracy, what exactly is this relationship?
  2. jcsd
  3. Jan 28, 2013 #2


    User Avatar

    Staff: Mentor

    Your Relevant Equation deals with the kinetic energy of the system. For part (a) you'll need to account for the change in potential energy, too. What's the expression for gravitational potential energy?

    You might want to take note that objects in higher circular orbits have lower orbital velocities; So check the order in which you specify the radius reciprocals in your KE formula.

    Hint: An expression for the Total Specific Mechanical Energy of an orbit is:
    $$\xi = \frac{v^2}{2} - \frac{\mu_E}{r}$$
    where ##\mu_E = G M_E##

    It combines the Kinetic and Potential energy contributions. The result is in J/kg, the energy per kg for the object in orbit. Multiply by the mass of the object in orbit to obtain the total energy in Joules.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook