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: Object in elliptical orbit around another object

  1. Apr 25, 2010 #1
    1. The problem statement, all variables and given/known data

    In a science fiction story, humans have built a large spherical spaceship for an interstellar journey of many generations; they have christened their ship Big Bird.

    The mass of Big Bird is 7.496×1011 kg (the mass of an aircraft carrier is about 10^8 kg). Most of Big Bird's journey takes place at constant speed through the vacuum of interstellar space.

    Big Bird is accompanied on its journey by a small satellite which is held in orbit about Big Bird solely by Big Bird's gravitational attraction. The satellite is called Little Bird and has a mass of 1000 kg.

    Little Bird is in an elliptical orbit about Big Bird; the distance of closest approach is 5 km and the greatest separation distance is 40 km.

    Find the difference in total mechanical energy for the system; i.e. mechanical energy at maximum separation minus mechanical energy at closest approach.

    2. Relevant equations

    E = K + U

    K = 1/2mv2

    U = -Gmm/r

    3. The attempt at a solution

    I am a little confused on this problem. i only seem to know the equation for V for a circular orbit so i am not sure how to go about this problem.

    would i maybe do this mL = mass of little bird and mB = mass of big bird

    1/2mLv2 + -GmBmL / r1 - [1/2mv2 + -GmBmL / r2 ]

    r1 = 40,000 m
    r2 = 5,000 m

    Thanks for any suggestions on what to do cause i am completely lost on how to go about this problem.
  2. jcsd
  3. Apr 25, 2010 #2


    User Avatar
    Homework Helper

    Mechanical energy should be conserved because no forces are acting on the system, shouldn't it?
  4. Apr 25, 2010 #3
    Yes it should be conserved.

    Am i even on the right track to find the difference in total mechanical energy for the system?

    I am a little confused on what equations to use i guess.

    I thought i should use:

    K1 + U1 - (K + U)

    Where K1 and U1 are:

    1/2mLv2 and GmBmL / r

    r = 40,000 m

    K and U are:

    1/2mLv2 and GmBmL / r

    r = 5000 m

    and v can be found using GmB / r

    r = 40,000 for the K1 and r = 5000 for K

    Am i on the right track with these equations and using the right "r" for each equation or am i not even close?

    Thank you
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook