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: Find velocity of rocket that obtains more gas from gas cloud

  1. Oct 12, 2016 #1
    1. The problem statement, all variables and given/known data
    To solve this problem you will need to construct a differential equation. A picture of the situation will help. Ignore all gravitational forces. A Rocket ship of structural mass M and fuel mass m, begins at rest relative to a gas cloud. The ship burns fuel at the rate ω which is exhausted with speed v relative to the ship. The ship obtains pure fuel from the gas cloud at the rate 2ω.
    Find the velocity of the ship as a function of time.

    2. Relevant equations

    3. The attempt at a solution
    All I can think of is using momentum... P initial = 0, but I do not now how to construct the equation for P final.
  2. jcsd
  3. Oct 12, 2016 #2


    User Avatar

    Staff: Mentor

    You need to make a better attempt. Perhaps do some research on the rocket equation and its derivation first so that you can at least prepare a credible list of relevant equations.
  4. Oct 12, 2016 #3
    I am having a hard time describing this situation, and getting proper cancelations\simplifications.
    To start we need to find an equation describing the momentum... I think. This would look like.
    P = P_ship + P_exhausted fuel + P_gas cloud = 0
    P_ship = (M_total)(v+dv)
    P_exhausted = (v_ship - v_fuel)(dm_exhausted)
    P_gas cloud = (v_ship)dm_intake
    we also know that the fuel m = m_o+2ω-ω=m_o+ω
    Using this information I am unable to get a differential equation that has an meaning.
  5. Oct 12, 2016 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    You need to be more systematic in the analysis, using differentials everywhere that is appropriate.
    In time dt, let the ship change speed from u to u+du.
    What mass of fuel did it burn in that time? What was the exhaust speed of that fuel relative to the gas cloud? What momentum did it take with it?
    What mass of new fuel did it take on?
    What is the new momentum of the ship plus its load of fuel? What was the momentum change in time dt?
    What equation can you now write?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted