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: Rocket Propulsion with gravity

  1. Dec 29, 2014 #1
    1. The problem statement, all variables and given/known data

    Earlier we considered a rocket fired in outer space where there is no air resistance and where gravity is negligible. Suppose instead that the rocket is accelerating vertically upward from rest on the earth's surface. Continue to ignore air resistance and consider only that part of the motion where the altitude of the rocket is small so that g may be assumed to be constant.

    a) How is eq. (8.37) modified in the presence of the gravity force?

    b) Derive an expression for the acceleration a of the rocket, analogous to Eq. (8.39)

    2. Relevant equations

    eq (8.37): m(dv/dt) = - vex(dm/dt)
    m= mass of rocket vex= velocity of rocket exhaust v= velocity of rocket

    eq (8.39): a= (dv/dt) = (-vex/m)(dm/dt)

    3. The attempt at a solution

    For part (a) I believe I need to factor in the impulse of gravity, Jg= mg(dt), but I don't know how. Then I would use my answer to part (a) to find an equation for acceleration.

    Thank you
  2. jcsd
  3. Dec 29, 2014 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    You need to include the effects of gravity, but not as you've stated. Try looking at units for 8.37 and rethink what you "believe."
  4. Dec 30, 2014 #3
    i believe that the trick lies in realising the fact that the net external force acting on the rocket is now, no longer the reaction due to its own engines, but also gravity acts on it. so we need to modify the LHS first
  5. Jan 2, 2015 #4
    sorry for the late response!

    so the units of the equation are kg*(m/s^2), which is the same as a newton. So do I just add the force of gravity?

    m (dv/dt)+mg=-v(dm/dt)
  6. Jan 2, 2015 #5


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    That'd be my first approach.
  7. Jan 2, 2015 #6
    okay, and then I just solve for dv/dt to get acceleration for part (b). thank you!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted