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 Thrust

  1. May 13, 2010 #1


    User Avatar

    1. The problem statement, all variables and given/known data

    Derive from Newton II the thrust (the force produced by the rocket exhaust) achieved by a rocket motor which ejects material with constant velocity u and mass ejection rate a.

    2. Relevant equations

    3. The attempt at a solution

    Not sure...I know how to derive vf = vi + u ln(mi/mf) but thats not what its asking for..

    So my attempt

    F = mdv/dt

    then using what we have for m i come to

    F = a(vf-vi)/ln(M/m-at)

    is this right?
  2. jcsd
  3. May 13, 2010 #2

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    Newton's second and third laws of motion apply. Rather than F = ma, write the second law as:

    [tex]\vec{F} = \frac{d\vec{p}}{dt}[/tex]

    The third law for an object on which no external forces act can be written:

    [tex]\sum \vec{F_i} = \sum {\frac{d\vec{p_i}}{dt} = 0[/tex]

    which in this case reduces to:

    [tex]\frac{d\vec{p_{rocket}}}{dt} = - \frac{d\vec{p_{gas}}}{dt}[/tex]

    For the gases fired out the end of the rocket, the velocity relative to the rocket is constant. What gives it a non-zero time rate of change of momentum?

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