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: Velocity of the rocket with changing mass

  1. Nov 16, 2006 #1
    I want to do something about the rocket. I have done the zero gravity point, escape velocity and the velocity of the rocket with constant mass. But I would like to expand it! PLEASE GIVE ME SOME IDEAS!
    It probably is not right, but please read through my notes.
    1) http://i53.photobucket.com/albums/g58/hunghoiyanl6/1.jpg" [Broken]
    2) http://i53.photobucket.com/albums/g58/hunghoiyanl6/2.jpg" [Broken]
    3) http://i53.photobucket.com/albums/g58/hunghoiyanl6/3.jpg" [Broken]
    3.1) http://i53.photobucket.com/albums/g58/hunghoiyanl6/3-1.jpg" [Broken]
    3.2) http://i53.photobucket.com/albums/g58/hunghoiyanl6/3-2.jpg" [Broken]
    3.3) http://i53.photobucket.com/albums/g58/hunghoiyanl6/3-3.jpg" [Broken]

    My physics teacher said that the mass (fuel) is changing only upto a point where the rocket passes the atmosphere. Afterwards, it will be a freefall process. If so, what other sort of ideas can I expand on it. Please give me some comments on this 'presentation', and give me some other recommandations on what else I can do.
    Thank you very much.
    Please contact me via email: annie.hung@tsmail.co.uk

    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Nov 16, 2006 #2


    User Avatar
    Science Advisor
    Homework Helper

    There are numerous sources on the internet for the derivation of the rocket equation. Some of them are good, and some are not. One that I think does a very good job of setting up the equation in terms of conservation of momentum is found here


    I think this approach is far superior to anything that involves F = d(M*V)/dt = V*dM/dt + M*dV/dt. This latter equation is only useful under special circumstances. For example, if fuel were being expelled out of both sides of the rocket perpendicular to its direction of motion dM/dt would be the rate of losing mass, but there would be no change in velocity and no net force.

    I'm not sure the point your teacher was making about the mass changing only up to the point where the rocket passes the atmosphere. While in the atmosphere there is an effect related to the pressure of the exhausted gas helping to push the rocket. This effect is ignored in the derivation I posted, and it is not needed for the roicket to have thrust. Ships landed on the moon where there is no atmosphere and lifted off again. A rocket does not need atmosphere to accelerate.
    Last edited by a moderator: May 2, 2017
  4. Nov 30, 2006 #3


    User Avatar
    Science Advisor
    Gold Member

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