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!

Force on rocket

  1. Oct 31, 2012 #1
    1. The problem statement, all variables and given/known data
    A rocket with mass 60 x 103 kg (25 % of it is "empty mass"). The rocket burns the fuel at rate 250 kg/s and expel the gas at relative speed 1.6 km/s. Find the force experienced by the rocket and the time until the force equals to zero

    2. Relevant equations
    Δp = F.Δt
    p = mv
    W = mg

    3. The attempt at a solution
    I am not sure what is the meaning of "empty mass". Is it the mass of rocket without fuel?

    This is my attempt:
    Force experienced by rocket
    = Δp / Δt
    = Δ(mv) / Δt
    = v . Δm / Δt
    = 1.6 x 103 x 250
    = 4 x 105 N

    Is that correct?

    For the time until the force ceases:
    Assume empty mass means the mass of rocket without fuel, so the mass of fuel = 75 % of 60 x 103 kg = 45,000 kg

    Time = 45,000 / 250 = 180 s ---> this means that the rocket accelerates for only 3 minutes???

  2. jcsd
  3. Oct 31, 2012 #2


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    That all looks right to me, except that it's not clear whether "the force experienced by the rocket" is supposed to be just that due to the engine or is to include gravity.
  4. Nov 2, 2012 #3
    If includes weight, it should be 4 x 105 N - weight, correct?
  5. Nov 2, 2012 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

  6. Nov 3, 2012 #5
    OK thanks for your help :)
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Force on rocket
  1. Rocket Thrust Force (Replies: 14)

  2. Net force on a rocket (Replies: 9)