1. Not finding help here? Sign up for a free 30min 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!

How long to fire a small rocket. Impulse problem.

  1. Oct 19, 2008 #1
    1. Small rockets are sued to make small adjustments in the speed of satellites. One such rocket has a thrust of 59N. If it is fired to change the velocity of a(n) 74000kg space craft by 54cm/s, howl ong should it be fired? Answer in units of s.

    F=deltaP, delta t;
    F=mVf-mVi (impulse equals change in momentum);
    F=m(delta v,delta t)

    So my derivation (if that's a word) is

    (m*delta v)/ delta t= net force = mVf-mVi. From there I get the formula in my attempt to solve the problem.

    3. I made delta t=mV/F= 74000kg(.054m/s)/ 59N.... I think my problem lies in that I don't have the initial velocity... but I'm not sure how to get it.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Oct 19, 2008 #2
    Oh in 2. It should be F= m(delta v/delta t)
  4. Oct 19, 2008 #3


    User Avatar
    Homework Helper

    They want the change in velocity to be 54cm/s

    and since Ft=mv-mu=m(v-u)

    What should v-u be equal to?
  5. Oct 19, 2008 #4
    Dang, I don't know what you mean by u in Ft=mv-mu=m(v-u). The change in velocity is definitely something I missed. THank you for pointing it out.
  6. Oct 19, 2008 #5
    Oh I got it. Thank you. Delta t= m(Vf-Vi)/F.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?