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Homework Help: 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


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    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.
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