# Homework Help: How long to fire a small rocket. Impulse problem.

1. Oct 19, 2008

### fanie1031

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.

2.
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. Oct 19, 2008

### fanie1031

Oh in 2. It should be F= m(delta v/delta t)

3. Oct 19, 2008

### rock.freak667

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?

4. Oct 19, 2008

### fanie1031

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.

5. Oct 19, 2008

### fanie1031

Oh I got it. Thank you. Delta t= m(Vf-Vi)/F.