1. Dec 6, 2013

wr8899

1. The problem statement, all variables and given/known data
Consider a rocket in space that ejects burned fuel at a speed of v_ex= 2.0 km/s with respect to the rocket. The rocket burns 10 % of its mass in 340 s (assume the burn rate is constant).

(a) What is the speed v of the rocket after a burn time of 170.0 s? (suppose that the rocket starts at rest; and enter your answer in m/s) ?
(b) What is the instantaneous acceleration of the rocket at time 170.0 s after the start of the engines?(in m/s^2 ) ?

2. Relevant equations

v_f = -v_ex*ln(m_final/m_initial) - g*burn time

3. The attempt at a solution

I plugged in the data, getting -2000*ln(0.95) - g*170, but got the velocity wrong. I have no idea where I went wrong, and I am stuck here, along with finding the acceleration. Can someone please help me get unstuck here. Thanks and sorry to be a bother.

2. Dec 6, 2013

BOYLANATOR

EDIT : What I wrote here was wrong

Last edited: Dec 6, 2013
3. Dec 6, 2013

BOYLANATOR

What is g in space?

4. Dec 6, 2013

Staff: Mentor

In addition to BOYLANATOR's post, where does the 0.95 come from?

5. Dec 6, 2013

wr8899

0.95 comes from the final mass over initial mass, since after 170.0 seconds, you have 95% of the original mass left (other 5% mass was burned already)

6. Dec 6, 2013

Staff: Mentor

Oh 10% in 340s... okay, I missed that part. Where is the point in the second 170 seconds?

7. Dec 6, 2013

wr8899

what you mean?

8. Dec 6, 2013

wr8899

Because, 10% of mass burned every 340 seconds, so 5% of mass burned every 170 seconds, therefore I got the 0.95

9. Dec 7, 2013

Staff: Mentor

I agree, I didn't read the problem statement careful enough.
I was just wondering why the problem statement gives the fuel burnt in 340 seconds and then just asks for the rocket after 170 seconds.

10. Dec 7, 2013

BOYLANATOR

Did you manage to solve the problem?