Rocket Problem: Determining the amount of fuel burned

[Ockham]

The set up:
A 20,000kg rocket has a motor that generates a thrust of 3x10^5 N.


The problem:
a)What is it's initial Acceleration?

b)At an altitude of 5,000m the rockets acceleration has increased to 6 m/s^2, what mass of fuel has it burned?

Relevant equations:

F = ma

The attempt at a solution:

I found the initial Acceleration to be 5.2 by taking the difference of the thrust and the rockets weight and dividing my the initial mass.

Part two is the one I can't figure out...I don't think the height of the rocket has anything to do with the solution does it?

 

[haruspex]

I agree, the altitude is unnecessary. In fact, it's a bit odd because you could calculate the altitude, and I'll bet it's not 5km exactly.

 

[Ockham]

How do I determine the amount of fuel burned?

 

[haruspex]

Recall how you calculated the initial acceleration. Why does the acceleration increase? What, in your calculation, will have remained constant?

 

[Ockham]

OH...thanks. I get it now. The Thrust is constant the only thing that changes is the mass. Which I can solve for.


Thank you!
Ockham