# Rocket/Variable mass

## Homework Statement

A rocket traveling 1950 m/s away from the Earth at an altitude of 6600 km fires its rockets, which eject gas at a speed of 1300 m/s (relative to the rocket).
If the mass of the rocket at this moment is 2.25×104 kg and an acceleration of 1.4 m/s^2 is desired, at what rate must the gases be ejected?

## Homework Equations

M*dv/dt = $$\sum$$ F(ext) + v(rel)*dM/dt

## The Attempt at a Solution

I thought i could just solve for dM/dt in the above equation and plug in the numbers to get the answer but that didnt work. I think my problem is with finding all the external forces, which i think is just mg. I just need a hint to start.

*bump*

ehild
Homework Helper
The external force at altitude of 6600 km is not mg.

ehild

The external force at altitude of 6600 km is not mg.

ehild

so when I find g is that all i have to find? ->for the external force?

Last edited:
ehild
Homework Helper
The external force is gravity that depends on the altitude.
F=GmM/r2, and r is the distance from the centre of Earth, r=radius or Earth + altitude

ehild

alright thanks