# Thrust of a rocket at lift-off

1. Jun 2, 2004

### Kodomut

Need help with an ancient A lvl question. Just cant visualise the whole process even though I drew out the diagram

A space research Rocket stands vertically on its launching-pad. Prior to ignition, gas is ejected from the rocker at a speed of 2.5x10^3 ms^-1 relative to the rocket, and fuel is consumed at a constand rate of 7.4kgs^-1. Find the thrust of the rocket and hence explain why there is an interval between ignition and lift-off

g = 10ms-2

I have totally no idea how to find the thrust. =(

2. Jun 2, 2004

### remcook

there's not much one can do with two numbers. Try some dimensional analysis to see if it fits.

But regardless...you can already have an idea about the second question. Take a guess.

3. Jun 2, 2004

### remcook

BTW: a force is a change of momentum (the derivative)