How Do You Calculate the Thrust of a Rocket at Lift-Off?

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SUMMARY

The thrust of a rocket at lift-off can be calculated using the formula Thrust = ejection velocity × mass flow rate. In this discussion, the ejection velocity is 2.5 x 103 m/s and the mass flow rate is 7.4 kg/s, leading to a thrust of 18,500 N. The interval between ignition and lift-off occurs because the rocket must build sufficient thrust to overcome gravitational forces before ascending.

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Kodomut
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Need help with an ancient A lvl question. Just can't 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. =(
 
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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.
 
BTW: a force is a change of momentum (the derivative)
 

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