Magnitude of thrust of rocket (q50)

[jack1234]

For this question:
http://tinyurl.com/ynkshx

I just use ma=(3000-800)*20 =44kN to get the answer, and the answer is correct.
and haven't make use of
thrust=exhause velocity * dm/dt
or
change in velocity=exhause velocity*ln|initial mass/final mass|

So, just want to confirm am I solved this problem with correct approach, if not, what is the correct solution?

 

[Dick]

That's right. It's not really much of a rocket problem, is it?

 

[ogb p]

Your approach to solving this problem is correct. Using the equation ma = (3000-800)*20 = 44kN is a valid method for calculating the magnitude of thrust of the rocket. However, if you want to use the equations thrust = exhaust velocity * dm/dt or change in velocity = exhaust velocity * ln|initial mass/final mass|, you will need to have additional information such as the exhaust velocity and the change in mass of the rocket over time. Without this information, it is not possible to use these equations to solve for the magnitude of thrust.