A small 120 kg rocket is fired vertically, proppelled by an engine with a thrust of 6500N. THe engine ejects gas at the rate of 4.5kg/s. Calculate the change in velocity of gases relative to the engine. You must use momentum as a last step to answer this question.

I am confused on how to answer this question because I am not sure if I should subtract 120-4.5 for mass. In addition, I am not sure if there is any Force of friction. THis is what I have done so far.

F delta T= m delta V
6500 (1)= 115.5 (Vf - 0)
Vf= 56 m/s

I would start with the concept that momentum is conserved, so



In other words the momentumn of the gas (g) goint out the back equals the momentumn of the rocket (r) moving forward. You also know



Put these together to get:



Now you should have it.