Answer to Question #81576 in Physics for Sayem

Question

In rocket physics why we write, (dm)×v=F×dt
Again,F=(v×(dm\dt))-mg
why we subtracted mg
Please tell me,
According to newtons 3rd law if mg is working downward and F is working upward if I give g upwards (like the cases of lift) what will happen?

Is it true a rocket can fly if the rate of gas Force is equal to rockets weight(mg)? If it is true then we are also at least able to fly (how F=mg our weight mg downwards and earth is giving F upwards)
Please kindly Ans Thank you

Answer 1

1) The net force is the difference between thrust and the weight of the rocket:
F_net=v (dm/dt)-mg
2) You cannot assume that the acceleration due to the gravity is directed upwards.
3) It is true that a rocket can fly if the rate of gas Force is equal to rockets weight:
F_net=mg-mg=0.