Force of Gravity on a Rocket

[KindofSlow]

Hello,
I’m working on drawing a free body diagram for a rocket.

The book says that the force of gravity is M*g where M is the mass of the rocket.

Since dM is the rate of expulsion of mass of fuel, I feel like the force of gravity should be
(M-dM)*g

Hopefully I am missing something obvious and simple regarding why force of gravity is M*g instead of (M-dM)*g.

As always, all insights are appreciated.

Thank you

 

[DrPhil]

Hello,
I’m working on drawing a free body diagram for a rocket.

The book says that the force of gravity is M*g where M is the mass of the rocket.

Since dM is the rate of expulsion of mass of fuel, I feel like the force of gravity should be
(M-dM)*g

Hopefully I am missing something obvious and simple regarding why force of gravity is M*g instead of (M-dM)*g.

As always, all insights are appreciated.

Thank you

F = M g
BUT neither M nor g is constant. M is changing with time because of expulsion of fuel, and g is changing with altitude because the distance from the center of the earth is changing.

 

[KindofSlow]

Wow, I thought both were constant - how wrong I was. Thank you very much.

 

[Bob Brown MSEE]

F = M g
BUT neither M nor g is constant. M is changing with time because of expulsion of fuel, and g is changing with altitude because the distance from the center of the earth is changing.

Newton said...
F = (d/dt)p where p = momentum