Question

rocket propulsion:
in deriving the equation for thrust, is the relative velocity of exhaust gasses and rocket:
1) -ver=-v-(v+dv) OR 2) -ver=v-(v+dv), while assuming the exhaust velocity is lower than the speed of the rocket. 2) makes more sense to me but for some odd reason, the textbook goes with 1)

Answer 1

Just mentioning this: I brought up Newton's Universal Law of Gravitation in the other question, but F = gM/^2 was what we were looking for. I'll be here in a sec.

Answer 2

but i though f was Gmm/r^2

Answer 3

Yup, totally right, it is, just referring to the special case where only one force is acting on a constant mass, and the force is gravity. Just confusion, my bad. \[F_g = mg = \frac {GmM}{R^{2}} = \frac {gM}{R^{2}}\]Alright, now this.

Answer 4

Lol, I'm sleepy, the above was written wrong..but from the other post, you know what I mean.

Answer 5

Yeah, I know next to nothing about rocket propulsion, I can't help. I think I'm going to go fall on a pillow now..

Answer 6

lol. thx anyway. good night. got uni tomorrow and its 12:30 here lol

Answer 7

Same thing, except none of my Physics classes have me needing to know rocket propulsion, so I can call myself lucky, haha

Answer 8

are u in engineering too?

Answer 9

I'm a Physics major-which is why it's all the more terrible I know nothing about this. I'm just now taking E&M along with Ordinary Diff Eq.

Answer 10

In Physics I (Classical Mechanics) we never really needed to deal with stuff like that.

Answer 11

nice!

Answer 12

downward direction has been taken as negative. Hence the -v. Note you have to find relative velocity (direction is important)