can someone confirm this idea on momentum (1 Viewer)

[wolf7]

momentum is the

mass x veloctiy of the rocket = mass x the velocity of exhast gas

so that mean, the law of converation of momentum

so the momentem before = the momentum after

another theory

momentum calculation and rockets

F - mg = ma

as you can f(thrust) is the rocket going up and mg(the force on it in other words negative) is going down on the rocket. when f increase, the mass decrease so would that mean f is going to be greater than mg

can someone confirm that

 

[wolf7]

just need someone to confrim this problem

 

[gman03]

I think the momentum is

mass of rocket x veloctiy of the rocket = mass gas x the velocity of exhast gas + new mass of rocket x new veloctiy of the rocket


The other one with F - mg = ma

it is not because F increase therefore m decrease. Is more like constant F, decrease m therefore increase in a (accelerating up at higher rate)

 

[Mountain.Dew]

might also like to consider this:

P = momentum

P(rockets) = P(gases)
change in P(rockets) = change in P(gases)

I(rockets) = I(gases), I = Impulse
then
I(rockets)*t = impulse(gases)*t

so, F(rockets) = F(gases)
since F(gases) is constant, THEN
F(rocket) = ma = k, some constant k
So, as mass decreases, acceleration increases.

this also works the other way, going up (going from Forces to momentum) to 'prove' that the law of conservation of momentum applies to rockets, to achieve a successful rocket launch.

 

[Halfasian89]

so the momentem before = the momentum after

This case i would regard that it is the conservation of momentum within a system, rather than before and after, for the event is simultaneous, the momentums exist at the same time. Just an observation.

And to clarify, it isn't really another theory, they are different applications of finding variables provided a set of given variables.

 

[Mountain.Dew]

And to clarify, it isn't really another theory, they are different applications of finding variables provided a set of given variables.

hmmm i didnt really say that its another 'theory', i just said to 'consider'.

 

[Halfasian89]

Sorry i was just trying to clarify wolf's argument, as he did state such. I was bit worried he might get the wrong idea that both applications share the same concepts. In fact i just noticed this:

anotanother theory

momentum calculation and rockets

F - mg = ma

as you can f(thrust) is the rocket going up and mg(the force on it in other words negative) is going down on the rocket. when f increase, the mass decrease so would that mean f is going to be greater than mg

This equation has to do with the equivelence of the net force of differing variables, not the conseervation of momentum. net force is equal to mass x acceleration, but can be further expressed as the net force between thrust and weight. Thus net force can equated as such.

 

[Mountain.Dew]

to be a big more succint,

from F - mg = ma, it should be written as

Net force (ma) = Thrust(F) - Weight-force of rocket(mg)

wolf7, u said that "when f increase, the mass decrease so would that mean f is going to be greater than mg"

one small thing...F doesnt increase. Thrust is a constant. and also, F MUST BE GREATER than mg IF u want to get rocket off the ground in the first place.

just an observation...

 

[Halfasian89]

gman beat you to it, but yeah that basically all the analysis we can do for you wolf. So, yeah.