How Does Rocket Ejection Mass Affect Its Velocity in Space?

AI Thread Summary
The discussion centers on how rocket ejection mass affects its velocity in space, specifically using conservation of momentum to derive the final velocity after mass is ejected. It is established that as the rocket ejects a small mass, the change in velocity can be expressed as v' = v + (udm)/(m-dm), where v is the initial velocity. The confusion arises when considering the limit as dm approaches zero, leading to the conclusion that the velocity remains unchanged. However, it is clarified that this limit cannot be applied when seeking the change in velocity (dv) due to mass ejection. Ultimately, the velocity does change, but the analysis must account for small contributions rather than ignoring them entirely.
Father_Ing
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Homework Statement
-
Relevant Equations
momentum conservation
Consider a rocket with mass ##m## in space is going to move forward. In order to do so, it needs to eject mass backwards. Let the mass that is ejected has velocity ##u## relative to the rocket. What is the equation for the final velocity?

It is said that after ##dt## second, the rocket will have mass ##m-dm##, and velocity ##v+dv##.But, isn't it also possible for the speed to increase in high sum after a very small amount of time, or even, not changing at all?

And I tried to find this out by using conservation of momentum.
Let ##v'## be the rocket's speed after ##dt## second, and ##v## is the initial speed.
$$mv = dm(v-u)+(m-dm)v'$$
$$v'=\frac {(m-dm)v +udm}{m-dm}$$

$$v'=v+\frac {udm}{m-dm}$$
Since ##dm## is small, we can take the limit of dm->0. Therefore,$$v'= v$$
It can be concluded that the velocity neither increase nor decrease.

But, I searched about this matter in the internet, and they said that ##v'## is ##v+dv##. Are there any mistakes in my method?
 
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Father_Ing said:
Homework Statement:: -
Relevant Equations:: momentum conservation

Since dm is small, we can take the limit of dm->0.
No you cannot. You are looking for dv, the small change in velocity when you eject mass dm. Obviously, if you do not eject any mass at all (ie, take the limit dm -> 0), then velocity does not change.

You want to find the change dv while ignoring higher order contributions (ie, ##dm^2## etc).
 
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