Rocket ejecting mass in a direction perpendicular to its velocity

AI Thread Summary
The discussion revolves around the physics of a rocket ejecting mass perpendicular to its velocity. The initial approach involved calculating momentum changes and deriving a relationship between the rocket's mass and the angle of ejection. A key point highlighted is that the change in speed is zero, suggesting that the focus should be on the perpendicular component of velocity rather than using the variable for speed changes. Clarifications were made regarding the direction of exhaust relative to the rocket's motion, emphasizing that it must be perpendicular to the original direction. The LaTeX formatting issues were resolved, allowing for clearer communication of the equations.
LCSphysicist
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Homework Statement
Find the relation between the mass of the rocket (initial mass = M_o) and the angle between the velocity and the initial velocity. The gas is ejected with velocity u with respect to the rocket in a direction perpendicular to rocket's velocity v.
Relevant Equations
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That was my approach:
$$P_f - P_i = [(m-dm)(v + dv) + dm(u+v+dv)] - [m(v)]$$
$$= mdv - dmv + dmu + dmv = mdv + dmu = 0$$

Since the variation of the rocket's velocity is perpendicular to itself, $$ dv = v d \theta => m v d \theta + dm u = 0$$

So we have $$\frac{dm}{m} = \frac{-v d \theta}{u}$$

And so, $$M = M_o e^\frac{-v \theta}{u}$$

Is that right? I am afraid that it can be wrong, but i am not sure why.
 
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Please fix the LaTeX. We cannot read the bottom line.
 
kuruman said:
Please fix the LaTeX. We cannot read the bottom line.
I am not sure where should i fix. In my pc everything is right:
latex.png
 
I am not sure at all in what I am going to say but I think the rocket's initial component of velocity (say ##\vec{V_{0y}}##will remain unchanged, and all you have to do is calculate the other perpendicular component ##\vec{V_x}## (setting as ##V_{0x}=0##) and then vector add the two components ##V_x,V_{0y}## as being perpendicular to each other to get the final velocity ##\vec{V}=\vec{V_x}+\vec{V_{0y}}##.
 
Herculi said:
Homework Statement:: Find the relation between the mass of the rocket (initial mass = M_o) and the angle between the velocity and the initial velocity. The gas is ejected with velocity u with respect to the rocket in a direction perpendicular to rocket's velocity v.
Relevant Equations:: .

That was my approach:
$$P_f - P_i = [(m-dm)(v + dv) + dm(u+v+dv)] - [m(v)]$$
$$= mdv - dmv + dmu + dmv = mdv + dmu = 0$$

Since the variation of the rocket's velocity is perpendicular to itself, $$ dv = v d \theta => m v d \theta + dm u = 0$$

So we have $$\frac{dm}{m} = \frac{-v d \theta}{u}$$

And so, $$M = M_o e^\frac{-v \theta}{u}$$

Is that right? I am afraid that it can be wrong, but i am not sure why.
You could motivate your treatment in a better way. For example, don’t use ##dv## because the change in the speed ##v## is zero so ##dv=0## by construction. Instead, use reasoning to say that you are taking the component of ##d\vec v## perpendicular to ##\vec v##.

Apart from that, it looks reasonable.

Delta2 said:
I am not sure at all in what I am going to say but I think the rocket's initial component of velocity (say ##\vec{V_{0y}}##will remain unchanged, and all you have to do is calculate the other perpendicular component ##\vec{V_x}## (setting as ##V_{0x}=0##) and then vector add the two components ##V_x,V_{0y}## as being perpendicular to each other to get the final velocity ##\vec{V}=\vec{V_x}+\vec{V_{0y}}##.
If this were the case, the exhaust would need to be perpendicular to the original direction of motion, not the current direction of motion (they are the same only at the beginning).
 
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Orodruin said:
If this were the case, the exhaust would need to be perpendicular to the original direction of motion, not the current direction of motion (they are the same only at the beginning).
Thanks, I now understand my faulty reasoning.
 
Herculi said:
I am not sure where should i fix. In my pc everything is right:
It is fixed now, thanks. I could see all the LaTeX lines except for the last, but now I see them all.
 
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