Is This Momentum Conservation Equation Correct for a Gas Ejection Problem?

AI Thread Summary
The discussion centers on the conservation of momentum in a gas ejection problem involving a trolley and a cylinder. The initial equation proposed for momentum conservation is debated, with clarifications on whether the speed of ejected gas (u) needs to be constant. Participants discuss the average speed of ejected gas and its relation to the mass of oxygen ejected, emphasizing that u can vary. The conversation highlights the importance of integrating to find average values in momentum calculations. Overall, the discussion seeks to clarify the correct application of momentum conservation principles in this specific scenario.
songoku
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Homework Statement
A trolley has cylinder on top of it and inside the cylinder there is oxygen gas. The trolley and cylinder has mass of 0.68 kg and the initial mass of oxygen is 12 g. Calculate the average ejection speed of the oxygen gas if the maximum speed of trolley + cylinder is 2.7 m/s
Relevant Equations
Conservation of momentum
I assume the system starts from rest so the total initial momentum is zero.

Let:
M = mass of trolley + cylinder
m = initial mass of oxygen
Δm = mass of ejected oxygen
u = speed of ejected oxygen

Conservation of momentum:
0 = (M + m - Δm) . vmax - u . Δm

1) Is my equation correct?

2) I suppose I need to assume u is constant?

3) Do I need to know u to answer this question?

4) Is it correct that the question asking to find ##\frac{\Delta m}{\Delta t}##

Thanks
 
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I am not sure, sorry.
Let's ask @haruspex
 
songoku said:
So the equation should be like this?

$$\vec F_{net}=\frac{M.v_{max}-m.u}{\Delta t}$$
$$0=\frac{M.v_{max}-m.u}{\Delta t}$$

And the question is asking about ##u##? Thanks
Yes, but you do not need to assume u constant.
The average speed is ##\frac{\int u.dm}{\int dm}##, and the momentum conservation is ##Mv=\int u.dm##.
 
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haruspex said:
Yes, but you do not need to assume u constant.
The average speed is ##\frac{\int u.dm}{\int dm}##, and the momentum conservation is ##Mv=\int u.dm##.
I am sorry I don't understand this. Why the average speed is ##\frac{\int u.dm}{\int dm}##?

And for momentum conservation, ##u## is function of ##m## where ##m## is mass of oxygen ejected? So if I give the limit to the integration, it would be like this?
$$Mv=\int_{m_o}^{0} u.dm$$

Thanks
 
songoku said:
I am sorry I don't understand this. Why the average speed is ##\frac{\int u.dm}{\int dm}##?

And for momentum conservation, ##u## is function of ##m## where ##m## is mass of oxygen ejected? So if I give the limit to the integration, it would be like this?
$$Mv=\int_{m_o}^{0} u.dm$$

Thanks
As a mater of definition, the formula for the average of variable Y with respect to variable X is ##\frac{\int Y.dX}{\int dX}##. The average velocity of a collection of masses is the average with respect to mass.

It is not a question of limits; it is the average over the collection of oxygen molecules.
 
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Thank you very much Lnewqban and haruspex
 
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