One of those 'think about' problems in textbook

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    Momentum Textbook
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Homework Help Overview

The discussion revolves around a scenario involving a water tanker on a railroad that is moving at a speed of 5.0 m/s while draining water. Participants are exploring the implications of the water's momentum and the effects on the tanker’s speed as water is released.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants consider the momentum of both the tanker and the water, questioning whether the tanker will speed up or slow down as water drains. There are discussions about the conditions under which the water exits the tanker and how that affects the overall momentum.

Discussion Status

There is an active exploration of different interpretations regarding the problem statement, particularly concerning the direction and speed of the water as it drains. Some participants suggest that the location of the drain hole is crucial to understanding the outcome, while others emphasize the need to clarify assumptions about the system's momentum.

Contextual Notes

Participants note that the original problem statement does not specify the location of the drain hole or the direction of the water's motion when it leaves the tanker, leading to varied interpretations and assumptions about the scenario.

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Homework Statement


A water tanker on a railroad has a plug that is opened.
The tanker is moving at 5.0m/s
Will the tanker speed up when the water is being drained?

Homework Equations


m1*v1 = m2*v2

The Attempt at a Solution


My original thought was yes it will speed up
But after looking at the answer it says that the water being drained is moving horizontally at 5.0m/s, so there is no change in momentum. So no change in speed.
Why is there no change in momentum? Isn't m1 decreasing?
Is it something to do with not being an isolated system?
 
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You have to think, the water has momentum as well. You can take your system to be the water and the tanker, or you can choose 2 systems with a mass flow, tanker and water inside, & water outside. (among others), if you do the first, mass is constant in the system, there is no change in momentum. If you do the second, there is an "exchange" of momentum between the two systems via the exchange of mass with momentum.
I think the second one helps visualize it better.
 
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Bennyp85 said:

Homework Statement


A water tanker on a railroad has a plug that is opened.
The tanker is moving at 5.0m/s
Will the tanker speed up when the water is being drained?
It depends on where the hole is. If it is at or near the bottom but on the side so the escaping water is directed backward the car will increase its speed.

AM
 
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In the OP, it says the water is moving at 5m/s w.r.t. the ground.
 
BiGyElLoWhAt said:
In the OP, it says the water is moving at 5m/s w.r.t. the ground.
Yes, but that is while it is in the tank. The issue is how the water moves when it leaves the tank.

AM
 
"The water being drained is moving at 5 m/s"
I assumed "being drained" = "as it leaves"
 
BiGyElLoWhAt said:
"The water being drained is moving at 5 m/s"
I assumed "being drained" = "as it leaves"
The original question does not say that the water being drained is moving at 5 m/s. It just says "The tanker is moving at 5.0m/s". The answer provided to the OP says that the water is moving at 5 m/s when being drained but that would appear to be an assumption. The question as stated does not provide any information as to the location of the drain hole or the direction of motion of the draining water.

AM
 
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Bennyp85 said:

Homework Statement


A water tanker on a railroad has a plug that is opened.
The tanker is moving at 5.0m/s
Will the tanker speed up when the water is being drained?
Is this the problem statement exactly as given?
 
Andrew Mason said:
It depends on where the hole is. If it is at or near the bottom but on the side so the escaping water is directed backward the car will increase its speed.

AM
And if water flows out in the forward direction the speed will decrease :smile:
 

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