# Homework Help: One of those 'think about' problems in textbook

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1. Jul 8, 2016

### Bennyp85

1. The problem statement, all variables and given/known data
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?

2. Relevant equations
m1*v1 = m2*v2

3. 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?

2. Jul 8, 2016

### BiGyElLoWhAt

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.

3. Jul 9, 2016

### Andrew Mason

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

4. Jul 9, 2016

### BiGyElLoWhAt

In the OP, it says the water is moving at 5m/s w.r.t. the ground.

5. Jul 9, 2016

### Andrew Mason

Yes, but that is while it is in the tank. The issue is how the water moves when it leaves the tank.

AM

6. Jul 9, 2016

### BiGyElLoWhAt

"The water being drained is moving at 5 m/s"
I assumed "being drained" = "as it leaves"

7. Jul 9, 2016

### Andrew Mason

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

8. Jul 9, 2016

### vela

Staff Emeritus
Is this the problem statement exactly as given?

9. Jul 10, 2016

### conscience

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