Correct statement about a reservoir with outlet pipe

[songoku]

Homework Statement

A reservoir has a constant water level as shown in picture below. For each statement, check whether it is true or false
(i) Velocity of the surface water going down is equal to the velocity of the water flowing through the outlet pipe
(ii) If the reservoir is sealed and the water will still be flowing but eventually it will stop.
(iii) If the outlet pipe moving to higher position, the velocity of the water will decrease
(iv) In the picture, pressure at the surface of the water in reservoir is equal to the pressure at the tap

Relevant Equations

Bernoulli Equation

Continuity Equation

The answer to this question is statements (ii) and (iv) are correct.

(i) This is FALSE because the speed of water in the tap is greater than speed at the water surface

(ii) I don't even understand this statement. What does the "seal" part have to do with water flowing out? Won't the water still flow out through the tap until the tank is empty whether the reservoir is sealed or not?

(iii) In my opinion, this statement would be correct. Increasing the gravitational potential energy of the pipe will reduce its kinetic energy so the speed of water is slower. Why is this statement not correct?

(iv) This is correct because both pressures are atmospheric pressure.


What is the logic to analyze statements (ii) and (iii)?

Thanks

 

[kuruman]

In (ii) I think "sealed" means that the container has a top cover and is no longer open to atmospheric pressure.
Why do you think (iii) is not correct? If the outlet pipe is moved all the way to the top, there will be no water flow. The water below the outlet does not count. The speed at the outlet depends on the height difference from the outlet to the surface.

 

[songoku]

In (ii) I think "sealed" means that the container has a top cover and is no longer open to atmospheric pressure.

But what this information has to do with "the water will still be flowing but eventually it will stop."? I mean even though the container is not sealed, the water will still flow out the tap when it is opened and will stop when the water runs out.

Why do you think (iii) is not correct? If the outlet pipe is moved all the way to the top, there will be no water flow. The water below the outlet does not count. The speed at the outlet depends on the height difference from the outlet to the surface.

I say statement (iii) is correct but the answer key says it is wrong.

Thanks

 

[kuruman]

But what this information has to do with "the water will still be flowing but eventually it will stop."? I mean even though the container is not sealed, the water will still flow out the tap when it is opened and will stop when the water runs out.

When the top is sealed, the air above the surface is initially at atmospheric pressure. Will it remain at atmospheric pressure as the level of the water drops? Why do people punch two holes to pour liquid out of a can? (See picture on the right.)

I say statement (iii) is correct but the answer key says it is wrong.

Does the answer key provide a reasoning for the answer? Who do you think is right, you or the answer key?

 

[jbriggs444]

Homework Statement: A reservoir has a constant water level as shown in picture below.

(i) Velocity of the surface water going down is equal to the velocity of the water flowing through the outlet pipe

If the water level is constant then the surface water is motionless. If it is supposed to be raining, then one needs to say so.

The people making up these questions need to invest in some proof readers.

 

[kuruman]

If the water level is constant then the surface water is motionless. If it is supposed to be raining, then one needs to say so.

The people making up these questions need to invest in some proof readers.

I interpreted that to mean "The velocity speed at which the surface drops is the same as the velocity speed of the water exiting at the bottom."

 

[jbriggs444]

I interpreted that to mean "The velocity speed at which the surface drops is the same as the velocity speed of the water exiting at the bottom."

Which would mean that no water is exiting at the bottom.

 

[kuruman]

Yes, I chose to disregard the word "constant" in the statement of the problem for it to make sense.