AAMC 9 BS item 22

[justhanging]

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This is the passage about sloshinig, where water oscillates in a container. The water level is flat in the container and one side of the water is higher then the other, and it basically just oscillates one side of the container water going from high to low to high... Hopefully you can get an idea.

QQ: The principle restoring force responsible for maintaining the sloshing oscillations in an ideal body of water is

A. Gravitational force
D. Hydrostatic pressure at the bottom of the container

The answer is A and I understand where there coming from but I think a more direct force should be D. I understand that the force of the pressure from the water comes from the weight of the water which ultimately comes from gravity so it all leads back to A but its just kinda dumb because D is a better description of the restoring force. Pressure is a force it just comes from gravity. Since one side of the container has a higher water level it has a higher pressure on the bottom which leads water to move from one side to the other. I don't even know what am asking but did anyone get tripped up in this question and found it dumb?


I feel like this question is similar to asking

What is the restoring force on a pendulum's oscillations?

then have answers like

A. Gravitational force
D. Weight of pendulum

 

[MT Headed]

Here are a few ways to eliminate answer D:

Answer D is the wrong units. The answer is supposed to be in newtons, not pascals.

Answer D is an effect, not a cause of the water restoring itself and creating simple harmonic motion.

Answer D is one of those answers that is undoubtedly true but doesn't answer the question.

This question tries to distract the test taker by confusing the pressure at the bottom of the tank (which is larger but irrelevant) with the pressure at the top of the tank (which is the same and therefore does not contribute to restoring anything).

 

[justhanging]

here is a pic of the diagram of sloshing. Its figure 1.

 

[justhanging]

Here are a few ways to eliminate answer D:

Answer D is the wrong units. The answer is supposed to be in newtons, not pascals.

Answer D is an effect, not a cause of the water restoring itself and creating simple harmonic motion.

Answer D is one of those answers that is undoubtedly true but doesn't answer the question.

This question tries to distract the test taker by confusing the pressure at the bottom of the tank (which is larger but irrelevant) with the pressure at the top of the tank (which is the same and therefore does not contribute to restoring anything).

I dont agree with you on the last 2. I think you could argue that the higher pressure on the bottom of the container pushes water to the other side thus causing oscialltions. I think that D does answer the question I just feel like this question is too nit picky. Just like the pendulum question, the weight of the pendulum is the effect of gravity but its hard to argue that it is not the cause of the oscillations.

 

[jrf794]

Pressure is not a force

 

[justhanging]

Pressure is not a force

Thank you for that, very enlightening.

 

[jrf794]

Thank you for that, very enlightening.

Glad I could help.

 

[whiteshadodw]

simple harmonic motion has only ONE restoring force that obeys Hooks law (kx for mass spring, mg*theta for pendulum for small angles). while pressure is actually a force/area, it is not just a force acting on the water.

also, for aamc questions, you'll often find that the correct answer is the answer that requires the least justification. narrowed it down to two? great, now which one requires the least justification.

everyone elses justification: "pressure isn't a force"

your justification: " but I think a more direct force should be D. I understand that the force of the pressure from the water comes from the weight of the water which ultimately comes from gravity so it all leads back to A but its just kinda dumb because D is a better description of the restoring force. Pressure is a force it just comes from gravity. Since one side of the container has a higher water level it has a higher pressure on the bottom which leads water to move from one side to the other. I don't even know what am asking but did anyone get tripped up in this question and found it dumb?"

speed and accuracy, i feel like you knew the answer to this, you just overcomplicated it or got nervous.

 

[AppleRind]

I think this question also has a lot to do with just knowing about water-waves. When you consider waves in liquid, the restoring force is gravity.

-AppleRind