Pressure Problem TPR

[qtsjoe]

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When I first saw this problem, I immediately thought of just plugging and chugging

P= F/A = Gauge pressure/Area = (1000kg)(10m/s2)(10m) / ( .5 ) ---> 2e6 (e=10^x)

In the soln manual they state you need to take into account atm pressure so... (atm pressure ~10e5)

e5 + 1e5(gauge pressure) / (.5)

Why do we need to take into account atmospheric pressure when (assuming the botttom of tank is not a vacuum) atmospheric pressure would cancel out...

 

[milski]

No good reason for that. You are correct that you'll have to take it into account only if there was vacuum under the plug. What's the source of the question?


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[qtsjoe]

TPR Test 5 ...i hate how sometimes the questions can be ambiguous.

 

[SaintJude]

I gotta be honest, I made the same mistake as you made. But the MCAT is partly about identifying clue words to solve problems.

In retrospect, (hindsight is so clear) there are 2 main clues:

"Open tank"--- I just started reviewing Fluid & Solids, and I think this question has hammered it into my head that "Open tank" usually indicates atmospheric pressure

"Total downward force" exerted by water As EK says" Additional fluids on top of the first fluid simply add their weight to to the total pressure & since air is a fluid it has to be accounted for"

And it's a stopper--so the bottom of tank is closed.

This question asks us to remember two things
1.) Air is a fluid.
2.) Total pressure is found by summing pressure of fluid in container & that sitting on top of water (atmosphere)

 

[milski]

They are right, after all. They're not asking for the net force on the plug, they're asking for the downward force on it. One of those lame questions where you have to read carefully. 😳

 

[ljc]

They are right, after all. They're not asking for the net force on the plug, they're asking for the downward force on it. One of those lame questions where you have to read carefully. 😳

But it says what is the total downward force exerted by the water, not the water and atmosphere. I'd have answered it without atmospheric added in. Oh well, just hope AAMC isn't so ambiguous.

 

[milski]

But it says what is the total downward force exerted by the water, not the water and atmosphere. I'd have answered it without atmospheric added in. Oh well, just hope AAMC isn't so ambiguous.

But the air exerts force on the water and that force is also added to the force that the water would exert on its own. I did not catch the whole 'total downward' force thing the first time, could have missed it on the exam as well. I wish they'd ask us in a bit more straightforward manner.