- #1

- 89

- 1

A. 58,000

B. 45,000

C. 36,000

D. 13,000

The answer key says 45,000 lbs. is the answer but I have no clue how they got there.

Total pressure - lbs./ft^2 - should be 12 x 62.4, or 5.2lbs/in^2. How do they get 45,000?

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- Thread starter jbowers9
- Start date

- #1

- 89

- 1

A. 58,000

B. 45,000

C. 36,000

D. 13,000

The answer key says 45,000 lbs. is the answer but I have no clue how they got there.

Total pressure - lbs./ft^2 - should be 12 x 62.4, or 5.2lbs/in^2. How do they get 45,000?

- #2

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- #3

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with perimeter 40 feet. So each side is 10 feet. I really am mildly ashamed to admit that I don't recall a formula for "horizontal" pressure. I thought pressure at the same depth was the same throughout a fluid. This question is for a State Civile Service exam. NY State. Maybe we do things differently.

- #4

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with perimeter 40 feet. So each side is 10 feet. I really am mildly ashamed to admit that I don't recall a formula for "horizontal" pressure. I thought pressure at the same depth was the same throughout a fluid. This question is for a State Civile Service exam. NY State. Maybe we do things differently.

Hi jbowers9!

Yes, pressure

Hint: force = pressure times area.

And how does the pressure change with the depth?

- #5

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OK

So the area of the tank is 100ft^2...

It's 12 ft high...

62.4 x 10 X 10 X 12 gives total force which is not 45,000 lbs.

- #6

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OK

So the area of the tank is 100ft^2...

It's 12 ft high...

62.4 x 10 X 10 X 12 gives total force which is not 45,000 lbs.

urgh! :yuck:

That's the

how does the pressure change with the depth?

- #7

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Actually, the density x volume IS the force in the English system. Lbs. is a measure of force (mass x g). The English system of mass is the slug.

I figured it out Saturday evening. 62.4y - if y is the verticle coordinate - is the PRESSURE at any depth y in the tank (lbs/ft^2). 10dy is an element of area along one side of the tank.

642 y dy integrated over the limits 0 - 12 give 642 x 72 = 46,224 which is close enough to the given answer of 45,000 for me. Notice that the dimension of the answer is pounds. Which is a unit of FORCE in the English system. Thank you.

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- #8

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Actually, the density x volume IS the force in the English system. Lbs. is a measure of force (mass x g). The English system of mass is the slug.

I did know that!

But the mass (in pounds) is a

I figured it out Saturday evening. 62.4y - if y is the verticle coordinate - is the PRESSURE at any depth y in the tank (lbs/ft^2). 10dy is an element of area along one side of the tank.

642 y dy integrated over the limits 0 - 12 give 642 x 72 = 46,224 which is close enough to the given answer of 45,000 for me. Notice that the dimension of the answer is pounds. Which is a unit of FORCE in the English system. Thank you.

erm … I think you'd get a lot closer if you used 624 instead of 642.

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