TPR fluids question

[sanguinee]

Is this solution worded correctly? I always thought that F= P*A, so if they want to find the force exerted, it would be the total pressure * the surface area. I could see how the answer would be correct if they were simply asking what the pressure being exerted by the balloon is, but assumed a different calculation for force.

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[bestinthewest]

Looks like it. Even the solution makes no reference to the force being exerted by the material inside. if they were in fact asking for the force would you have still gone with the answer you picked?

 

[sanguinee]

@bestinthewest I probably would've gone with A, since I'd multiply the Pressure (a little more than 101,300PA) * 0.5m^2.

 

[Mantis Toboggin]

Anyone mind explaining this to me? I've spent a good ten minutes trying to figure it out but I'm totally lost

 

[sanguinee]

nvm

 

[Eaglefan]

It made sense to me. You know they're in equilibrium, so the pressures on the inside of the balloon will be the same as that on the outside. the pressure on the balloon is gauge (100,000) plus 1 atm (100,000). 200,000 Pa is the pressure inside the balloon; F=P*A so 200,000*.5=100,000 or 100 kN

Main problem I had was them expecting you to have atm pressure memorized in Pa... *adds to memorization list*

 

[sanguinee]

^ that makes so much sense. I don't even know why I was confused to begin with

 

[Mantis Toboggin]

Wow I feel like such an idiot. I kept thinking the shaded answer (C) was correct-- now it makes sense.

The only thing confusing me is the gauge pressure, which is pgh, right?

So in any fluid question we have to use the density of water in kg/m^3, so we would get pgh= (1000)(10)(10)= 100,000 for gauge pressure?

 

[sanguinee]

@Mantis Toboggin yes but you also have to add atmospheric pressure (101,300 Pa).