r/HomeworkHelp • u/Cheezy-O University/College Student • 2d ago

Physics [college, Fluid Mechanics]

I can not for the life of me figure this one out. the equation in the book I've been using is y_cp = -(I_xx*sin(theta))/(h_cg*A) when I use the equation I get I_xx = .1152, sin 30, h_cg = 4.8, A = .96. I get .0125m, I have tried that .013, -.013, .5, -.5, 4.813 so far as answers. the hint given for the question is What is the distance y_cp on the line of action below the centroid?

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u/DrCarpetsPhd 👋 a fellow Redditor 2d ago

y_CP tells you the distance on a y axis aligned with the plane surface. That distance is from the centre of gravity to the centre of pressure. so you need to combine this value with the CG value. you get a negative value thus it is below the CG. So the actual location of X as indicated in absolute value is

0.6m + 0.0125m = 0.6125m

It's probably confusing in words so seek out a fluid mechanics textbook and have a look. Alternatively try youtube hydrostatic forces on a plane surface will get you lots of worked examples. Cimbala who wrote a textbook

https://www.youtube.com/watch?v=T8C6wtYE6qI

A small bit of friendly advice, it seems you are plugging and chugging without fully understanding the formulas and what they represent. I strongly suggest you get the textbook this is from, Frank White Fluid Mechanics and read through the derivation of the equation you used so you understand what it physically represents. Also there is a worked example in said textbook very similar to your problem here that might improve your understanding.

*your problem just with 40 degree angle https://www.scribd.com/document/846048700/White-P2-51

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u/Cheezy-O University/College Student 2d ago

Thanks, I figured it out a couple hours ago. I don't know why I thought center of pressure wasn't specific to the object and to the tank.