Re: [Phys-L] Fluids problem

[Brian Whatcott <betwys1@sbcglobal.net>]

 In a sentence: what is the work to raise  the center of mass of a body from 9 meters down to  surface level?
    On Tuesday, December 8, 2020, 08:17:53 AM CST, Carl Mungan via Phys-l <phys-l@mail.phys-l.org> wrote:  
 
 You have it right except it should be [12-3] instead of [15-3].

-Carl

> On Dec 8, 2020, at 8:59 AM, Peter Schoch <pschoch@fandm.edu> wrote:
>
> I need some help in trying to understand why my "intuition" appears to have
> misled me.
>
> The problem my students bought me is:
>
> A cylindrical tank is buried underground.  It is 3m in radius and 12 m in
> depth.  It is half full of water, and the upper edge of the tank is at
> ground level.  What is the work required to pump the water out of the tank?
>
> They had gotten this problem in Calculus class and wanted my help because
> they thought it was more a Physics problem.  The Calculus solution is:
>
> W = integral from 0 to 6 { density*pi*3^2*g(12-y) dy}
> which is 1.4978 x 10^7 if I didn't punch the wrong button on my calculator.
>
> I thought this would be a simple potential energy problem.  Move the CM of
> the "block" of water from where it is to above the ground.  So, I thought
> it would be
>
> [density*g*(pi*3^2*6)]*[15-3] = 1.995  10^7
>
> What about my deltaPE assumption was wrong?
>
> Peter Schoch
> _______________________________________________
> Forum for Physics Educators
> Phys-l@mail.phys-l.org
> https://www.phys-l.org/mailman/listinfo/phys-l


-----
Carl E. Mungan, Professor of Physics  410-293-6680 (O) -3729 (F)
Naval Academy Stop 9c, 572C Holloway Rd, Annapolis MD 21402-1363 
mailto:mungan@usna.edu ;   http://usna.edu/Users/physics/mungan/

_______________________________________________
Forum for Physics Educators
Phys-l@mail.phys-l.org
https://www.phys-l.org/mailman/listinfo/phys-l