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Math Help - Just for a little clarity

  1. #1
    Senior Member
    Jan 2009

    Just for a little clarity

    The problem is :

    Calculate the work against gravity required to build the structure out of brick, with density 80 lb/ft^3

    Structure is a cylindrical tower of height 20 ft and radius 10 ft.

    The solution has that the area is 100 pi ft^2 and so the volume per layer is simply 100 pi delta y ft^3

    The weight would then be 8000 pi delta y lb. (just multiplying by density)

    So the total work against gravity is

    integral (0 to 20) 8000 piydy

    The only thing I don't understand is why you don't have to multiply by most other work problems you it because this says the work done AGAINST gravity?
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  2. #2
    MHF Contributor

    Apr 2005
    You don't need to multiply by g because weight (in pounds), as opposed to mass (in "slugs"), already is the force of gravity on a object. IF you were given mass density instead of weight density, then you would have to multiply by g.
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