Tuesday
September 2, 2014

Homework Help: Physics (Liquids)

Posted by Physics (Liquids) on Wednesday, October 21, 2009 at 8:39pm.

I have a question about fluids ok...

I've been told that
Pressure = (density of fluid) (gravity) (height)

ok that's great in all but I don't see how it works...

Pressure = (Force perinduclar to area force is acting on)/(Area force is acting on)

ok this great and all but leads me to several questions ok the force perpendicular to area it acts on is gravity

1.) WHY NOT THE BOUYANCY FORCE?

assuming that the force is gravity then

Force of gravity = (mass)(gravity)

for objects near surface of planet

so plug into our pressure equation

Pressure = ( (mass of object)(gravity) )/(Area the force is acting on)

my book then goes on to say

density of an object = (mass of object)/(volume of object)
ok rearaning for mass

(density of an object)(volume of object) = objects mass

plug and chug into our Pressure equation...

Pressure = ( (density of an object)(volume of object)(gravity) )/(Area the force of gravity is acting on)

Volume of uniform solid = (area of bottom)(height of object)

pluging into our Pressure equation

Pressure = ( (density of mass)(gravity)(area force of gravity is acting on)(height of object) )/(area force of gravity is acting on)

area cancels out

Pressure = (density of mass)(gravity)(height of ojbect)

ok I followed this derivation step by step but it's wrong WHY???

my book gives me this equation

Pressure = (density of fluid object is in)(gravity) (how far submerged the object is in the fluid, the depth i believe the term is like how far under the surface the object is)

this does not match my equation

2.) The density of the object changed to the denisty of the fluid it's in

3.) The height of the object changed to how far under the surface the object is

4.) How do I use this equation when it's not a uniformed block? I couldn't right? Like if it were a person...

THANK YOU
Please I need to understand this... I don't see how such a simple derivation just changes variables around and such...

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