Forces in FluidsForces in Fluids

Fluid pressureFluid pressureBouyancyBouyancyHydraulicsHydraulics

Bernoulli’s PrincipleBernoulli’s Principle

Fluid PressureFluid Pressure

Exerts a “Exerts a “pushpush” on an object” on an object Acts in Acts in all directionsall directions

PressurePressure

Force that acts over a certain area.Force that acts over a certain area.

FF P AP A

Pressure = Force Pressure = Force ÷ Area ÷ Area Force is measured in Newtons, area in Force is measured in Newtons, area in

cmcm22, so pressure is N/ cm, so pressure is N/ cm22..

Example problemExample problem

A woman weighs 750N. She is standing A woman weighs 750N. She is standing on high heels with a total area of 2cmon high heels with a total area of 2cm22 touching the ground. How much pressure touching the ground. How much pressure does she exert?does she exert?

FF 750N 750N P A P 2cmP A P 2cm22

750N 750N ÷ ÷ 2cm2cm22 = 375N/cm = 375N/cm22

Example problem 2Example problem 2

An elephant weighs 75,000N. She is standing on An elephant weighs 75,000N. She is standing on 4 big feet. Each foot is 50cm in diameter. How 4 big feet. Each foot is 50cm in diameter. How much pressure does she exert?much pressure does she exert?

Area of feet = 4Area of feet = 4··ππ·d = 628·d = 628cmcm22

FF 75,000N 75,000N P A P 628 cmP A P 628 cm22

75,000N 75,000N ÷ ÷ 628cm628cm22 = 119 N/cm = 119 N/cm22

How can this be?How can this be?

An elephant weighs 100 times as much as An elephant weighs 100 times as much as a woman, yet it exerts less pressure on a woman, yet it exerts less pressure on the floor?the floor?

What puts a greater force on a bamboo floor?

                                    

    

                                

        

                                    

    

A two ton car?An African elephant?

A 125 pound woman with high heels?

Measured in pounds per square inch, (PSI), a car has a load of 28 to 30 PSI, an elephant 50 to 100 PSI, and a 125 pound woman with high heels, when she takes a step and those hit the floor, 2,000 PSI! 

BouyancyBouyancy

Force of a fluid that pushes up on an Force of a fluid that pushes up on an object.object.

Makes the object seem lighterMakes the object seem lighter

Archimedes PrincipleArchimedes Principle

The buoyant force on an object is equal to The buoyant force on an object is equal to the weight of the fluid displaced.the weight of the fluid displaced.

An object will float in a liquid if the density An object will float in a liquid if the density of the object is less than the density of the of the object is less than the density of the liquid.liquid.

•If the weight of the water displaced is less than the weight of the object,

the object will sink•Otherwise the object will float,

with the weight of the water displaced equal to the weight of the object.

Archimedes' Principle explains why steel ships float

                                                                                                                                                                                                                  

       

Will It Float?Will It Float?

The density of the granola bar is greater than the density of the water.