Transcript Forces in Fluids

Forces in Fluids
Fluid pressure
Bouyancy
Hydraulics
Bernoulli’s Principle
Fluid Pressure
 Exerts
a “push” on an object
 Acts in all directions
Pressure
 Force


that acts over a certain area.
F
P A
= Force ÷ Area
 Force is measured in Newtons, area in
cm2, so pressure is N/ cm2.
 Pressure
Example problem
 A woman
weighs 750N. She is standing
on high heels with a total area of 2cm2
touching the ground. How much pressure
does she exert?

F
750N

P A
P 2cm2
 750N
÷ 2cm2 = 375N/cm2
Example problem 2

An elephant weighs 75,000N. She is standing on
4 big feet. Each foot is 50cm in diameter. How
much pressure does she exert?
 Area of feet = 4·π·d = 628cm2



F
P A
75,000N
P 628 cm2
75,000N ÷ 628cm2 = 119 N/cm2
How can this be?
 An
elephant weighs 100 times as much as
a woman, yet it exerts less pressure on
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,
Bouyancy
 Force
of a fluid that pushes up on an
object.
 Makes the object seem lighter
Archimedes Principle
 The
buoyant force on an object is equal to
the weight of the fluid displaced.
 An object will float in a liquid if the density
of the object is less than the density of the
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?
The density of the granola bar is greater
than the density of the water.