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Pressure relates to anything exerting
force on another object by pressing on
Pressure = Force/Area
Units will be in Newton per squared
The amount of pressure you exert
depends on the area over which you
exert a force.
– Snow shoes vs. regular shoes
– Light pressure used with needles for shots
– High heels in grass vs. regular shoes
Fluid Pressure (1)
Remember back to fluids…
Fluids are a material that can easily
flow and change shape.
If you understand the pressure in
fluids, you can predict whether an
object will float or sink.
**** Fluids are not just liquids!! They can
also be gasses- look at the definition!!!
Fluid Pressure (2)
In a fluid, all of the forces exerted by
the individual particles combine to
make up the pressure exerted by the
– The denser the fluid, the more pressure it
will exert (because there are more
particles to exert pressure).
Fluid Pressure (3)
Air pressure is also considered to be a
type of fluid pressure.
We are constantly surrounded by this
pressure of the air around us.
Although it changes at different
elevations, our bodies are designed to
withstand it without it crushing us.
– Sea Level atmospheric pressure is a
whopping 101,300 Pa!!!!! (Pascal units)
Fluid Pressure (4)
Pressure gets greater, the lower you
are in a fluid.
– You have the weight of all of the fluid on
top of you and the lower you get, the more
weight is on top of you.
– It’s like playing “dog pile”. You never want
to be on the bottom or you get crushed!
We use Barometers to measure
People who make weather predictions
use Barometers to help them
understand weather patterns and
Floating and Sinking (1)
Remember Density? Mass per unit
We can calculate density by dividing
mass by volume!
Density = Mass / volume
Floating and Sinking (2)
If you know the densities of objects you
can predict whether they will sink or
float when placed in a fluid of a given
Remember if the density is larger than
the fluid it’s placed in it will sink, if it’s
smaller it will float. (let’s see what
Floating and Sinking (3)
If I had water (1g/ml) would the
following float or sink?
– Wood (.7 g/cm3
– Tar (1.02 g/ml)
– Corn Oil (.925 g/ml)
– Mercury (13.6 g/ml)
– Corn Syrup (1.38 g/ml)
Buoyancy is the ability to float.
Water and other fluids exert an upward
force on any object submerged into it.
Buoyancy acts in the opposite direction
of gravity, so it makes an object feel
– Ever notice how it’s much easier to lift or
carry a friend in a pool than it is outside
Weight of submerged objects is a
downward force and the buoyancy of
an object is an upward force.
If an objects weight is greater than the
buoyant force it will sink.
If an objects weight is equal to the
buoyant force it will not sink.
Archimedes Principle (1)
Archimedes Principle states that the
buoyant force acting on a submerged
object is equal to the weight of the
volume of the fluid displaced by the
Meaning, if your hand displaces 50 ml
of water when I stick in a cup (stupid, I
know) then the buoyant force acting on
it would be equal to the weight of 50ml.
Archimedes Principal (2)
So how does this apply to ships?
If the buoyant force of an object is
equal to the weight of the displaced
fluid, then a large fluid that displaces a
lot of fluid should have a strong
If you are confused, check out the
picture on page 429 in your text book.
Pascal’s Principle (1)
Fluids exert pressure on any surface
that it touches.
Pascal’s Principle states that when a
force is applied to a confined fluid, the
change in pressure is transmitted
equally to all parts of the fluid.
– An example of Pascal’s principal at work
is hydraulic pistons.
Pascal’s Principle (2)
Hydraulic systems multiply forces by
applying the force to a small surface
area. The increase in pressure is then
transmitted to another part of the
confined fluid, which pushes on a
larger surface area.
– Hydraulic lifts
– Hydraulic breaks
Bernoulli’s Principle (1)
Fluids (remember, not just liquids) flow
from areas of high pressure to low
pressure. This is an attempt to keep
The pressure of a moving fluid is
different than the pressure of a fluid at
rest (not moving).
Bernoulli’s Principle (2)
Bernoulli’s principle states that the
faster a fluid moves, the less pressure
the fluid exerts.
– Therefore, as the speed increases, the
pressure exerted by the fluid decreases.
Bernoulli’s Principle (3)
This principle is used to explain the
flight of objects (kites, airplane, frisbee
If the air moves faster above the
object, fluid pressure pushes the object
– The air moves faster due to the
shape/design of the object (typically a
curved shape with a slightly rounded top)