Transcript Fluid Mechanics - Clark Pleasant Community School Corp

Forces in Fluids
Chapter 13
What is pressure?
• The result of a
force acting over
a given area.
• Pressure =
Force/Area
• What label?
• N/m2
• 1 N/m2 is known
as a pascal (Pa)
Blaise Pascal
• 1623-1662
• French physicist
and
mathematician
• Performed some
of the first
experiments
dealing with
pressure
13.1 Pressure
• The force should be in newtons (N).
• The area should be in square meters (m2).
• The resulting unit would be N/m2.
• One N/m2 is a pascal (Pa).
• The SI unit of pressure is the pascal.
13.1 Fluid Pressure
Pressure: is the force per unit area.
A seat that reduces pressure will
be more comfortable than a chair
with higher pressure.
How can you reduce pressure on a
chair?
13.1 Pressure
• To calculate pressure, divide the
force by the area over which the
force acts.
P=F/A
Pressure in Liquids
• Deeper the more pressure.
• Weight of water (and air) above
pushing against you.
– Twice weight, twice pressure.
– Pressure of air above transmitted down
through water and adds pressure.
Density and Depth
• Liquid pressure = weight density x depth.
• Pressure does not depend on amount of
liquid, just the depth.
Density and Depth
• Pressure exerted by a liquid is the
same at any given depth below
surface no matter what its shape.
• Exerted equally in all directions.
Pressure on Dams
Liquid Pressure = Weight density x depth
Pressure Increases with Depth
Pressure Increases with Depth
• Why are the metal
bands round the water
tower closer together
at the bottom?
Air Pressure
• Air pressure at
sea level is
approx. 101 kPa.
• Air pressure will
decrease with
increases in
altitude.
• Examples: flying
in a plane, driving
in the mountains
Air Pressure &
the Atmosphere
• Just as water pressure
increases with depth the weight
of the atmosphere results in air
pressure.
• Air pressure decreases as the
altitude increases.
Air pressure
Air pressure is equal to
the weight (per unit
area) of the column of
air extending above
that location to the top
of the atmosphere.
"Standard Pressure"
• 1 atmosphere (at sea-level)
* 1 atm = 101.3 kPa = 14.7 psi
• kPa is kiloPascals.
Air Pressure &
the Atmosphere
• Your ears pop when you go up a
hill because the pressure
changes.
– Pressure inside our bodies equal
surrounding air
– Pressure inside a balloon is equal
to the pressure of the surrounding
air
13.2 Forces and
Pressure in Fluids
• Pressure is exerted equally in all
directions.
• Pascal’s Principle states that: a
change in pressure at any point
in a fluid is transmitted equally
and unchanged in all directions.
Pascal’s Principle
Pascal’s Principle
• A change in
pressure at
any point in a
fluid is
transmitted
equally and
unchanged in
all directions
throughout the
fluid
hydraulics
• Uses Pascal’s principle
and moving pistons with
an enclosed pressurized
fluid:
• Examples:
– Car brakes, jacks, and
loaders
Hydraulic Systems
• Hydraulics is the science of applying
Pascal’s principle.
• Hydraulic systems use pressurized
fluid acting on a piston to change the
force.
Hydraulic Systems
Bernoulli’s Principle
• When the speed of a fluid
increases, pressure in the fluid
decreases.
Why is wing shaped the
way it is?
Fast air!
Low pressure.
Slow air!
High pressure.
Bernoulli’s principle
• Bernoulli's Principle is an example of
an inverse relationship.
• An inverse relationship means that
when one value goes down, the
other one goes up.
Other applications of
Bernoulli’s principle:
Applications of
Bernoulli’s principle
“OOOh”
B-58 Husler
Applications of
Bernoulli’s principle
“OOOh”
B-58 Husler
Applications of
Bernoulli’s principle
“Go IRL”
Applications of
Bernoulli’s principle
“Go John”
Applications of
Bernoulli’s principle
HAIRDRYER AND PING PONG BALL
Applications of
Bernoulli’s principle
Air
Applications of
Bernoulli’s principle
A Hose-End Sprayer
Straw Pressure
• Pressure in
straw
– You reduce air
pressure in
straw
– Atmospheric
pressure pushes
liquid into
reduced
pressure region
Interest Grabber
What Makes Objects Float and Sink?
Do heavy objects always sink when placed
in water?
Interest Grabber
What Makes Objects Float and Sink?
Consider a dime and a large cruise ship.
When placed in water the dime quickly
sinks, while the cruise ship floats.
1. Which object is heavier, the cruise ship or
the dime?
Interest Grabber
What Makes Objects Float and Sink?
Consider a dime and a large cruise ship.
When placed in water the dime quickly
sinks, while the cruise ship floats.
2. Knowing that weight acts downward,
what can you infer about other forces
acting on a floating object?
13.3 Buoyancy
• Buoyancy is the ability of a fluid
to exert an upward force on an
object placed in it.
• Buoyancy causes the apparent
loss of weight when an object is
placed in a fluid.
Buoyant Force
• The pressure on
the bottom of the
ball is greater than
the pressure on
the top.
• This produces the
buoyant force.
13.3 Buoyancy in a
Liquid
• Buoyancy
– The apparent loss of
weight of submerged
objects.
• 4.9 N object in air.
• 4.3 N object in water.
– Buoyant force = 0.6 N
4.9 N
4.3 N
0.6 N
Buoyant force
• Buoyant force- a consequence of
pressure increasing with depth
– Pressure is greatest at bottom
– Upward force against the bottom are
greater than the downward forces
against top
13.3 Archimedes
Archimedes’ Principle
• An immersed body is buoyed up
by a force equal to the weight of
the fluid it displaces.
– True to all fluids, liquids and gases
– Ex. 7lb object displaced 3lb of
water, the buoyant force is 3lbs
and the apparent weight is 4lbs.
Archimedes’ Principle
Buoyant force
• Weight is greater than the
buoyant force the object sinks
• Weight is still greater than the
buoyant force
• Buoyant force is = to the weight
and the object floats
Weight and the Buoyant Force
Buoyant force
A sunken object displaces its own
volume of liquid.
A floating object displaces its own
mass of liquid.
Buoyant force is equal to the weight
of the displaced liquid, whether the
object is submerged or floating.
Partial Submersion
Density of the fluid makes things float
Alcohol
Water
Glycerin
Why do steel ships float?
•The weight of the water displaced is less than
the weight of the cube, the cube will sink.
•The weight of the water displaced equal to the
weight of the ship, the ship will float.
Principle of flotation
• Floating object
displaces a weight
of fluid equal to its
own weight.
• A ship must be built
to displace enough
fluid to equal its
weight.
Density and Buoyancy
• The weight of a floating object
equals the weight of the water
displaced by the submerged
part.
Principle of flotation
• If a ship weighs
100 tons, it must
displace 100 tons
of water.
• Ship floats higher
in salt than fresh
water.
• Salt is denser.
Principle of flotation
• Weight of water displaced equals the
weight of the cargo.
SCUBA Divers
Buoyancy Largest = Ascend
Forces Balanced = Hover
Weight Largest = Sink
Ascend
Hover
Sink
Make sinkers floaters
• By either removing ballast
(weight) or increasing the size.
• Removing ballast by while
keeping the same size makes
the object a better floater.
Make floaters sinkers
• By either adding ballast or
making their size smaller.
• Adding ballast makes them
heavier than the buoyant force
and the object sinks.
Good Floaters
• Objects which are big and weigh
little are good floaters.
Buoyancy in Gas
• Archimedes principle in gas
– An object surrounded by air is buoyed up
by a force equal to the weight of the air
displaced.
• Rise
– Object has mass less than mass of
equal volume of air rises
– Only rise so long as it displace volume of
air that weighs more than it does
Hot Air Balloons
Buoyancy Largest = Ascend
Forces Balanced = Hover
Weight Largest = Sink
Ascend
Hover
Sink
Hot Air Balloons