Transcript Ch.11 Forces In Fluids

Why is the electricity
produced at the bottom of
dams?
When you catch a deep-sea
fish, why does its eyes popout?
Why do your ears pop on an
airplane or up in the
mountains?
Chapter 11 Notes
Pressure
• Pressure is equal to the force
applied to a surface, divided
by the area.
Equations for Pressure
• Pressure = Force/surface area
•Pressure = Newtons (Kg x m/s/s)
side x side
•Units are in Pascals or N/m²
• A substance that can easily
change its shape, such as liquids
and gases.
•The molecules in a fluid have
a certain amount of force (mass
and acceleration) and exert
pressure on surfaces they
touch.
•All the molecules add
up together to make up
the force exerted by the
fluid.
•Air has a
mass of
1Kg/m³
•Gravity creates an air pressure
of 10.13N/m³ at sea level.
1 atmosphere = 760 mmHg = 29.92 inHg = 14.7 lb/in2 = 101.3 KPa
•Air Pressure
decreases as
elevation
increases.
Very Low
pressure
Higher Pressure
The whole system
is a low pressure,
but it dramatically
decreases towards
the eye of the
hurricane.
Pressure always
flows from high to
low, which creates
the high velocity
winds.
Barometric Pressure
• The barometer is used to forecast
weather.
• Decreasing barometer means
stormy weather and an
increasing barometer means
warmer weather.
START AT 3:10
•Bill Nye
Atmosphere
30
The atmosphere has the most air
pressure…
1. At the bottom or at
sea level.
2. At the top or up in
the mountains.
0%
0%
0
1
2
The units for pressure are…
30
1.
2.
3.
4.
N
m/s/s
N/m² or Pa
Kgm/s
0%
0%
0%
0%
0
1
2
3
4
30
The pressure in a lake is the
highest at…
1. The bottom.
2. The top.
0%
0%
0
1
2
The equation for pressure is…
30
1.
2.
3.
4.
Mass x velocity
Mass x acceleration
Force ÷ area
Force ÷ mass
0%
0%
0%
0%
0
1
2
3
4
The equation for force is…
30
1.
2.
3.
4.
Mass x velocity
Mass x acceleration
Force ÷ area
Force ÷ mass
0%
0%
0%
0%
0
1
2
3
4
•Water pressure
increases with
depth.
• c88c.pdf
•When a force is applied to a
confined fluid, the increase in
pressure is transmitted
equally to all parts of the
fluid.
Transmitting Pressure in a Fluid
• When force is applied to a confined fluid,
the change in pressure is transmitted equally
to all parts of the fluid.
Hydraulic Devices
• In a hydraulic device, a
force applied to one
piston increases the
fluid pressure equally
throughout the fluid.
Hydraulic Devices
• By changing the size of
the pistons, the force
can be multiplied.
activeart/hydraulic_systems/hydraulic_systems.html
4N
3. What is the total force of
the right Piston?
F=Pa= 2000N/m2 x 20m2 = 40,000N
20m
.002m2
1. What is the pressure of the left P= F/a = 4/.002 = 2000Pa
piston?
2. What is the pressure of the right 2000Pa
Piston?
Hydraulic Brakes
• The hydraulic
brake system of a
car multiplies the
force exerted on
the brake pedal.
The tendency or
ability of an object
to float.
Buoyancy
• The pressure on the
bottom of a
submerged object is
greater than the
pressure on the top.
The result is a net
force in the upward
direction.
Buoyant Force
The upward force exerted by a
fluid on a submerged or floating
object.
Buoyancy
• The buoyant force works opposite the
weight of an object.
Archimedes’ principle:
• Buoyant Force on an object immersed in a
liquid equals the weight of the liquid displaced
and the weight of the object if it floats.
Archimedes' Principle
Hmm! The crown seems lighter under water!
The buoyant force on a
submerged object is equal
to the weight of the liquid
displaced by the object. For
water, with a density of one
gram per cubic centimeter,
this provides a convenient
way to determine the
volume of an irregularly
shaped object and then to
determine its density
•Density and buoyancy: An
object that has a greater
density than the fluid it is in,
will sink. If its density is less
than the fluid it will float.
• A solid block of steel sinks in water. A steel
ship with the same mass floats on the
surface.
Density
• Changes in density cause a submarine to
dive, rise, or float.
Density
• Changes in density cause a submarine to
dive, rise, or float.
Density
• Changes in density cause a submarine to
dive, rise, or float.
1g/cm³
2 Different ways to find volume in order
to find density:
1) Regular objects- Length x width x height = volume
2 x 2 x 2 = 8cm³
D= m/v
D= 16g/8cm³ = 2g/cm³
2cm
16g
2cm
2cm
2) Irregular objects:
Submerge object and
subtract before volume from after volume.
1-Before
Volume
is 4mL
2-After
Volume is
now 7mL
3-Volume
of object is
3mL
4- Now find
density.
You have two different liquids
and a piece of plastic floats in
liquid A and the same piece of
plastic sinks in liquid B, this
must mean…
Container
1. Liquid B has a greater
density.
2. Liquids A and B have
the same densities.
3. Liquid A has a greater
density.
A
Container
B
0%
1
0%
2
0%
3
What is the density of the block?
1. 640g/cm³
2. 0.5g/cm³
4cm
16g
2cm
4cm
0%
1
0%
2
The weight of the water displaced
is 5N. What is the buoyant force
acting on the object?
1. 5N
2. 25N
The weight of
the water
displaced is
5N.
0%
1
0%
2
A piece of wood has a mass of 4g
and a volume of 2cm³, what is the
density?
1. D=mass x volume = 8g/cm³
2. D=mass/volume = 2 g/cm³
3. D=volume/mass = .5g/cm³
0%
1
0%
2
0%
3
How can a huge metal ship float
and a small rock sink?
1. The rock has a small volume
compared to its mass.
2. The ship has a large volume
compared to its mass.
3. The ship has a huge mass
compared to its volume.
0%
1
0%
2
0%
3
What allows ships to float?
1. Gravity is stronger
than the buoyant
force.
2. Buoyant force is
stronger than the
gravity.
0%
1
0%
2
•The pressure exerted
by a moving stream of
fluid is less than its
surrounding fluid.
•Therefore, as the speed
of the fluid increases its
pressure decreases.
Bernoulli’s
and Baseball
A non-spinning baseball or a
stationary baseball in an
airstream exhibits symmetric
flow. A baseball which is
thrown with spin will curve
because one side of the ball
will experience a reduced
pressure. This is commonly
interpreted as an application
of the Bernoulli principle. The
roughness of the ball's surface
and the laces on the ball are
important! With a perfectly
smooth ball you would not get
enough interaction with the
air.
Bernoulli’s and Air Foil
The air across the top of a conventional
airfoil experiences constricted flow
lines and increased air speed relative to
the wing. This causes a decrease in
pressure on the top according to the
Bernoulli equation and provides a lift
force. Aerodynamicists (see Eastlake)
use the Bernoulli model to correlate
with pressure measurements made in
wind tunnels, and assert that when
pressure measurements are made at
multiple locations around the airfoil
and summed, they do agree reasonably
with the observed lift.
• Others appeal to a model based on Newton's
laws and assert that the main lift comes as a
result of the angle of attack. Part of the
Newton's law model of part of the lift force
involves attachment of the boundary layer of
air on the top of the wing with a resulting
downwash of air behind the wing. If the wing
gives the air a downward force, then by
Newton's third law, the wing experiences a
force in the opposite direction - a lift. While the
"Bernoulli vs Newton" debate continues,
Eastlake's position is that they are really
equivalent, just different approaches to the
same physical phenonenon. NASA has a nice
aerodynamics site at which these issues are
discussed.
MORE EQUATIONS!!!
Liquid Pressure = ρgh
where…..
ρ = mass/volume = fluid density
g = acceleration of gravity
h =height or depth of fluid
The =pressure
from the
weightx of
Fluid Pressure
gh = 1000Kg/m³
x 9.8m/s²
1ma=column
9,800 Pa
of liquid of area A and height h is
The most remarkable thing about this expression
is what it does not include. The fluid pressure at a
given depth does not depend upon the total mass
or total volume of the liquid. The above pressure
expression
easy to see xfor
the straight,
Fluid Pressure
= gh =is1000Kg/m³
9.8m/s²
x 3m = 29,400 Pa
unobstructed column, but not obvious for the
cases of different geometry which are shown.
•As temperature increases,
pressure increases.