Chapter 11 notes 05

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Transcript Chapter 11 notes 05

Why is the electricity
produced at the bottom of
When you catch a deep-sea
fish, why does its eyes popout?
Why do your ears pop on an
airplane or up in the
Chapter 11 Notes
• 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
•All the molecules add
up together to make up
the force exerted by the
•Air has a
mass of
•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
Very Low
Higher Pressure
The whole system
is a low pressure,
but it dramatically
decreases towards
the eye of the
Pressure always
flows from high to
low, which creates
the high velocity
Barometric Pressure
• The barometer is used to forecast
• Decreasing barometer means
stormy weather and an
increasing barometer means
warmer weather.
•Bill Nye
•Water pressure
increases with
• c88c.pdf
•When a force is applied to a
confined fluid, the increase in
pressure is transmitted
equally to all parts of the
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.
3. What is the total force of
the right Piston?
F=Pa= 2000N/m2 x 20m2 = 40,000N
1. What is the pressure of the left P= F/a = 4/.002 = 2000Pa
2. What is the pressure of the right 2000Pa
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.
• 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
Buoyant Force
The upward force exerted by a
fluid on a submerged or floating
• 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
• Changes in density cause a submarine to
dive, rise, or float.
• Changes in density cause a submarine to
dive, rise, or float.
• Changes in density cause a submarine to
dive, rise, or float.
•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.
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
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
Liquid Pressure = ρgh
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
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
easy to see xfor
the straight,
Fluid Pressure
= gh =is1000Kg/m³
x 3m = 29,400 Pa
unobstructed column, but not obvious for the
cases of different geometry which are shown.
•As temperature increases,
pressure increases.