Download fluid and air pressure

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
shape or flow.
(example: liquids and gases)
•The molecules in a fluid have a
certain amount of force (mass &
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.
-A Barometer is used to measure atmospheric
pressure.
- It can measure the pressure exerted by the
atmosphere by using water, air, or mercury.
-Some types of Barometers are water-based,
Mercury, Barographs and Aneroid.
-A barometer is commonly used for weather
prediction, as high air pressure in a region indicates
fair weather while low pressure indicates that storms
are more likely.
-Evangelista Torricelli, who is generally credited with
inventing the barometer, went on to build the first
instrument with mercury in 1643.
Barometric Pressure
M
E
R
C
U
R
Y
B
A
R
O
M
E
T
E
R
• The barometer is used to forecast
weather.
• Decreasing barometer means
stormy weather and an
increasing barometer means
warmer weather.
Aneroid vs. Mercury Barometer
•Water pressure
increases with
depth.
Water Pressure and Depth
• Water pressure acts in the opposite direction
compared to air pressure
• Water pressure increases as depth increases!
•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.
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 acting on an object immersed
in a liquid equals the weight of the liquid
displaced and the weight of the object if it
floats.
• This force does not depend on the weight of
the object!
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
•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.