Download Fluids Notes - Net Start Class

Fluids Notes
Fluid- Any material that flows and offers little resistance to a change in its shape when
under pressure. Both liquids and gases are fluids.
Three basic assumptions of Kinetic Theory of Gases:
1. Gases are made up of a large number of very small particles.
2. The particles are in constant, random motion. They are widely separated and
make only elastic collisions.
3. The particles make perfectly elastic collisions with the walls of the container
that hold them.
The kinetic theory pictures a volume of gas as a large number of very small moving
particles. According to Kinetic theory, the pressure exerted on the walls of the box is the
result of the collisions of the particles with the walls of the box.
Pressure- is the force on each unit area of a surface.
P = F/A
P- Pressure (N/m2)
F- Force (N)
A- Area
(m2)
Unit: 1N/m2 = 1 Pascal
Hydrostatics- Fluids at rest
Pascal’s Principle- any change in pressure applied to a confined fluid at any point is
transmitted undiminished throughout the fluid.
Examples: squeezing toothpaste out of a tube, hydraulic lift.
Hydraulic Lift
F1
A1
A2
F2
Fluid
Pressure is the same throughout the fluid. Thus P1 = P2
So, F1/A1 = F2/A2
Hydrostatic pressure - pressure due to a fluid’s depth. Since Pressure depends on the
weight of the fluid above you.
P = gh
Where  is density of fluid (kg/m3)
h- height of fluid
g 9.8m/s2
Gauge Pressure Pressure gauges measure the pressure over and above
atmospheric pressure. This is called gauge pressure.
Pgauge = gh
Absolute Pressure (total pressure) To get the absolute pressure at a point, one
must add the atmospheric pressure to the gauge pressure.
PAbsolute = Patm + gh
where Patm = 1.01 x 105 Pa
Density of water:  = 1000 kg/m3
Buoyant force- An upward force of the fluid on an object.
FB =  g V
where  is the density of the fluid in kg/m3
V is the volume of the displace fluid
FB is the buoyant force
Archimedes Principle- An object immersed in a fluid is buoyed up by a force equal to
the weight of the fluid displaced by the object.
Apparent weight of object in a fluid = weight of object – Buoyant force
Wapp = mg-FB
Fluid Dynamics - study of fluids in motion
Bernoulli’s Principle- As the velocity of a fluid increases, the pressure exerted by the
fluid decreases.
Equation of Continuity -the volume of fluid passing two points per second is equal
A1 v1 = A2 v2
Where A-area (m2) and v-velocity (m/s)
In a narrow tube, the velocity of the liquid is high; in a wide tube, the velocity of
the liquid is low.
Torricelli's theorem- Velocity of water depends on the depth of water.
V = (2gh)
Example 1. What is the guage pressure in a 40 m tall column of water?
Example 2. What is the absolute pressure in a 50 m tall column of water?
Example 3. A 60 kg object is in a lake. Its volume is .02m3.
a) What is the buoyant force exerted on the object by the water?
b) What is the object’s apparent weight?
Example 4. What is the height of a column of water that exerts a gauge pressure of
60,000 Pascal?
Example 5. A buoyant force of 20 N acts on an object in alcohol (density = 800 kg/m3).
What is the volume of the object?
Example 6. A force of 5 N is applied to a piston of .5m2 cross-sectional area of a
hydraulic piston. What force must be applied to a 5 m2 cross-sectional area?
Example 7. A ship strikes an underwater rock that punctures a hole 30cm2 in area in its
hull 4 m below the water line.
a) What is the velocity of the water as it enters the hull?
b) At what rate does the water enter the hull?