Download Fluids Chp. 10 - Marlington Local Schools

Fluids Chp. 10
AP Physics Groff 2013
Pressure
 A fluid is a Liquid or a gas, it can flow and fill the
shape of any container.
 Pressure is defined as the ratio of a force applied
to a specific area.
 P=F/A
 P=Pressure…measured in Pascal..Pa
 F=Force…measured in Newtons..N
 A is Area…Measured in m^s
Pressure Cont.
 Hydrostatic Pressure
 The pressure applied by the weight of a
fluid. Think of a column, A column of fluid
applies pressure due to the weight of the
fluid above it.
Calculating Pressure
 Since P=F/A where F is the weight =mg
 P= mg/A
 Mass is not commonly used to measure fluids, Volume is
most commonly used. The relationship between mass and
volume is Density.
 ρ=m/v
 ρ=Density…kg/m^3
 M=mass…kg
 V is volume…m^3
Calculating Pressure cont…
 Therefore the mass of a fluid can be described in
terms of the density and the volume of the fluid.
 M=ρV
Substituting this for m in the pressure equation
 P=ρVg/A
The volume of the column is the Area of the base of the
column multiplied by the height of the column.
V=Ah
Calculating pressure cont.
 Substituting this expression in for the
volume
 P=ρAhg/A
This can be simplified to
 P=ρgh
 Height of column measured in …meters m
Gauge Pressure and it’s Calculation

The pressure due to just the column of fluid is often referred to as the gauge pressure.
Gauge pressure is the difference in the pressure applied to the top of the column of
fluid,(the atmospheric pressure), and the absolute pressure, the total pressure at the
bottom of the column.

Pgauge =Papplied/atmos - Pabsolute
(Pgauge= PA-Po )

Pgauge=ρgh

P=Po+ ρgh

Pgauge is gauge pressure

P is the absolute pressure

Po is the atmospheric pressure, 1.01 x 10^5 Pa @ sea level.
 Buoyancy
 When an object is placed in a fluid, the object will float or sink. In both cases
fluid is displaced.
 Archimedes principle states: the weight of the fluid displaced is equal to
the upward force exerted on the object by the fluid. This is called the
buoyant force.
 Fbuoy= mf g;
Most fluids are not measured in terms of mass, but in terms of Volume, so:
 M=ρv
 Then buoyant force is

Fbuoy=ρfvfg
 Force…Newtons
 ρ…density
 Vf…is the volume of the fluid displaced
 Buoyancy Cont…
 When an object sinks, the buoyant force is less than the
weight of the object. The volume of the fluid displaced is
equal to the volume of the object. Since the forces are
unbalanced, the object will accelerate downwards (until it
reaches terminal velocity)drag cannot be ignored!
 Common Situations Involving Buoyant ForceObject
 Situation
FBD
Force Equations
Fbuoy
Fg
 Floating, partially submerged
ΣF=ρfVfg – mg=0
 Cont..
Fbuoy
ΣF=ρfVfg – mg=0
Vfluid =Vobject
Fg
Object floating, completely submerged (max weight of the object for floating)
 Cont..
F buoy
ΣF=ρfVfg – mg=ma
Fg
Object Sinking…Jack “don’t let go” Rose “I’ll never let go Jack”
-Rose lied.
 Cont.
Fspring
Fbuoy
Fgrav
Object suspended in a fluid, attached to a spring.