Why is the electricity produced at the bottom of dams?When you catch a deep-sea fish, why does its eyes pop-out?Why do your ears pop on an airplane or up in the mountains?

Chapter Three Notes pages 70-92

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

ρ = mass/volume = fluid densityg = acceleration of gravityh =height or depth of fluid

MORE EQUATIONS!!!

Liquid Pressure = ρgh where…..

The pressure from the weight of a column 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 is easy to see for the straight, unobstructed column, but not obvious for the cases of different geometry which are shown.

Fluid Pressure = gh = 1000Kg/m³ x 9.8m/s² x 1m = 9,800 Pa

Fluid Pressure = gh = 1000Kg/m³ x 9.8m/s² x 3m = 29,400 Pa

• 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.

Air pressure and differences in pressure are among the most important weather makers.

The centers of storms are areas of relatively low air pressure, compared to pressures around the storm. High air pressure generally brings good weather. Keeping track of how the pressure is changing is important for forecasting the weather.

Differences in air pressure between places cause the winds to blow - air moves from high toward low pressure.

The instruments that measure air pressure are called barometers, from Greek words for weight and measure.

The U.S. National Weather Service reports air pressure at the surface in inches of mercury while air pressure aloft is reported in millibars, also known as hectopascals (hPa). Scientists, however, generally use pressures in hectopascals.

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.

•Water pressure increases with depth.

•As temperature increases, pressure increases.

•When a force is applied to a confined fluid, the increase in pressure is transmitted equally to all parts of the fluid.

Pages 78-81

Pascal's Principle

Pressure is transmitted undiminished in an enclosed static fluid.

Any externally applied pressure is transmitted to all parts of the enclosed fluid, making possible a large multiplication of force (hydraulic press principle).

The pressure at the bottom of the jug is equal to the externally applied pressure on the top of the fluid

plus the static fluid pressure from the weight of the liquid.

Hydraulic Brakes

The wheel cylinder of hydraulic drum brakes acts as a double hydraulic press, multiplying the force on the fluid by the ratio of the area of the cylinder to the area of the supply line.

Besides the muliplication of force achieved, Pascal's principle gaurantees that the pressure is transmitted equally to all parts of the enclosed fluid system. This gives straight-line braking unless there is a fluid leak or something to cause a significant difference in the friction of the surfaces.

                                                             

•Buoyant force acts in an upward direction against the force of gravity.

Pages 82-88

•The buoyant force on an object is equal to the weight of the fluid displaced by 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 = mass / volume

•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.

1g/cm³

•The pressure exerted by a moving stream of fluid is less than its surrounding fluid.

Pages 89-92

•Therefore, as the speed of the fluid increases its pressure decreases.

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 Baseball

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.

                                   

Bernoulli’s and Air Foil

• 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.