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QUICK WRITE. 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?. Forces in Fluids. Chapter 11 Notes. Pressure.

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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?


Forces in Fluids

Chapter 11 Notes

  • Pressure is equal to the force applied to a surface, divided by the area.
equations for pressure
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 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.

Pressure and Elevation

  • Air Pressure decreases as elevation increases.

The whole system is a low pressure, but it dramatically decreases towards the eye of the hurricane.

Very Low pressure

Pressure always flows from high to low, which creates the high velocity winds.

Higher Pressure

barometric pressure
Barometric Pressure
  • The barometer is used to forecast weather.
  • Decreasing barometer means stormy weather and an increasing barometer means warmer weather.
start at 3 10
  • Bill Nye Atmosphere

Pressure and Depth

  • Water pressure increases with depth.

Pascal's Principle

  • 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
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
Hydraulic Devices
  • In a hydraulic device, a force applied to one piston increases the fluid pressure equally throughout the fluid.
hydraulic devices1
Hydraulic Devices
  • By changing the size of the pistons, the force can be multiplied.

3. What is the total force of the right Piston?

4 N

F=Pa= 2000N/m2 x 20m2 =




1. What is the pressure of the left piston?

2. What is the pressure of the right Piston?

P= F/a = 4/.002 = 2000Pa


hydraulic brakes
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 direction.

Buoyant Force

The upward force exerted by a fluid on a submerged or floating object.

  • The buoyant force works opposite the weight of an object.
archimedes principle
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.


  • 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.

Bernoulli's Principle

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

Bernoulli's Principle

  • 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.


Liquid Pressure = ρgh where…..

ρ = mass/volume = fluid density

g = acceleration of gravityh =height or depth of fluid


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


Pressure and Temperature

  • As temperature increases, pressure increases.