Fluid

• Fluid - any substance that “flows”… liquids and gases.

• Ex. Water and Air

• Pressure - force per unit of area.

• A force that exist in fluids are caused by the mass and motion of the particles making up the fluid.

Fluid PressureFluid Pressure

Calculate Fluid Pressure

• P = ρ · g · h Units: N/m2 or Pa

• Air pressure at sea level is 14.7 psi = 760 mm Hg = 101 kPa = 1 atm.

• ρ of H2O is 1000 kg/m3

• What is the pressure due to water 3.0 m below the surface of a lake?

• A truck falls through the “too thin” ice at Lake Poinsett, and sinks 7 m to the bottom.

– What is the pressure on the truck at this depth?

– If the area of one of the doors on the truck is 0.7 m2, calculate the force that the water exerts on it.

Calculate Fluid Pressure

Calculate Fluid Pressure

• Fluid pressure acts in all directions.

• Force of gravity causes the pressures in a fluid to vary with depth only. Not with horizontal position.

• Fluids will move from areas of high pressure to areas of low pressure.

Properties of PressureProperties of Pressure

Hydraulics• Pascal’s

Principle - pressure is transmitted equally throughout a confined liquid.

• Multiplies force!

• Ex. Brakes, hydraulic lifts, etc.

Calculate Hydraulics• P = F1 = F2

• The small piston of a hydraulic lift has a cross sectional area of 3 cm2, and the large piston has an area of 200 cm2. What force must be applied to the small piston to raise a load of 15,000 N?

A1 A2

Buoyancy

• The upward force exerted by a fluid on a substance partly or completely immersed in it.

• Fb< wt. of sub. SINK

• Fb ≥ wt. of sub. FLOAT!

• Weight Difference method~ Fb= Dry weight – Wet weight

• Weight of displaced water~ Fb= ρ · g · v

Calculate Buoyant ForceCalculate Buoyant Force

Archimedes Principle

• Archimedes Principle - The buoyant force acting in a substance in a fluid at rest is equal to the weight of the fluid displaced by the substance.

• Fb= weight of displaced fluid

or

Fb= ρ · g · v

Sink or Float?

• Density (object)> Density (fluid) SINKS

• Density (object) ≤ Density (fluid) FLOATS

Bernoulli’s Principle

• For a fluid undergoing steady flow, the pressure is lower where the fluid is flowing faster.

• Ex. Shower curtain, airplane wing, etc.