AP Physics

Mr. Jean

September 20th, 2011

The plan:

• Fluid Mechanics– Pressure– Atmospheric Pressure– Standard Pressure formula in fluids– Understanding pressure

Pressure

• Most basic pressure equation– Pressure = Force / Area

– What exerts more pressure-per-square inch when walking a 100 lb woman in high heels or a 6,000 lb elephant?

– At the moment when only the heel rests on the ground.

– Stiletto heels have an area of about 0.5cm2. Elephants, unlike humans, walk with two feet on the ground at a time. Each foot is about 400cm2.

– Under normal conditions a woman’s heeled shoe can apply 20 times the pressure compared to an elephant’s foot.

Pressure

• 1 Pascal = 1N / m2

• 1 Kilopascal = 1000N/m2

• 1 atm = 1.013105 Pa

Pressure Formula:

• Standard pressure equation

• Pf = Pi + ρgh

• Pf = Final pressure reading (Pa)• Pi = Initial pressure reading (Pa)• ρ = density of matter (kg/m3)• g = gravity (m/s2)• h = total height (m)

Demos

• Collapsing can video

• Magdeburg hemispheres video– von Guericke and the first vacuum pump,

1654 demo for Ferdinand III

Density

• Units?

• Density of water: – 1000 kg/m3

= 1 g/cm3

• “Specific gravity”

Air, standard conds 0.0013

Wood (oak) 0.6 – 0.9

Liquid nitrogen 0.81

Ice 0.92

Water 1.00

Bricks 1.84

Aluminum 2.70

Steel 7.80

Silver 10.50

Lead 11.30

Gold 19.30

Platinum 21.40

Demos

• See Demo videos from AP folder.

For next day:

– Buoyancy– The Bernoulli Effect– Archimedes’ Principle

The Bernoulli Effect• When a constantly flowing fluid has regions of

different speed in its flow, the pressure of the fluid will be lowest in regions where the speed is fastest.

• Why would the fluid change speed? The “garden hose equation”

Terminology: “volume flow rate”

Archimedes’ Principle

FBuoyant = B = mdisplaced fluid g

= fluidVobjectg

FB

mg

FB = “weight of the displaced fluid”

Proof: imaginary plastic bag filled with water

Demos

• Bernoulli red fluid

• Floating ball

• Chimney effect

• Ball in funnel