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AP Physics Chp 11. Density (mass density) ρ =m/V Solids and liquids are fairly constant at our range of temp/pressure but gases vary. Pressure P = F/A the force is perpendicular to the surface area but pressure is not a vector 1 Pa = 1N/m 1 atm = 101,325 Pa bar = 10 5 Pa - PowerPoint PPT Presentation

### Transcript of AP Physics Chp 11

AP Physics Chp 11

AP Physics Chp 11Density (mass density)

=m/V

Solids and liquids are fairly constant at our range of temp/pressure but gases vary.

Pressure

P = F/A the force is perpendicular to the surface area but pressure is not a vector

1 Pa = 1N/m 1 atm = 101,325 Pabar = 105 Pa The english system used lb/sqin = psiThe pressure we experience now is due to the fluid above us being pulled down by gravity.

14.7 lb/si or 101,325 Pa is the standard at sea level

The higher up you go the lower the pressure gets because of less fluid above youThus in a static fluid

P2 = P1 + gh

is the density and h is the depth in m

We use this in constructing a barometer (measure air pressure).

As well as manometers which measure other fluid pressure.

Gauge pressure is the pressure compared to atmospheric pressure

Absolute pressure is the actual pressureHow high can a submersible pump raise water if it has a maximum pressure of 110 kPa?

P2 = P1 + gh

110000 = 101325 Pa + (1000kg/m3)9.8m/s2(h)

h = 0.89 mHow much force would it have at the pump if the pipe was 2.3 cm in diameter? How much at the maximum height with the same size pipe?At the pump

P = F/A P = 110 kPa or 110000 Pa A = r2 r = 2.3cm/2 = 1.15cm = 0.0115mA = (0.0115m)2 = 0.00042m2

F = PA = (110000 Pa)(0.00042m2) = 46 NAt the maximum hieght

P = F/A P = 101325 Pa A = r2 r = 2.3cm/2 = 1.15cm = 0.0115mA = (0.0115m)2 = 0.00042m2

F = PA = (101325 Pa)(0.00042m2) = 43 N

Why cant a surface mounted pump pull water up out of a well that is 12 m deep?

P2 = P1 + gh

101325 Pa = 0 Pa + (1000kg/m3)(9.8m/s2)(h)

h = 10 mPascals Principle

Any pressure applied to a completely enclosed fluid is transmitted undiminished to all parts of the fluid and enclosing walls.

P1 = P2 if the level of the fluid is equal at each point otherwise we add gh as neededF1/A1 = F2/A2 useful in hydraulic jacks

Arcimedes Principle

Fluids apply a buoyant force on any object partially or totally submerged in it with a magnitude equal to the weight of the displaced fluid.Fb = Wfluid

Fb = gV

Why do supertankers float?The supertankers design displaces enough water/salt water such that Fg = Fb

Why will filling a ship to its limit in Boston create problems when it tries to go to Panama?As fluid enters a tube that is open on both ends the mass of the fluid going in has to be equal to the mass coming out.

A1v1 = A2v2 continuity equation

How does this explain the effect on kinking a hose?Bernoullis Equation

P1 + 1/2pv12 + pgh1 = P2 + 1/2pv22 + pgh2

When the fluid isnt moving this gives the pressure equation P = Po + pgh

This equation is used often to explain the affects of moving air like wind over a roof.What is the pressure above the roof sheeting if the wind is blowing 32 m/s and there is no wind inside your attic space. Assume the height difference is negligible. Density of air is 1.29 kg/m3. P1 + 1/2pv12 + pgh1 = P2 + 1/2pv22 + pgh2

P1 + (1.29 kg/m3)(32m/s)2 = 101325 Pa

P1 = 100004 Pa