13 Fluids

• density

• pressure in a fluid

• buoyancy and archimedes’ principle

• Fluids in motion

• Hk: 21, 31, 33, 41, 43, 55.

Density

• = mass/volume

• SI Unit: kg/m3

• common unit: g/cm3

• 1,000 kg/m3 = 1 g/cm3

Pressure, p

• p = force/area

• Unit: pascal, Pa

• 1 Pa = 1N/m2.

• Example: 100N applied to 0.25m2. Pressure = 100/0.25 = 400N/m2.

Pascal’s Principle

AdF

AdFMA

Bulk Modulus

• B = -P/(V/V)

• compressibility = 1/B

• liquids have large B

• gasses have smaller B

Depth and Fluid Pressure• P = Po + gd

• P is the pressure at depth d.

• Po is the pressure at the fluid surface

• is the fluid density

• Example: increase in pressure at a depth of 1m in water = (1,000)(9.8)(1.0) Pa

Buoyancy and Archimedes’ Principle

8

vv AA

9

At same velocity: lower height fluid is at greater pressureAt same height: faster moving fluid = less pressure

Summary

• Density = mass/volume

• pressure in a fluid varies with depth

• Buoyant force = weight of displaced fluid

• Fluids in motion

• /

Gauge Pressure is Differential Pressure

)76.0)(/8.9)(/600,13(0 3 mkgNmkgghP

Barometer

13