Fluids

Definition Has no fixed shape Ability to flow

Density (ρ) Mass per unit volume

ρ = m/V SI unit – kg/m3

Specific to different materials (pg. 301)

What is the mass of a solid iron wrecking ball of radius 18cm?

Specific Gravity Ratio of density of a substance to

the density of water (1000 kg/m3)

What is the specific gravity of whole blood?

Pressure Force per unit area: P = F/A Unit N/m2 = pascal (Pa) Fluids exert pressure in all directions Pressure due to liquid at any depth, h, is

due to the weight of the liquid above it

Equation F = mg m = ρV V = Ah F = ρAhg P = F / A P = ρAhg/A P = ρhg (for liquids) Called Gauge Pressure

Pressure is directly proportional to density and depth

Pressure at equal depths in the same liquids is the same

At the same height, which experiences a greater water pressure at the bottom?

Example The surface of a water storage

tank is 30m above the faucet in the kitchen. Calculate the water gauge pressure in the faucet.

Atmospheric Pressure Unit – atmosphere (atm)

1atm = 101.3 kPa

If the surface of a liquid is open, the total pressure is the sum of the atmospheric and gauge pressures

P = Pa + ρgh

Example What is the total pressure on a

scuba diver in a lake at a depth of 8.00? (standard atmospheric pressure)

Pascal’s Principle Pressure applied to a confined fluid

increases the pressure throughout by the same amount

Pout = Pin

F1 = F2

A1 A2

Small force over small area is the same as a large force over a large area - Hydraulics

Example The area of the output piston is

20x that of the input cylinder. What force would need to be applied to lift a 1800kg car?

Buoyancy Upward net force on objects within

a liquid Occurs because pressure in a fluid

increases with depth Since the bottom is deeper, more

force is pushing up than force on the bottom pushing down

Net upward force pushes objects up

Archimedes Principle Weight of fluid displaced is equal

to the volume of the object

FB = ρfluidgVsubmerged

Example A 70kg statue lies at the bottom of

the sea. Its volume is 3.0 x 10-2 m3. How much force is needed to lift it?

When a crown of mass 14.7kg is submerged in water, an accurate scale reads only 13.4kg. Is the crown gold?