L 13 Fluids [2]: Statics



fluids at rest

 More on fluids.

 How can a steel boat float.

 A ship can float in a cup of water!

 Today’s weather

The deeper you go, the higher the pressure P Top A P Bottom A W hypothetical volume of water inside a larger volume. density = mass/volume  = mass/Vol or mass =   Vol

Problem: how much does 1 gallon of water weigh?

• At 20 C the density of water is 998 kg/m 3 • there are 264 gallons in one liter, so the volume of 1 gal is 1/264 m 3 • the mass of 1 gal of water is then 998 kg/m 3 x (1/264) m 3 /gal = 3.79 kg/gal • weight = mass x g = 3.79 kg x 9.8 m/s 2 = 37.1 N x 0.225 pounds/N =

8.3 pounds

Forces in a

STATIC

fluid (at rest) H F BOTTOM F TOP W • W is the weight = mg of this volume • F TOP is the force on the top of the volume exerted by the fluid above it pushing down • F BOTTOM is the force on the volume due to the fluid below it pushing up • For this volume that

not to move

(Static fluid) we must have F BOTTOM = F TOP + mg

Variation of pressure with depth

F BOTTOM F - F TOP BOTTOM = mg = (density x Vol) x g - F TOP =  A H g rho Since pressure is Force / area, Force = P x A P Bottom A – P Top A =  A H g, or

P Bottom – P Top =



H g

The pressure below is greater than the pressure above.

How much does P increase

• At the surface of a body of water the pressure is 1 atm = 100,000 Pa • As we go down into the water, at what depth does the pressure 100,000 Pa h double, from 1 atm to 2 atm or 200,000 Pa • Want  g h = 100,000 Pa 1000 kg/m 3 x 10 x h = 100,000 • So h = 10 meters or about 30 feet

Variation of pressure with depth in a liquid • Anybody the does scuba diving knows that the pressure increases as then dive to greater depths • The increasing water pressure with depth limits how deep a submarine can go (about 2000 ft)

this layer of fluid must support all the fluid above it the block on the bottom supports all the blocks above it

Inverted closed tube filled with liquid P ATM Measuring atmospheric pressure - Barometers P ATM The column of liquid is held up by the pressure of the liquid in the tank. Near the surface this pressure is atmospheric pressure, so the atmosphere holds the liquid up.

P liquid

Barometric pressure

Atmospheric pressure can support a column of water 10.3 m high, or a column of mercury (which is 13.6 times as dense as water) 30 inches high  the mercury barometer Today’s weather

Pascal’s Vases

• The fluid levels are the same in all each tube irrespective of their shape

Pressure depends only on depth Dam • The pressure at the bottom of the lake is higher than at the top • The dam must be thicker at its base • The pressure does not depend on how far back the lake extends

Blood Pressure

• The blood pressure in your feet can be greater than the blood pressure in your head depending on whether a person is standing or reclining

Water pumps

• A ground level pump can only be used to cause water to rise to a certain maximum height since it uses atmospheric pressure to lift the water • for deeper wells the pump must be located at the bottom

Pascal’s Principle

• If you apply pressure to an enclosed fluid, that pressure is transmitted equally to all parts of the fluid • If I exert extra pressure on the fluid with a piston, the pressure in the fluid increases everywhere by that amount • Cartesian diver

A hydraulic car lift

• Pressure is F x A • At the same depth the pressures are the same • so F 1 /A 1 = F 2 /A 2 , or

F

2

A

1  • with a little force you can lift a heavy object!

• the jack

Buoyancy – why things float

TITANIC

• The trick is to keep the water on the outside of the ship, and • to avoid hitting icebergs (which also float), and • are easy to miss since 90 % of it is submerged.

P Top A h P Bottom A W

Buoyant Force

submerged object that has a mass density ρ O The density of the water is ρ W

Buoyant force

• The water pushes down on the top of the object, and pushes up on the bottom of the object • The difference between the upward force and the downward force is the buoyant force

F B

• since the pressure is larger on the bottom the buoyant force is UP

Archimedes principle

• the pressure difference is ρ W buoyant force is g h, so the • F B = P x A = ρ W • = ρ W g h A g (volume of object) • = ρ W (volume of object) g • = mass of displaced water x g h object • weight of displaced water • This is Archimedes principle

Will it float?

• The object will float if the buoyant force is enough to support the object’s weight • The object will displace just enough water so that the buoyant force = its weight • If it displaces as much water as possible and this does not match its weight, it will sink.

• Objects that have a density less than water will always float.

Floating objects

lighter object heavier object

Floating in a cup of water Only a thin layer of water around the hull is needed for the ship to float!

Oil Tankers

empty tanker full tanker