Chapter 12Chapter 12

Forces & Fluids

Ch 12.1Ch 12.1 - Pressure - Pressure

A. Pressure – force per unit area that is applied on the surface of an object

B. Pressure Formula:

Pressure (Pa) = force (N) / area (m2)

or

P = F/A

Calculating PressureCalculating Pressure

A water glass sitting on a table weighs 4N. The bottom of the water glass has a surface area of 0.003m2. Calculate the pressure the glass exerts on the table.

• P = F = 4N = 1,333N/m2 = 1,333Pa

A 0.003m2

WeightWeight

Weight (in Newton's) is equal to an objects mass times its acceleration due to gravity

•Weight(N) = mass(kg) x 9.8 m/s2

1. SI Unit for pressure is the pascal (Pa)

a) One pascal is equal to 1N of force applied over an area of 1m2

b) The weight of a dollar bill resting flat on a table exerts the force of 1Pa

c) 1Pa is so small that pressure is sometimes expresses as kPa which equals 1,000Pa

2. Changes in pressure

a) Pressure increases if the force applied increases or if the area decreases

b) Pressure decreases if the force applied decreases or if the area increases

Pressure vs. AreaPressure vs. Area

Pressure vs. AreaPressure vs. Area

Pressure vs. AreaPressure vs. Area

C. Pressure in Fluids

1. Fluid – any substance that has no definite shape and the ability to flow

2. Fluids include liquids, gases and plasma

3. When the height of a fluid above a surface is greater, the pressure exerted by the fluid is greater

4. Pressure exerted by a fluid increases with depth

5. The pressure on objects in a fluid is exerted perpendicular to all sides of the object

D. Atmospheric Pressure

1. The atmosphere exerts about 100,000 N of force over every square meter on Earth

2. Atmospheric pressure decreases as elevation in the atmosphere increases

3. A barometer is a device that measures atmospheric pressure

Aneroid Barometer

E. Changes in Gas Pressure

1. Pressure & Volume

a) Volume = Pressure

b) Volume = Pressure

Pressure & VolumePressure & Volume

2. Temperature & Pressure

a) Temperature = Pressure

b) Temperature = Pressure

Pressure & TemperaturePressure & Temperature

Ch 12.2Ch 12.2 – Why do Objects Float? – Why do Objects Float?

A. Two properties influence if an object will sink or float when submerged in a fluid

1. Buoyant force

2. Density

B. Buoyant Force – an upward force that is exerted by a fluid on any object in the fluid

1. The buoyant force is caused by the pressure exerted by the fluid

2. The force exerted on the bottom of an object is greater than on the top

3. Buoyant force is an unbalanced force since the portion of the object that is deeper will have a greater force pushing up than down

a) If the weight of an object is greater than the buoyant force the object will sink

b) If the buoyant force is equal to or greater than the weight the object will float

4. Buoyant force depends on the shape of the object

a) Weight can be spread over a larger surface area to increase buoyancy

Buoyant Force & Surface AreaBuoyant Force & Surface Area

C. Archimedes’ Principle

1. The buoyant force on an object is equal to the weight of the fluid it displaces

Archimedes' PrincipleArchimedes' Principle

D. Density – mass of an object divided by its volume

1. Any object with a density greater than the density of the fluid will sink

2. Any object with a density less than the density of the fluid will float

Density of WaterDensity of Water

Density of pure water is 1 g/mlIf you add a solute it increases the

density

The Great Salt Lake

The Dead Sea

3. A boat floats because its volume is large enough to displace enough water so that the buoyant force is greater than its weight and its density is less than the water

Ch 12.3Ch 12.3 – Work with Fluids – Work with Fluids

A. Fluids at rest and in motion exert forces that can do useful work

1. Pushing on a fluid can increase pressure

B. Pascal’s Principle

1. When a force is applied to a fluid in a closed container the pressure throughout the fluid increases

2. A device that makes use of this principle is called a force pump

Force PumpForce Pump

C. Hydraulic System – uses fluid to increase pressure

D. Bernoulli’s Principle – when the speed of a fluid increases the pressure exerted by the fluid decreases

1. A fluid will move from a higher pressure environment to a lower pressure environment

2. Wind can increase the rate at which smoke rises in a chimney

3. Hurricanes tornadoes and high winds can decrease air pressure outside a house causing the roof to be pushed off due to higher pressure still inside

E. Lift

1. Moving air produces this force which allows planes and birds to fly

2. This force is created by downward flowing air which creates an upward reaction force on the bottom of the wing

Action Force(Gravity)

Reaction Force (Lift)