CHAPTER 13Forces in Fluids

Physical Science

Fluid and Pressure

Fluid and Pressure 13.1• Pressure – The result of force distributed over

an area– Pressure = Force(in Newton's – N)/area (m2)

• Pascal (Pa) – SI unit for Pressure– Named after French scientist, Blaise Pascal (1623 – 1662)

• Pressure in Fluids– Fluid – substance that assumes the shape of its

container• Liquid and gas

– Depth and type of fluid = 2 factors that affect pressure

• As depth increases, pressure increases– Pressure at 25 cm in bathtub and lake are the same

» Amount doesn’t affect

Air Pressure and the Atmosphere 13.1

• Weight of Earth atmosphere at sea level = 101kPa– Air pressure increases with depth of

atmosphere just like water– AS ALTITUDE INCREASES, AIR PRESSURE

DECREASES– Why do your ears pop?

• Unbalanced air pressure inside and outside your ear b/c pressure changes more quickly than ears adjust

– Pressure equalizes when air passes thru a small tube and you hear a pop

– Body is not crushed by atmosphere b/c pressure inside your body balances outside pressure

Forces and Pressure in Fluids 13.2• Transmitting Pressure in a Fluid 13.2

– Pascal’s Principle• A change in pressure at any point in a fluid is

transmitted equally and unchanged in all directions throughout the liquid

– Hydraulic Systems• Device that uses pressurized fluid acting on

pistons of different sizes to change a force• Input force applied to small piston, pushes on fluid

of seal system pushes large piston = output force

Bernoulli’s Principle 13.2• Daniel Bernoulli (1700 – 1782)• As speed of fluid increases, the pressure within

fluid decreases– Blow across the top of a piece of paper, paper lifts

upward… WHY?• Pressure is constant below and decreases above

• Wings and Lift– Why can a bird and airplane fly

• Aerodynamics so air travels faster over the wings• Lift – the pressure difference between the top and bottom

of the wing creates and upward force• Birds can flap wings to produce some lift and forward

motion– Sometimes do opposite with cars to keep

downward force for traction

Buoyancy 13.3• Buoyant Force

– Buoyancy – the ability of a fluid to exert an upward force on an object lace in it

– Apparent loss of weight • Water exerts and upward force on object, making

easier to lift– Acts in opposite direction of gravity– Water pressure increases with depth SO

forces pushing up on bottom are greater than forces pushing down on top

Archimedes’ Principle• Archimedes – Greek mathematician –

died 212 B.C.• The buoyant force on an object is equal to

the weight of the fluid displaced by the object

Density and Buoyancy 13.3• Density = mass/volume

– g/cm3

– If object less dense than fluid = float• 2 forces always act on an object in a fluid

– Weight• Equals force of gravity (mass X 9.8 m/s2)• Acts downward

– Buoyant force• Equals weight of volume of displace fluid• Acts upward

– When force is equal – object floats or is suspended

• Suspended – Same density as the fluid it is submerged in – Buoyancy = weight

• Sinking– weight > buoyancy

• Floating– Buoyancy > weight– Why does a piece of steel sink and a huge

steel ship float?• Ship

– Shape of hull» Shaped to displace large volume of water = large

buoyant force and it increases volume and decreases density

http://www.teachertube.com/view_video.php?viewkey=cafa030c827d58b2cc85

– 8 minuteshttp://videos.howstuffworks.com/discovery/6540-mythbusters-lets-talk-buoyancy-video.htm - 3 minutes