Forces in FluidsChapter 11

State Standard: Unbalanced forces cause changes in velocity. All

objects experience a buoyant force when immersed in a fluid.

Pressure

• Force between two objects

• Two objects come into contact with each other

• Gravity “pulls” object down

• Based on area and force exerted

• Formula is Pressure = force ÷ Area

• SI unit is “pascal”

Fluid Pressure• Fluid: material that

easily flows– (ex. Oil, water, air)

• Changes shape• Causes

– Atoms push against each other

– Pull or push– Sum of all particle

forces

Air Pressure• Mixture of gases in

Earth’s atmosphere– Nitrogen,

oxygen, argon, carbon dioxide

• Each cubic meter = 1 kilogram

• Atmospheric gases press down

Variations in Air Pressure

• Symptoms: ears pop, hard to breathe, cold

• Atmospheric pressure decreases with land elevation– Decrease in inside and outside objects – Pressure wants to balance

Measuring Air Pressure

• Barometer

Water Pressure

• Balanced pressure

• P exerted equally in all directions

• Water is 800 times denser than air

• P increases with depth

• Divers need to reach surface at slow pace or risk obtaining the bends

Density

• Formula is D= mass ÷ volume– Volume is the length x width x height – Volume is the amount of space an object takes up

How do you change density?

• Removal or adding of a fluid– Adding water increases mass (__ __ __ __)– Adding air decreases mass (__ __ __ __ __)– The larger the surface area the less P

Buoyancy• Acts in the opposite the direction of the

force of gravity• Pushes up on an object• In water, object feels lighter• Submerged objects have unbalanced forces

due to fluid exerting pressure on all surfaces of object/organism– Pressure increases with depth so upward

pressure is greater than downward pressure

Buoyancy Video

• Fluids

• Buoyancy

Buoyancy

Archimedes’ Principle

• Applies to sinking and floating objects

• Buoyant force acting on submerged object is equal to the weight of the volume of the liquid displaced by an object and the buoyant force.

Archimedes’ Principle• Fluid air

• Air in container is denser than surroundings because air is under pressure (_ _ _ _)

• Heated air becomes less dense(_ _ _ _ _)

Pascal’s Principle• Fluid exerts P on

any surface it touches

• P increases the same amount all over the enclosed or contained fluid– Up, down and

sideways

Pascal’s Principle

Bernoulli’s Principle• Fluid moves from area of high pressure to

low pressure• Faster fluid moves the lower the pressure• Moving air exerts less pressure than still

air• In a plane, moving air creates “lift” under

plane which forces the plane up• Remember “heat” increases air pressure

Bernoulli’s Principle

Bernoulli’s Principle

Application of Bernoulli’s Principle