Forces in FluidsForces in Fluids

Chapter 3Chapter 3

Buoyancy in a GasBuoyancy in a Gas

An object surrounded by air is buoyed up by a force equal to the weight of the air displaced.

Exactly the same concept as buoyancy in Exactly the same concept as buoyancy in water. Just substitute air for water in the water. Just substitute air for water in the statementstatement

If the buoyant force is greater than the If the buoyant force is greater than the weight of the object, it will rise in the air!weight of the object, it will rise in the air!

Can a Balloon Rise Forever?Can a Balloon Rise Forever?

Since air gets less dense as altitude increases, the buoyant force decreases as you go up. So helium balloons don’t rise forever!!!

Bernoulli’s PrincipleBernoulli’s Principle

When the speed of a fluid increases, internal When the speed of a fluid increases, internal pressure in the fluid decreases.pressure in the fluid decreases.

Bernoulli’s PrincipleBernoulli’s Principle

Flow is faster when the pipe is narrowerFlow is faster when the pipe is narrower This is why water flows faster when you put This is why water flows faster when you put

your thumb over the end of a garden hoseyour thumb over the end of a garden hose So pressure is So pressure is lowerlower in a gas that is moving in a gas that is moving

faster!faster!

Four Forces on an AirplaneFour Forces on an Airplane

A Plane Needs LiftA Plane Needs Lift

Faster moving air above the wing exerts less Faster moving air above the wing exerts less pressure than slower moving air below the wingpressure than slower moving air below the wing

The increased pressure below exerts an upward The increased pressure below exerts an upward force called lift.force called lift.

Lift is the force that is opposite gravityLift is the force that is opposite gravity

How Do You Get a Big Lift?How Do You Get a Big Lift?

Lift depends on the size of the wing and the Lift depends on the size of the wing and the speed that the air is moving around itspeed that the air is moving around it

Speed is determined by thrustSpeed is determined by thrust Thrust is a force created by a power source Thrust is a force created by a power source

and gives a plane a forward motion.and gives a plane a forward motion. More thrust = moving fasterMore thrust = moving faster Moving faster = faster moving airMoving faster = faster moving air Faster moving air = more liftFaster moving air = more lift

What about WingsWhat about Wings

Big engines create a Big engines create a lot of thrust and speedlot of thrust and speed

You can get enough lift You can get enough lift with a small wingwith a small wing

A glider has no engine A glider has no engine and less thrustand less thrust

A glider must have A glider must have large wings to have large wings to have enough liftenough lift

What about Birds?What about Birds?

A bird with a big wing size is a glider. An example A bird with a big wing size is a glider. An example is a hawk or an eagleis a hawk or an eagle

A bird with smaller wings has to work harder by A bird with smaller wings has to work harder by flapping its wings to stay in the airflapping its wings to stay in the air

Overcoming DragOvercoming Drag

Drag is the force or fluid Drag is the force or fluid friction that slows forward friction that slows forward movementmovement

The movement can be of The movement can be of an airplane, boat, or an airplane, boat, or person.person.

It is in the opposite direction It is in the opposite direction of motion (remember of motion (remember Newton’s #3)Newton’s #3)

Reducing TurbulenceReducing Turbulence

Drag is caused by an irregular flow of fluid Drag is caused by an irregular flow of fluid called turbulencecalled turbulence

Turbulence reduces liftTurbulence reduces lift Airplane flaps change the shape of the wing Airplane flaps change the shape of the wing

to reduce dragto reduce drag Birds adjust their wing feathers in the same Birds adjust their wing feathers in the same

way!way!

Bernoulli’s Principle and CurveballsBernoulli’s Principle and Curveballs

Slower air (where the air and direction of the ball are opposite) has higher pressure and pushes the ball.

Bernoulli’s Principle and an AtomizerBernoulli’s Principle and an Atomizer

Boyle’s LawBoyle’s Law

Boyle’s LawBoyle’s Law

Density is mass divided by volume.

Halve the volume and you double the density and thus the pressure.