PHYS 100: Ch. 6

A favorite college prank involves simultaneously flushing several toilets while someone is in the shower. The cold water pressure to the shower drops and the shower becomes very hot. Why does the cold water pressure suddenly drop?

Flushing makes the water in the pipes flow faster, dropping the pressure and there is more water output

On hot days in the city, people sometimes open up fire hydrants and play in the water. Why does this activity reduce the water pressure in nearby hydrants?

The water in the pipes flows faster, dropping the pressure.

Why does a relatively modest narrowing of the coronary arteries, the blood vessels supplying the blood to the heart, cause a dramatic drop in the amount of blood flowing through them?

the blood flow is proportional to the diameter of the blood vessel to the fourth power

Why does hot maple syrup pour more easily than cold maple syrup?

it has a lower viscosity at a higher temperature

Why is "molasses in January" slower than "molasses in July," at least in the northern hemisphere?

its viscosity is higher at lower temperatures

Why is it so difficult to squeeze ketchup through a very small hole in its packet?

The rate at which ketchup flows is proportional to the diameter of the hole to the fourth power

A baker is decorating a cake by squeezing frosting out of a sealed paper cone with the tip cut off. If the baker makes the hole at the tip of the cone too small, it's extremely difficult to get any frosting to flow out of it. Why?

frosting has a high viscosity and squeezing it through the narrow pipe requires a large pressure difference across the pipe

Why is the wind stronger several meters above a flat field than it is just above the ground?

The air just above the ground is kept from moving by viscous forces.

Pedestrians on the surface of a wind-swept bridge don’t feel the full intensity of the wind because the air is moving relatively slowly near the bridge’s surface. Explain this effect in terms of a boundary layer.

The air just above the bridge is kept from moving by viscous forces, forming a boundary layer. Viscous forces act like friction of the air with itself.

An electric valve controls the water for the lawn sprinklers in your backyard. Why do the pipes in your home shake whenever this valve suddenly stops the water but not when the valve suddenly starts the water?

When the valve closes the water flow stops suddenly which causes water hammer, but when the valve opens the flow does not instantly reach full speed.

If you drop a full can of applesauce and it strikes a cement floor squarely with its flat bottom, what happens to the pressures at the top and bottom of the can?

The pressure at the top is lower than the pressure at the bottom

moving water experiences __ forces that oppose its motion relative to a hose

frictional

most water in a hose never actually touches the hose; forces have to occur __ the water, the water must exert frictional forces on ITSELF

within

what are the internal frictional forces that water experiences called?

viscous forces

viscous forces

forces that appear whenever one layer of a fluid tries to slide across another layer of that fluid

the measure of resistance to relative motion within a fluid is called…

viscosity

the hotter a fluid is, the __ viscosity it has.

less

hotter fluids have less viscosity because their molecules have more __ energy and as a result, bonds are broken more easily.

thermal

water flow rate is __ proportional to diameter

directly

water flow rate is __ proportional to length (and viscosity)

Inversely

if fluid has to travel a large length will it experience more or less friction?

more

Poiseuille’s Law

volume = pi x pressure difference x pipe diameter^4 / 128 x pipe length x fluid viscosity

what law does the equation show?

poiseuille’s law

what does poiseuille’s law state?

flow rate is proportional to the radius to the fourth power

even in steady state flow, water can accelerate but for it to accelerate it must involve __

turning

acceleration toward the side (turning) requires:

obstacles, involves pressure imbalances, and changes in speed

bending the flow of water requires a __ imbalance

pressure

toward what pressure does water accelerate to?

low

on the outside of the bend, the pressure is __ while the speed is __

high; low

on the inside of the bend, the pressure is __ while the speed is __

low; high

what is the pressure and speed like at the end of the nozzle (the red colored area)?

low pressure and high speed

what would speed and pressure be if the color were purple/violet and the streamlines were extremely far apart? (in other words, what is the pressure and speed like at the start of the nozzle?)

low speed and high pressure

flow type (laminar or turbulent) depends on the __ number

reynolds

equation for reynolds number

density x obstacle length x speed / viscosity

the more viscous water is, the more difficult it is to __ through a hose

flow

a longer hose has __ viscous forces to slow water down

more

if you have a hose and let the water flow out without your thumb blocking it or without a nozzle, why does the water pour out gently?

the water will move quickly out and encounter large viscous forces which causes most of the waters ordered energy to be wasted as thermal energy. the water pours out gently.

why does water spray out if you block the hose’s opening with your thumb?

water travels slowly and encounters smaller viscous forces which helps it keep most of its ordered energy and be at high pressure when it meets your thumb. the high pressure water accelerates to great speed as it passes through the small opening and sprays out.

laminar flow

the smooth, silent flow that is characterized by simple streamlines; adjacent portions of fluid have same velocity

when viscosity dominates the fluids motion what is the flow like?

laminar

turbulent flow

noisy flow in which adjacent regions on fluid soon become separated from one another as they move independently in unpredictable directions

what is the flow when inertia dominates the fluids motion?

turbulent

viscous forces keep nearby regions of fluid moving together. Would high viscosity would favor laminar or turbulent flow?

laminar

why does honey follow more of a laminar flow than water?

honey has a higher viscosity than water

whether flow is laminar or turbulent depends on several characteristics of the fluid and its environment, name the 4.

the fluids viscosity, the fluids speed past a stationary obstacle, the size of the obstacle the fluid encounters, fluids density

2 nearby regions of a fluid can become separated and it becomes harder for viscous fluids to keep them together if the fluid moves (a) faster, or (b) slower?

(a) faster

the __ the obstacle a fluid encounters, the more likely it is to cause turbulence because viscous forces are unable to keep the fluid ordered over such a long distance.

larger

how does a fluid respond to viscous fluids if it is denser? its flow is more likely to turn laminar or turbulent?

it responds less to viscous fluids and it is likely to turn turbulent

as reynolds number increases, flow goes from __-dominated to __-dominated (laminar to turbulent)

viscous; inertia

turbulence usually appears when reynolds number exceeds…

2300

what is a vortex?

a common feature of turbulent flow, it is a swirling region of fluid that moves in a circle around the central cavity

when a spoon stirs coffee quickly, the spoon moves fast enough through the fluid to create turbulence forms, this is an example of…

vortex

turbulent flow exhibits chaotic behavior or…

chaos, you can’t predict exactly where any drop of water will go

water hammer

sound heard when the flow of water is suddenly stopped

forces exerted on a ball by the air because of their relative motion are called…

aerodynamic forces

2 types of aerodynamic forces

drag and lift

drag forces push the ball __

downwind

lift forces push the ball to the __

sides

airflow around a slow moving ball is laminar or turbulent?

laminar

air speed __ in the front and behind the ball but the pressure __

decreases; rises

in a slow ball, air speed __ at the sides of the ball and the pressure __

increases; decreases

an air trail behind a ball that is smooth with no turbulence is known as…

wake

because of the symmetrical arrangement of a slow moving ball, the only aerodynamic force acting on the ball is…

viscous drag

when air flowing around a ball is turbulent, the air pressure distribution is no longer symmetrical and the ball experiences what drag?

pressure drag

pressure drag

downstream force exerted by unbalanced pressures in the moving air

if air has a low reynolds number, airflow is laminar because __ dominated over its __

viscosity; inertia

air’s inertia dominated viscosity and air flow is turbulent if reynolds number is (a) high, or (b) low

(a) high

Air effected by ball’s surface is the…

boundary layer

the boundary layer is laminar or turbulent in a fast moving ball?

turbulent

Why do balls experience air resistance?

Balls interact with and transfer momentum to air

surface friction causes __ drag

viscous

turbulence causes __ drag

pressure

why do some balls have dimples?

to produce a turbulent boundary layer

in a fast ball, a small __ forms behind the ball

wake

in a fast ball, the ball experiences a small __ drag force

pressure

how does a turbulent boundary layer affect a ball?

there is more ordered energy and creates more forward momentum for the ball to travel farther

pressure __ in the front of a fast ball while it __ on the sides

rises; drops

why do spinning balls curve in flight?

they experience magnus force and wake deflection force

magnus force

Turning surface pushes/pulls on the air flow

when does wake deflection force appear?

when ball's fast rotation deforms the wake that is created behind it and a high reynolds number

airflow bends __ above a frisbee

inward

there is __ pressure but __ speed above a frisbee

low; high

airflow bends __ below frisbee

outward

there is __ pressure but __ speed below a frisbee

high; low

what causes a frisbee to lift?

pressure imbalance

Airplanes use the __ to support themselves

air

what do airplanes need to stay aloft?

airspeed

Airplanes seem to follow their __, up or down

nose

how does an airplane support itself in air?

it deflects air downward and the air pushes it upward

when a plane is in the air, there is an __ pressure force on the wing and the wing transfers downward __ to the air.

upward; momentum

How does the airplane “lift off” the runway?

The airplane sheds a vortex and is lifted upward.

Why does plane tilt up to rise; down to descend?

The wing’s angle of attack affects its lift

A wing’s lift depends on what 2 things?

the shape of its airfoil and its angle of attack

angle of attack

the angle at which a wing approaches the onrushing air

greater angle of attack causes greater __

lift

Why are there different wing shapes?

Airspeed and performance influence wing design

Asymmetric airfoils produce large lifts and are well-suited to __-speed flight

low

Symmetric airfoils produce small lifts and are well suited to __-speed flight

high

too great of an angle of attack causes

stalling and severe pressure drag appears

when the boundary layer stalls, it comes to a stop and thereby spoils __ __ flow

steady state