Chp 5-6 PHY 101

The onset of turbulent flow happens for a Reynolds number of about

2300

Tennis ball A is dry and fuzzy. Tennis ball B is wet, and the fuzz is matted down. What will happen when the two balls are hit horizontally so as to have the same initial horizontal velocity?

A goes farther than B because the fuzz reduces drag.

Pressure potential energy

Has to do with describing moving water and is only an abstract concept.

When a fluid is in steady-state flow and its path bends, the pressure on the outside of the bend is always

Higher than the pressure on the inside of the bend.

At what place in or near the jet engine is gas moving the fastest relative to the flying airplane?

In the plume of exhaust gas flowing out of the engine's outlet duct.

Lead sinkers actually float on the surface of liquid mercury. Which takes up more space: 1 kg of mercury or 1 kg of lead?

Lead

You have just set up a weather station for your daughter to observe wind speeds at home. You had to move it up from the ground because when it was too close to the ground the wind speed readings are all too small. This is because

The boundary layer of a moving fluid moves slower than at other points farther into the fluid.

Why does fuzz on a tennis ball help it travel farther when hit?

The fuzz "trips" the boundary layer, causing turbulence everywhere around the ball.

For an object that sinks in a fluid,

The weight of fluid displaced is less than its weight.

For an object that is floating on a fluid,

The weight of the fluid displaced equals the object's weight.

Suppose that we are able to send astronauts to Mars and they engage in a game of golf on the red planet's surface. Given that the atmosphere on Mars is considerably less dense than our own, how will the path of the ball be affected?

There is less drag and gravity, so the ball will go further than if there were air.

Cities have to use water towers to supply water to because

Water pressure at the bottom of the tower is much higher than at the top.

Why are failures in dams more likely to occur closer to the bottom of the dam?

Water pressure is higher at the bottom.

Suppose you had a very long straw that could reach from a cup of water on the ground level to the top of a very tall building. When you suck on the straw in order to bring water up,

You could bring the water up to only a certain height in the straw because the atmosphere pushes down on the surface of the water by a given amount.

Your apartment window opens 20 meters (65 feet) above a lemonade stand. Your friend lowers a long plastic tube out the window until its end enters the tank of delicious lemonade far below. She then begins to suck on the other end of the tube in hopes of getting a free drink. To her dismay, she never tastes a drop because

atmospheric pressure cannot support a column of lemonade 20 meters tall.

On a windy day you notice that a tall light pole is bent away from its equilibrium position. If the wind speed were to increase the pole would

bend further from its equilibrium position because the aerodynamic drag forces that are bending the pole will increase with the speed of the air moving around the pole.

Aerodynamic forces consist of

drag and lift.

As the Reynolds number increases,

flow is going from being viscous dominated to inertia dominated and therefore turbulent.

You are taking a shower in your dormitory when someone flushes a toilet nearby. The pressure in the cold water line drops and you find yourself showering in what feels like molten lava. This loss of cold water pressure occurs when the toilet lets cold water flow through the pipes delivering it to the bathroom and the water's speed in those pipes increases. Assuming all the piping to be about on the same level, the cold water's faster motion in the delivery pipes reduces its pressure in the shower head because faster moving water

has less pressure than slower moving water.

When a Frisbee is flying horizontally through the air, the speed of the air flowing over the Frisbee's top is

higher than the speed of the air beneath it and the pressure on top of the Frisbee is less than the pressure beneath it.

Flowing honey is less likely to become turbulent than flowing water because

honey's large viscosity favors laminar flow.

You stop for a cappuccino at a coffee shop and notice that the tiny white bubbles of steamed milk remain on the surface of the coffee. These air-filled bubbles stay where they are, rather than descending into the coffee or rising into the air, because they are

less dense than the coffee but more dense than the air above the coffee.

When you blow on a pinwheel, it starts to spin. Even when you aim the air directly toward the wheel's pivot you produce a torque on the wing-like blades and they undergo angular acceleration. This torque is produced by

lift forces on the pinwheel's blades.

A curve ball in baseball curves because of

lift forces.

Suppose you are a teacher, and, to your excitement, the local weather person comes for a classroom visit! She is talking about tornadoes and says that there is air moving very rapidly upwards through the funnel but air standing basically still outside the funnel. Such motion would cause

low pressure inside the funnel.

If a golf ball were smooth rather than dimpled, it would

not travel as far after being hit by a golf club.

Gravity creates

pressure gradients in fluids.

You are caulking a window. The caulk is rather thick and, to lay the bead correctly, the exit nozzle is small. A caulking gun uses a plunger which is operated by pulling back on a handle. You must squeeze the handle very hard to get the caulk to come out of the narrow opening because

pressure is distributed uniformly throughout the fluid and the area of the plunger is much larger than the area of the opening.

Dimples on a golf ball hit without spin

reduce drag.

Imagine sitting on a merry-go-round and riding along as it spins. Assuming you are not grabbing it anywhere and are not moving with respect to the platform,

static friction (directed inwards) causes you to accelerate.

Fluids have viscosity because

the atoms and molecules within the fluid interact with each other, producing internal frictional forces.

The Coelocanth is a very interesting, quite primitive fish that live deep in the ocean. Watching these beautiful creatures on a nature documentary, you notice that they are able to stay basically at one position in the water with only slight movements of its lobed fins. This indicates that

the density of the fish is about equal to the density of the water.

A squirt gun is a simple type of water pump in which a plunger attached to the trigger forces water out of a nozzle and across the room. When you squeeze the trigger of the gun, water squirts out of the nozzle because

the pressure inside the gun is higher than atmospheric pressure.

You are filling a jar of honey from the spigot at the bottom of a large barrel at the grocery store. The honey flows extremely slowly, so the store manager has the barrel refilled. Now the honey flows much more rapidly from the spigot because

the pressure of the honey at the bottom of the barrel increases as the height of honey in the barrel increases.

Suppose you have an aquarium and you notice that you have a solid plastic decoration and a rock of about the same volume but twice as dense, and both are totally submerged in the water. The buoyant force on the plastic object is about ___________as that on the rock.

the same

The reason fast-moving water makes noise is

turbulence

Suppose you have an aquarium and you notice that you have a solid plastic decoration and a rock of about the same mass but the decoration is twice as large as the rock. Both are totally submerged in the water. The buoyant force on the plastic object is about ___________as that on the rock.

twice as big

When you pour honey into a bowl, it flows smoothly. If you did the same with water, it would splash. These different behaviors occur because honey's high

viscosity keeps it flowing smoothly while water's low viscosity allows inertia to break its flow into many separate pieces.

In a popular classroom demonstration, a cotton ball is placed in the bottom of a strong test tube. A plunger fits inside the tube and it makes an air-tight seal. It is then pushed down very rapidly, and the cotton flashes and burns. This happens because

when the air was compressed its temperature increased.