Deck 6: Fluids and Motion

Question 1

The reason fast-moving water makes noise is

A) vortices.
B) chaos.
C) laminar flow.
D) turbulence.

Answer 1

turbulence.

Question 2

The pressure in a water pipe is always lower in regions where the water moves faster only when

A) the pipe increases in altitude towards the faster fluid end
B) the pipe is level.
C) anything other than water is in the pipe
D) the pipe is made of plastic

Answer 2

the pipe is level.

Question 3

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

A) honey's large density favors laminar flow.
B) water's large viscosity favors turbulent flow.
C) water's large pressure favors turbulent flow.
D) honey's large viscosity favors laminar flow.

Answer 3

honey's large viscosity favors laminar flow.

Question 4

To set the world land speed record and travel faster than the speed of sound, the Thrust SSC vehicle used two jet engines that produced about a 250,000 horsepower. The principal reason why this car needed so much power to travel so fast is that

A) a car's momentum is proportional to its power, so reaching very high momentum requires very high power.
B) the pressure drag on a car increases dramatically as the car's speed increases.
C) the force of the car's momentum was enormous and the jet engines were needed to supply that force.
D) Newton's second law requires a very fast moving object to have a very large acceleration and thus a very large force.

Question 5

As air flows through the horizontal ductwork in a warehouse, it passes through a fan inside the duct and the air's total energy increases. Since the air's speed doesn't change as the result of going through the fan, you know that the air's

A) kinetic energy has decreased but its pressure has remained the same.
B) kinetic energy has increased but its pressure has remained the same.
C) pressure has decreased but its kinetic energy has remained the same.
D) pressure has increased but its kinetic energy has remained the same.

Question 6

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

A) has less pressure than slower moving water.
B) has less kinetic energy than slower moving water.
C) has less gravitational potential energy than slower moving water.
D) losses more energy to viscous drag as it flows through the delivery pipes.

Question 7

A stream of smoothly flowing water arcs through the air and hits the side window of a house. At the surface of the window, right where the stream of water hits it, the water's pressure is

A) lower than atmospheric and the water's speed is faster than in the stream.
B) higher than atmospheric and the water's speed is faster than in the stream.
C) higher than atmospheric and the water's speed is slower than in the stream.
D) lower than atmospheric and the water's speed is slower than in the stream.

Question 8

The top surface of a calm, smoothly flowing stream is always at atmospheric pressure. As water in this stream runs into a tree stump and slows almost to a stop, the water's top surface

A) shifts upward slightly above the normal stream level.
B) shifts downward slightly below the normal stream level.
C) stays at the same height and does not begin to rotate.
D) stays at the same height but begins to rotate clockwise as viewed from above.

Question 9

You construct a water-powered go-cart by attaching a large tank of water to a cart. The water can be forced out a nozzle by connecting a tank of high-pressure air to the water tank. In preparation for your first ride, you position this contraption facing away from a brick wall. You climb on and start spraying the water against the wall. To your delight you accelerate away from the wall and begin to move. When you have moved far enough away from the wall that the stream of water can no longer hit it, you

A) continue to accelerate as the water sprays out the back of the cart.
B) continue to move, but stop accelerating since the water can no longer push against the wall.
C) feel lighter than normal since the water hitting the ground will produce an upward force.
D) continue to accelerate, but less since it takes more force to accelerate a moving object than a stationary one.

Question 10

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

A) pressure is distributed uniformly throughout the fluid and the area of the plunger is much larger than the area of the opening.
B) viscous drag between the walls of the tip and the caulk causes the caulk to swirl around chaotically.
C) Newton's third law requires most of the energy in the caulk to be used to push back on the plunger rather than moving it through the tip.
D) the high density of the caulk impedes its flow through the small opening.

Question 11

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

A) viscosity keeps it flowing smoothly while water's low viscosity allows inertia to break its flow into many separate pieces.
B) momentum keeps it moving in a straight line while water's low momentum allows it to turn abruptly in many different directions.
C) density keeps it flowing smoothly while water's low density allows it to float upward and splash about.
D) mass keeps it flowing smoothly while water's low mass allows it to acquire lots of angular momentum.

Question 12

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

A) the pressure inside the gun is higher than atmospheric pressure.
B) the water is compressed at the plunger so it must expand out the nozzle.
C) the Bernoulli effect causes the pressure of water leaving the nozzle to be less than atmospheric pressure.
D) the Bernoulli effect causes the water's overall energy to increase as it travels through the narrow nozzle.

Question 13

Reliving the "good old days" several physics faculty decide to spend a warm balmy summer's day squirting each other with garden hoses right in front of the physics building. They place their thumb over the end of the hose, and the water squirts out very quickly, because their thumb increases

A) the energy of the water.
B) the potential energy in the hose.
C) the pressure of the water in the hose
D) the speed of the water in the hose.

Question 14

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

A) the pressure of the honey at the bottom of the barrel increases as the height of honey in the barrel increases.
B) the density of the honey decreases as the height of the honey in the barrel increases.
C) the viscosity of the honey decreases as the height of honey in the barrel increases.
D) the viscosity of the honey increases as the height of honey in the barrel increases.

Question 15

As the Reynolds number increases,

A) flow is becoming more laminar.
B) flow is going from being viscous dominated to inertia dominated and therefore turbulent.
C) flow is slowing down.
D) flow is speeding up.

Question 16

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

A) The air near the ground is too dense to move well.
B) The ground is hot so air movement there is always upward, never sideways.
C) The boundary layer of a moving fluid moves slower than at other points farther into the fluid.
D) Someone kept stepping on the wind speed gauge.

Question 17

Briefly discuss four properties of a fluid that determine whether it exhibits turbulent or laminar flow.

Question 18

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

A) 2300
B) 5000
C) 0
D) Infinity

Question 19

The diameter of a pipe is tripled while the pressure difference across the pipe remains the same. The volume flow rate of the pipe increases by a factor of

A) 3
B) 9
C) 27
D) 81

Question 20

Suppose you work in the E.R. and a choking patient comes in with their trachea half obstructed. (The diameter is half of what it was before the obstruction.) You manage to correct the problem and send them on their way safely, explaining that such obstructions are particularly dangerous because

A) a half - blocked airway passes only 1/4 the regular amount of air.
B) a half - blocked airway passes only 1/16 the regular amount of air.
C) a half - blocked airway passes only 1/2 the regular amount of air.
D) a half - blocked airway passes only 1/32 the regular amount of air.

Question 21

Fluids have viscosity because

A) the atoms and molecules within the fluid interact with each other, producing internal frictional forces.
B) the atoms and molecules within the fluid are heavy and hard to push..
C) molecules in fluids are long and get tangled.
D) they are always in contact with some boundary.

Question 22

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

A) Equal to the pressure everywhere else in the fluid.
B) Lower than the pressure on the inside of the bend.
C) Higher than the pressure on the inside of the bend.
D) Darned near infinite.

Question 23

Honey is about _______ times more viscous than air.

A) 10
B) 1,000
C) 1,000,000
D) 10,000,000

Question 24

One of the many things that make an artery blockage dangerous is

A) The amount of fluid passing through the blockage is proportional to the square of the diameter of the opening.
B) The amount of fluid passing through the blockage is proportional to the fourth power of the diameter of the opening.
C) The amount of fluid passing through the blockage is proportional to the area of the opening.
D) The amount of fluid passing through the blockage is proportional to the diameter of the opening.

Question 25

The pressure in a water pipe may not necessarily be lower in regions where the water moves faster when

A) the pipe is not level
B) the pipe is level.
C) the pipe is in zero gravity.
D) the pipe is in free fall.

Question 26

You are out watering the lawn and your neighbor decides to strike up a conversation with you. She says that she bought an extra long hose because of all the factors that affect how much water travels through a hose, the length is clearly not one of them because the water is incompressible so what goes in equals what comes out. Please comment on the scientific accuracy of her statement.

Question 27

The four things that affect the amount of water flowing through a hose are

A) air viscosity, hose length, inlet and outlet pressures and hose diameter.
B) water viscosity, hose composition, inlet and outlet pressures and hose diameter.
C) water viscosity, hose length, inlet pressure and hose diameter.
D) water viscosity, hose length, inlet and outlet pressures and hose diameter.

Question 28

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) B goes farther than A due to the extra weight of the water.
B) B goes farther than A due to the smoother surface.
C) A goes farther than B because the fuzz reduces drag.
D) Both go the same distance.

Question 29

You are at the park throwing a stick for your dog to fetch. When you throw a smooth straight stick by one end, it tends to tumble end over end as it flies through the air. However, when you throw a stick that still has some leaves attached to one end, it flies through the air without tumbling and travels in a smooth arc with the stick leading and the leaves trailing. This automatic alignment occurs because

A) the center of mass of the stick is no longer near its geometric center.
B) the center of pressure is at the same point as the center of mass in the stick, stabilizing its flight.
C) the viscosity of the air near the leaves is highest, producing torques which oppose any rotation the stick had after leaving your hand.
D) the drag forces acting on the leafy stick are not uniform around the stick's center of mass.

Question 30

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

A) remain bent by the same amount since aerodynamic drag forces, like sliding friction, are independent of the relative speed of the objects.
B) straighten out because the airflow would become laminar and reduce the drag force.
C) remain bent by the same amount because the larger drag force will be canceled by a larger restoring force.
D) 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.

Question 31

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

A) The fuzz causes turbulence on the leading edge of the ball only.
B) The fuzz causes turbulence on the trailing edge of the ball only.
C) The fuzz "trips the boundary layer, causing turbulence everywhere around the ball..
D) The fuzz causes laminar flow everywhere.

Question 32

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?

A) There is less drag and gravity, so the ball will go further than if there were air.
B) There is less lift, so the ball doesn't go as far than if there were air.
C) Less air means lower gravity, so it will go further than if there were air.
D) There is no significant effect.

Question 33

Dimples on a golf ball hit without spin

A) result in purely laminar flow.
B) increase drag.
C) reduce drag.
D) are merely decorative.

Question 34

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

A) travel much too far after being hit by a golf club.
B) curve toward the left after being hit by the club of a right-handed golfer.
C) curve toward the right after being hit by the club of a right-handed golfer.
D) not travel as far after being hit by a golf club.

Question 35

A curve ball in baseball curves because of

A) lift forces.
B) drag forces.
C) gravitational forces.
D) buoyant forces.

Question 36

You have two golf balls that differ only in their surfaces. One has dimples, while the other is smooth. Which ball has a greater pressure difference between the leading and trailing edges?

A) The pressure difference is greater for the smooth ball.
B) The pressure difference is greater for the dimpled ball.
C) The pressures are exactly the same.
D) there is no leading or trailing pressures.

Question 37

Suppose you have gotten a new job where you have to design balls that fly through the air with as little resistance as possible. You know that in front of the ball there is laminar flow, so in back of the ball there should be

A) Turbulent flow, because there will be lower density.
B) Turbulent flow, because there will be less drag.
C) Laminar flow, so the leading and trailing pressure environments are similar.
D) No air flow at all.

Question 38

A curve ball's path bends to the right as it flies toward home plate because it is experiencing an aerodynamic force to its right. One reason why the ball is experiencing this aerodynamic force is that the ball is spinning and that spin

A) causes the air to flow more rapidly around the left side of the ball than around the right side of the ball.
B) exerts a torque on the ball that causes its angular momentum to shift toward the left.
C) exerts a torque on the ball that causes its angular momentum to shift toward the right.
D) causes the air to flow more rapidly around the right side of the ball than around the left side of the ball.

Question 39

Suppose you are practicing baseball and throw the ball with a spin so that it moves forward and curves to the right. Now when the actual game takes place you are pitching with the wind behind you. Please explain how you could alter your throw so as to make the ball take the same path.

Question 40

Suppose you are practicing baseball and throw the ball with a spin so that it moves forward and curves to the right. Now when the actual game takes place you are pitching into the wind. Please explain how you could alter your throw so as to make the ball take the same path.

Question 41

Suppose your instructor staggers into class after a week - long science lab equipment convention. She proceeds to explain Magnus lift in the following way. She has you consider a ball thrown to the left and spinning clockwise, having the wind velocity profile shown below. This is a "fastball" so it will rise, as correctly indicated. Please explain why the force F_m is inconsistent with the wind velocity profile.

Question 42

In some motion pictures, airplane passengers are sucked out of the plane when a door is opened and the airplane is in flight. Comment on whether such an effect is scientifically realistic or not.

Question 43

Consider a baseball thrown to the left, spinning clockwise and experiencing laminar flow as shown in the picture below. Based upon the wind speed experienced at the top and bottom of the ball, comment on whether or not this would account for the ball's obtaining a lift (the vertical component of the purple arrow) from spinning. Also, mention whether the ball shown is a good candidate for the Magnus force or for wake deflection.

Question 44

You are out for a bicycle ride on a calm, windless day. You are heading northward on a level road and are experiencing a pressure drag force that pushes you toward the south. This air resistance explains why you have to keep pedaling to maintain your constant speed. If someone were to examine the air that you have left behind you after you have passed through it, they would find that the air's velocity is

A) zero everywhere.
B) straight northward everywhere.
C) zero on average but that it is swirling rapidly in all directions.
D) straight southward everywhere.

Question 45

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

A) higher than the speed of the air beneath it and the pressure on top of the Frisbee is greater than the pressure beneath it.
B) lower than the speed of the air beneath it and the pressure on top of the Frisbee is less than the pressure beneath it.
C) lower than the speed of the air beneath it and the pressure on top of the Frisbee is greater than the pressure beneath it.
D) higher than the speed of the air beneath it and the pressure on top of the Frisbee is less than the pressure beneath it.

Question 46

Aerodynamic forces consist of

A) drag and lift.
B) drag, lift and buoyancy.
C) drag and gravity.
D) lift and drop.

Question 47

As the Reynolds number decreases,

A) flow is becoming more laminar.
B) flow is going from being viscous dominated to inertia dominated and therefore turbulent.
C) flow is slowing down.
D) flow is speeding up.

Question 48

Surface roughness help golf and tennis balls travel with less drag, but yet objects that travel under water (submarines by human design and animals by natural design) generally have streamlined, smooth surfaces. Please explain such a difference.

Question 49

The top surface of a calm, smoothly flowing stream is always at atmospheric pressure. As water in this stream suddenly speeds up, the water's top surface

A) shifts upward slightly above the normal stream level.
B) shifts downward slightly below the normal stream level.
C) stays at the same height and does not begin to rotate.
D) stays at the same height but begins to rotate clockwise as viewed from above.

Question 50

Consider a baseball thrown to the left, spinning clockwise and experiencing laminar flow as shown in the picture below. Based upon the wind speed pattern shown, comment on whether or not this would account for the ball's obtaining drag. Also, mention whether the ball shown is a good candidate for the Magnus force or for wake deflection.

Question 51

Suppose your instructor staggers into class after a week - long physics joke convention. She proceeds to explain drag in the following way. She has you consider a ball thrown to the left, having the wind velocity profile shown below. She goes on to say that the air is compressed in front of the ball and rarefied behind it, so there is air pressure pushing backwards, which is exactly drag. Is the picture shown consistent with her explanation?

Question 52

A fan can be used to circulate air around a room. The pressure at the inlet side of the fan is

A) less than the pressure at the outlet side and less than atmospheric pressure..
B) less than the pressure at the outlet side but more than atmospheric pressure.
C) equal to the pressure at the outlet side.
D) more than the pressure at the outlet side.

Question 53

Which of the following would cause a household fan to stall?

A) The blades are too sharply angled for the speed the fan rotates.
B) The fan is rotating too fast.
C) (A) and (B)
D) The fan is rotating too slowly and can't push the air around.

Question 54

You are holding two identical-looking balloons, one filled with air and one filled with water. You drop these two balloons from a very tall bridge and notice that the water-filled balloon hits the ground first because its terminal velocity is larger. The terminal velocity of the water balloon is larger than that of air balloon because

A) conservation of momentum requires the lighter air-filled balloon to travel more slowly.
B) although the drag forces on the two equally shaped balloons are the same, the buoyant force on the air balloon is larger so the net force on that balloon is smaller and it falls more slowly.
C) the larger force of gravity on the water balloon must be balanced by a larger drag force, which occurs at a higher speed.
D) although the force of gravity on the two balloons is the same, the water balloon has more inertia and travels downward more quickly.

Question 55

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

A) high pressure inside the funnel.
B) low pressure inside the funnel.
C) the funnel pressure to be equal to the pressure outside.
D) houses on the ground to implode.

Question 56

When a fish hovers over the bottom of a lake, what forces act on it, and what is the net force?

A) Lift only, for a net force up.
B) Lift only, for a net force zero.
C) Lift and weight, for a net force up.
D) Lift and weight, for a net force zero.

Question 57

You are on an airplane trip to the islands for vacation. Your first flight is on a propeller driven airplane. The air flowing toward the spinning propeller blades doesn't slow down before it encounters those blades. Your second flight is on a turbofan-driven jet airplane. The air flowing toward the spinning fan blades slows down significantly before it encounters those blades. The air's pressure just before it encounters the turbofan's fan blades is

A) much higher than the air's pressure just before it encounters the propeller's blades.
B) much lower than the air's pressure just before it encounters the propeller's blades.
C) equal to atmospheric pressure at the plane's altitude.
D) the same as the air's pressure just before it encounters the propeller's blades.

Question 58

In old western movies, after having fired their guns people would blow across the end of their guns and smoke would come out the barrel. Please explain.

Question 59

Please consider the funnel of a tornado which holds its size (radius) and therefore its rotation rate. Now imagine that the funnel is made up of many tiny particles moving around in circles. You are examining one of those particles, which represents a tiny portion of the funnel cloud. You may ignore gravity for this problem.
-Please identify the direction of the net force acting on one of the particles.

Question 60

Please consider the funnel of a tornado which holds its size (radius) and therefore its rotation rate. Now imagine that the funnel is made up of many tiny particles moving around in circles. You are examining one of those particles, which represents a tiny portion of the funnel cloud. You may ignore gravity for this problem.
-Why would it make sense if we said that air is moving very rapidly up through the center of the funnel and almost stationary outside the funnel for it to remain in uniform circular motion?

Question 61

As you know, dimples on golf balls and fuzz on tennis balls are helpful because that gives the objects longer range. Please explain why implementing the same design for airplane wings is not a good thing.

Question 62

Suppose you are in a car and you fixed one of the windows using a plastic bag. As you drive down the highway fairly rapidly would you expect the bag to bow outwards away from you, bow inwards towards you or do nothing? Please explain.

Question 63

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

A) In the air flowing into the engine's inlet duct.
B) In the plume of exhaust gas flowing out of the engine's outlet duct.
C) In the engine's turbine section.
D) In the engine's compressor section.

Question 64

An airplane pilot must be very careful to maintain a proper angle of attack during flight because

A) too low an angle of attack will cause the Bernoulli effect to be pre-empted and hence the plane will never take off.
B) too steep an angle of attack will cause too much energy to be wasted in friction with the air via laminar flow.
C) too low an angle of attack will cause turbulent flow around the wing and hence stalling.
D) too steep an angle of attack will cause turbulent flow around the wing and hence stalling.

Question 65

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

A) lift forces on the pinwheel's blades.
B) a deflection of the air stream directly away from the pivot.
C) the buoyant force due to the high pressure air hitting the blades of the pinwheel.
D) drag forces on the pinwheel's blades.

Question 66

A typical cruising speed for a jet is

A) 24,000 km/h
B) 800 km/h
C) 100 km/h
D) 800 km/sec

Question 67

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 and air moving at the same speed outside the funnel. Such motion would cause

A) high pressure inside the funnel.
B) low pressure inside the funnel.
C) the funnel pressure to be equal to the pressure outside.
D) houses on the ground to implode.

Question 68

Suppose you are operating a fan and it is stalling. One thing that would not prevent stalling would be to

A) reduce the pitch of the fan.
B) thicken the air.
C) slow down the fan.
D) speed up the fan.

Question 69

When a flat flounder moves rapidly, touching the level bottom of the ocean, what forces act on it and what is the net vertical force?

A) Lift only, for a net force up.
B) Lift only, for a net force zero.
C) Lift and weight, for a net force of zero.
D) Lift, weight, and normal force for a net force zero.

Question 70

You are holding two identical-looking balloons, one filled with air and one filled with water. You drop these two balloons from a very tall bridge and notice that the water-filled balloon hits the ground first because it traveled faster. The object that experienced the larger amount of drag is

A) neither balloon, as it was the same for both.
B) neither balloon, as there is no drag force on an object moving at constant speed.
C) the water - filled balloon because it moved faster.
D) the air filled balloon because it slowed down more.

Question 71

You are at a truck stop and overhear some young physics professors talking about drag, and in particular how large trucks don't experience much drag because they can coast far but something like a piece of paper has a lot of drag. What could you add to their discussion to make it more accurate?