Physics Exam 3 (HW 2)
The onset of turbulent flow happens for a Reynolds number of about
2300
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
81
Why does an airplane have a "flight ceiling," a maximum altitude above which it can't obtain enough lift to balance the downward force of gravity?
Above that altitude the air density is too low to generate sufficient lift.
If you put your hand out the window of a moving car, so that your palm is pointing directly forward, the force on your hand is directly backward. Explain why the two halves of the airstream, passing over and under your hand, don't produce an overall up or down force on your hand.
Any lift forces cancel each other out because of a symmetric air flow.
A water-skier skims along the surface of a lake. What types of forces is the water exerting on the skier, and what is the effect of these forces?
Drag forces slow the skier, and lift forces hold the skier up.
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. The faster a fluid moves, the lower the pressure.
Why does hot maple syrup pour more easily than cold maple syrup?
Its viscosity is lower at higher temperatures. Lower viscosity means faster flow, according to Poisseuille's Law.
A skillful volleyball player can serve the ball so that it barely spins at all. The ball dithers slightly from side to side as it flies over the net and is hard to return. What causes the ball to accelerate sideways?
Lift forces caused by irregular air flow around the seams of the volleyball.
When a car is stopped, its flexible radio antenna points straight up. But when the car is moving rapidly down a highway, the antenna arcs toward the rear of the car. What force is bending the antenna?
Pressure Drag
Explain why a parachute slows your descent when you leap out of an airplane.
The air flow around the parachute is turbulent with a large turbulent wake, leading to huge pressure drag forces and a relatively small terminal velocity.
Pedestrians on the surface of a windswept 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.
A poorly designed household fan stalls, making it inefficient at moving air. Describe the airflow through the fan when its blades stall.
The airflow is turbulent rather than laminar.
When a plane enters a steep dive, the air rushes toward it from below. If the pilot pulls up suddenly from such a dive, the wings may abruptly stall, even though the plane is oriented horizontally. Explain why the wings stall.
The angle of attack becomes too large. The airflow is not horizontal in this case, and angle of attack is the angle between the direction of the airflow and the angle of the wing.
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.
You have two golf balls that differ only in their surfaces. One has dimples on it while the other is smooth. If you drop these two balls simultaneously from a tall tower, which one will hit the ground first?
The dimpled ball.
Why do flyswatters have many holes in them?
The holes cause the boundary layer to become turbulent.
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.
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?
The pressure difference is greater for the smooth ball.
Why does a relatively modest narrowing of the coronary arteries, the blood vessels supplying blood to the heart, cause a dramatic drop in the amount of blood flowing through them?
The rate at which blood flows is proportional to the diameter of the blood vessel to the fourth power. Blood is a viscous fluid, and Poiseuille's Law governs the flow of viscous fluids.
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.
When you mix milk or sugar into your coffee, you should move the spoon quickly enough to produce turbulent flow around the spoon. Why does this turbulence aid mixing?
The turbulence causes the coffee to move quickly in random directions, aiding mixing.
How does running directly behind another runner reduce the wind resistance you experience?
The turbulent wake behind the other runner shields objects in that wake from the air flow around the other runner.
If you ride your bicycle directly behind a large truck, you will find that you don't have to pedal very hard to keep moving forward. Why?
The turbulent wake behind the truck shields objects in that wake from the air flow around the truck.
If you let a stream of water from a faucet flow rapidly over the curved bottom of a spoon, the spoon will be drawn into the stream. Explain this effect.
The water flowing over the spoon moves faster and so is at lower pressure than the water flowing on the other side.
If you try to fill a bucket by holding it in a waterfall, you will find the bucket pushed downward with enormous force. How does the falling water exert such a huge downward force on the bucket?
The water goes from high speed and low pressure to low speed and high pressure when it hits the bucket.
When you swing a stick slowly through the air, it's silent, but when you swing it quickly, you hear a "whoosh" sound. What behavior of the air is creating that noise?
There is turbulence behind the rapidly-moving stick.
In 1971, astronaut Alan Shepard hit a golf ball on the moon. How did the absence of air affect the ball's flight?
There was zero drag on the ball, only the moon's gravity.
Racing bicycles often have smooth, disk-shaped covers over the spokes of their wheels. Why would these thin wire spokes be a problem for a fast-moving bicycle?
They would create turbulence and increase drag.
If you start two identical paper boats from the same point, you can make them follow the same path down a quiet stream. Why can't you do the same on a brook that contains eddies and vortices?
Turbulent water flows in unpredictacle directions. Eddies and vortices mean turbulence is present.
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, resulting in a water hammer, but when the valve opens the flow does not instantly reach full speed.
If you put your hand out the window of a moving car, so that your palm is mostly forward but tipped slightly downward, the force on your hand will be both backward and upward. How is the airstream exerting an upward force on your hand?
Your hand pushes the airflow downward, and as a reaction the airflow pushes your hand upward.
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
causes the air to flow more rapidly around the right side of the ball than around the left side of the ball.
As the Reynolds number increases,
flow is going from being viscous dominated to inertia dominated and therefore turbulent.
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.
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
pressure has increased but its kinetic energy has remained the same.
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
shifts upward slightly above the normal stream level.
The pressure in a water pipe is always lower in regions where the water moves faster only when
the pipe is level.
The pressure in a water pipe may not necessarily be lower in regions where the water moves faster when
the pipe is not level
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
the pressure drag on a car increases dramatically as the car's speed increases.
The reason fast-moving water makes noise is
turbulence.
