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Balls, Birdies, and Frisbees
Page 1 of 2 (19 Questions and Answers)

138. Does the decreased density of the air in Denver make it easier to achieve turbulent flow at the boundary layer of a baseball and therefore make the ball fly farther?
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Whew, this is a toughie. The air in Denver is less dense, so it tends to respond better to viscous forces. On that account, it would tend to be less turbulent. But it is also "thinner" and less viscous, so it would tend to be more turbulent. I think that those two effects essentially cancel, so that the ball experiences the same degree of turbulence at any altitude. However, the air in Denver has less pressure, so it exerts smaller forces on the ball than air at sea level. Thus, although the flow properties aren't affected by the increased altitude, the pressures involved are. The ball should certainly carry farther in Denver than at sea level. Imagine playing on the moon, where there's no air at all. The ball wouldn't experience any drag at all!

139. Does the design of balls (pentagons on soccer balls, lines on basketballs, panels on volleyballs, etc.) have a purpose or are they merely there for design?
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In most cases they are simply design. However, they do affect the flow of air over the ball and will change its motion. The classic examples of balls with designs that matter are golf balls and baseballs. A golf ball has dimples because they dramatically change the airflow over the ball and allow it to travel much farther. A baseball's stitching also affects its flight from the pitcher to the mound and is very important to pitches like the knuckle ball and the spitball.

140. How does a boomerang work?
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The correct way to throw a boomerang is overhand and, unlike a Frisbee, in a nearly vertical plane. (Usually the ideal angle is about 15° from vertical.) The boomerang is essentially a rotating airplane wing, and its shape produces lift using the Bernoulli effect in the same way an airplane wing does. But when it is thrown, notice that the top blade of the boomerang is moving faster through the air than the bottom blade, because of the rotation. This results in there being more lift on the top blade than on the bottom. From a right-handed thrower's perspective, there is a lift up and to the left, more so at the top than at the bottom. The upward lift is what keeps the boomerang in the air. You might think the leftward twist flips the boomerang over, but wait! The boomerang is also a flying gyroscope. Leaning the gyroscopic boomerang over results in its turning to the left, much the same way that leaning a moving bicycle leftward toward the horizontal causes the front wheel to turn and not fall over. (This is also why spinning tops start to slowly turn their axis of rotation when they lean, a process called "precession".) The boomerang doesn't flip over, but instead turns its axis of rotation around in a large horizontal circle, and it comes back to you.

After a moment's thought, you might wonder whether helicopters suffer the same effect. (How would a boomerang fly if thrown in a horizontal plane?) In fact, they do, and there is a tendency to pitch the helicopter upward (tip the nose up) precisely from this same effect, which the pilot instinctively corrects for.

(Thanks to Prof. Paul Draper, from the Physics Department of the University of Texas at Arlington, for writing this explanation.)


141. How does a Frisbee fly?
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As you begin to move a Frisbee forward, the air in front of the Frisbee splits to flow either over the Frisbee or under it. Because of the Frisbee's shape and the angle at which it's held, the air that flows over the Frisbee has a longer distance to travel and arrives late at the back of the Frisbee. The air flowing under the Frisbee reaches the back first and initially flows upward, around the rear surface of the Frisbee. But once the Frisbee is moving fairly rapidly, this funny upward-flowing tail of air blows away from the back of the Frisbee. As it leaves, it draws the air flowing over the Frisbee with it and speeds that air up. As a result, the air over the Frisbee travels faster than the air under the Frisbee. But the airs above and below the Frisbee have the same amounts of total energy per gram. Since the faster moving air above the Frisbee has more kinetic energy than the slower moving air below the Frisbee, the air above the Frisbee must have less of some other form of energy than the air below the Frisbee. In fact, the air above the Frisbee has less pressure potential energy than the air below it—the air pressure above the Frisbee is less than that below the Frisbee. And since the pressure pushing on the bottom surface of the Frisbee is greater than the pressure pushing on the top surface of the Frisbee, there is a net upward pressure force on the Frisbee. This upward pressure force balances the downward weight of the Frisbee and keeps the Frisbee from falling.

142. How does fuzz on a tennis ball make it fly faster? It seems counterintuitive to me.
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Yes, it is counterintuitive. The reason for the fuzz is that a swirling layer of air close to the ball makes a good buffer between the ball and the main airstream. As a result, the main airstream flows most of the way around the ball before it breaks away as a turbulent wake. Without the swirling layer of air on the ball's surface, the main airstream encounters backward-flowing air near the ball's surface and breaks away from the ball early, leaving a larger turbulent wake.

143. I am a little confused on how air pressure can drop as air flows through a small hole. I understand that speed and pressure are inversely proportional but aren't volume and pressure inversely proportional as well?
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If you squeeze air into a small space, its pressure will rise (yes, pressure and volume of stationary air are inversely proportional). But flowing air is a different story. When an airstream passes through a narrow channel, it must speed up and when it does, its pressure drops. That's aerodynamics, not statics.

144. I understand how dimples on a golf ball reduce pressure drag, but wouldn't they also increase viscous drag? (i.e. a rougher surface experiences more "friction" from the air).
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Yes, the dimpled golf ball probably does experience more viscous drag than a smooth golf ball. But viscous drag is only a small fraction of the total drag on the ball. Pressure drag is much more important (and much larger).

145. If you put a light plastic ball in the stream of air rushing upward from an open air hose, the ball will float in the airstream. How does this trick work?
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In this trick, the airstream is rather narrow. When the ball is centered in the stream, air flows around all sides of the ball. But when the ball drifts off center, the airstream flows mainly on the side of the ball nearest the airstream. That air still has to flow around the sides of the ball and speeds up as it does. The air pressure there drops. The result is that the pressure drops on the side of the ball closest to the airstream and the ball is pushed back toward the airstream.

146. In a sealed car driving down the road, when would you have the lowest pressure outside the car: when a window was just a little open, or all the way open? Or would the overall pressure be constant once the window was opened at all?
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The pressure outside the closed front windows of a moving car is lower than atmospheric pressure because the air flowing past the car is moving particularly fast as it arcs around the front portions of the car. When you open the front windows of the car slightly, you don't disturb this airflow very much, but you allow air from inside the car to flow outward toward the low-pressure air passing the windows. As a result, the air pressure inside the car drops below atmospheric pressure and you may feel your ears "pop." But if you open the windows wide, the air flowing around the car will probably be seriously disturbed and the low-pressure regions may vanish. As a result, the air pressure inside the car will probably be about atmospheric. However, there are times when the airflow past an open window becomes unstable and the moving air can actually fluctuate in direction, so that it's deflected in and out of the window. When that happens, the whole car begins to act like a giant whistle and you feel the air pressure inside it rise and fall rhythmically. This oscillation is irritating to your ears.

147. Is pressure drag the same thing as "air resistance"?
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Yes. The air resistance you experience when you bicycle into the wind or hold your hand out the window of a car or jump from a plane with a parachute on is just pressure drag. In each of these cases, the air flowing around you slows in front of you (so that its pressure rises), speeds up on the sides of you (so that its pressure drops), and then becomes turbulent behind you (so that its pressure hovers near atmospheric pressure). With more pressure in front of you than behind you, you experience a net force in the downwind direction...the force of pressure drag.

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