## How does fuzz on a tennis ball make it fly faster? It seems counterintuitive to …

#### How does fuzz on a tennis ball make it fly faster? It seems counterintuitive to me.

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.

## I am a little confused on how air pressure can drop as air flows through a small…

#### 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?

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.

## I understand how dimples on a golf ball reduce pressure drag, but wouldn’t they …

#### 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).

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).

## If you put a light plastic ball in the stream of air rushing upward from an open…

#### 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?

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.

## In a sealed car driving down the road, when would you have the lowest pressure o…

#### 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?

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.

## Is pressure drag the same thing as “air resistance”?

#### Is pressure drag the same thing as “air resistance”?

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.

## My big square truck creates a lot of turbulence when it moves. Does my roof rack…

#### My big square truck creates a lot of turbulence when it moves. Does my roof rack (a factory-installed one, close to the roof) actually improve aerodynamics, like fuzz on a tennis ball? (Also, what about the air dam at the back end?)

I’m sure that modern car designers consider aerodynamics when building a car or truck. They do structure the trailing edge of the car to minimize its turbulent wake. But I doubt that a roof rack helps much. It’s probably too tall for the boundary layer on the car and extends into the free flowing stream beyond. As a result, it probably experiences its own pressure drag. The “fuzz” that trips the boundary layer has to be no taller than the boundary layer itself, otherwise it causes turbulence in the main airstream rather than preventing it. The same goes for the air dam.

## Please explain what “lift” is.

#### Please explain what “lift” is.

Suppose that a horizontal wind is approaching a smooth, stationary ball from the right. The ball will experience a drag force that pushes it toward the left. We call it a drag force because it acts to slow the ball’s motion through the air—in other words because it pushes the ball directly downwind. But if the ball isn’t uniform or if the ball is spinning, it may experience a force that isn’t directly downwind. If the ball experiences an aerodynamic force (a force due to the motion of the wind near its surface) that pushes it to the side, or that pushes it up or down, then it is experiencing a lift force. This lift force isn’t necessary up…it’s just to the side—at right angles to the downwind direction.

## Does the decreased density of the air in Denver make it easier to achieve turbul…

#### 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?

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!