How Everything Works
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MLA Citation: Bloomfield, Louis A. "How Everything Works" How Everything Works 16 Jul 2018. Page 14 of 160. 16 Jul 2018 <http://www.howeverythingworks.org/prints.php?topic=all&page=14>.
131. How does a water aspirator pump work?
The water aspirator pump is essentially a pipe with a narrowing in it. As water flows through that narrowing, it speeds up and its pressure drops—it's exchanging its pressure energy for kinetic energy. A tiny opening in the side of the narrowing allows water or air to enter the high-speed flow. Since the pressure in that high-speed flow is very low, atmospheric pressure pushes fluids through the tiny opening and into the flow. The flow pumps fluids through the opening and into the water stream. If you connect a hose to the tiny opening, you can suck chemicals up the hose and into the water stream.

132. How does an airbrush work? Can you briefly explain it again.
In an airbrush, slow-moving but high-pressure air from a hose is allowed to pass through a very narrow channel. As the air enters this channel, it speeds up and its pressure drops—it has exchanged its pressure potential energy for kinetic energy. The channel is so narrow and the air moves so quickly through it that the pressure inside the channel drops below atmospheric pressure! There is a tiny pipe that attaches to this channel at right angles and that dips into a bottle of paint. As the pressure inside the channel falls below atmospheric pressure, the atmospheric pressure in the paint bottle pushes the paint toward the channel. The paint begins flowing into the channel and it collides with the high-speed stream of air. The paint is ripped into tiny droplets and these droplets travels through the channel along with the air. As the air emerges from the narrow channel, its pressure rises and it slows down, but it still moves fast enough to carry the paint droplets to the object that's being painted.

133. How does the fan in a vacuum cleaner boost the pressure back up so that the air flowing through the vacuum cleaner the air will go back into the room?
The fan is a rotating assembly of ramps. As the ramps move, they sweep the air from one side of the fan to the other and do work on that air. The air either accelerates as the fan blades spin past, or its pressure builds up. Either way, its total energy increases. The fan can take low-pressure air from one side and whisk it over to the other side where the pressure is higher. It can push air against the natural direction of flow (from high pressure to low pressure). It's essentially a pump for air.

134. Suppose that you fall out of a plane about 30 seconds after your parachute pack fell out. Is it really possible to catch up to your parachute pack and save yourself?
The answer depends on how high the plane was flying and just how much air resistance the pack experiences as it falls. After a few seconds of falling, an object reaches a terminal velocity—it stops accelerating downward. That's because the upward force that air resistance exerts on it grows stronger as its downward velocity increases. Eventually, the upward force it experiences exactly balances its downward weight and it has no net force on it—it doesn't accelerate. For a person, this terminal velocity ranges from about 100 mph to 200 mph, depending on the person's shape. Curling into a compact ball should allow you to reach a relatively high terminal velocity of 200 mph. Since the parachute pack is relatively light but has substantial surface area for the wind to push against, it probably has a lower terminal velocity of say, 100 mph. This arrangement would allow you to approach the pack at a relative velocity of 100 mph. In order to actually overtake the pack, you'll still need some time, so the higher the plane was when you started, the better your chances are. Since the pack has a 30 second head start and descends at 100 mph, it will be about 0.83 miles below you when you leave the plane. You'll catch up to it 30 seconds later, during which time you will have dropped a total of 1.67 miles. Thus in principle, you could catch the pack so long as the plane's altitude was more than about 1.67 miles. To allow time to put the pack on, for the parachute to open, and for your terminal velocity to then become low enough to avoid injury, you'd better have the plane at more than about 2.5 miles. Still, this doesn't sound like a fun experiment.

135. What is drag force?
A drag force is a force that opposes an object's motion through a fluid. Like sliding friction, drag always pushes the object in the direction opposite its motion though the fluid. Air resistance is really a drag force. You feel drag pushing you backward when you ride a bicycle fast. You also feel drag when you hold your hand out the window of a fast-moving car—it pushes your hand toward the back of the car and in the direction opposite your hand's motion through the air. If you were to fall downward, you would feel a drag force upward, in the direction opposite your motion through the air. And leaves experience a drag force when wind blows on them—pushing them downwind and in the direction opposite their motion through the air (they are moving upwind through the air, so it pushes them downwind). Incidentally, the object pushes back on the fluid with drag force, too, and this force on the fluid pushes the fluid in the direction opposite its motion past the object. This force tends to stop moving fluids and to turn their kinetic energies into thermal energy.

136. When you suspended the Ping-Pong ball in the stream of air from the pipe, why did the ball spin? The same thing happened to the two flat pieces of plastic that were held together when air flowed out between them.
The Ping-Pong ball spun because the viscous drag forces it experienced weren't equal on all sides. As we'll see shortly, there are a variety of different drag forces and they can act at different locations on an object. In the case of viscous drag, it acts locally at each point where air slides across the surface of the object. Since the airflow from the pipe wasn't perfectly uniform, the air swept past the ball faster in some places than it did in others. These differences in airspeed became most significant when the ball began to drift away from the airstream—the sudden increase in airspeed on the side of the ball nearest the center of the airstream is what created the low pressure that allowed the surrounding air to push the ball back toward the center of the airstream. But minor differences in airspeed also exerted unbalanced torques on the ball and caused it to spin. Similar flow imperfections between the two plates created differences in viscous drag and exerted torques on the two plates. That's why they began to spin around slightly.

137. Why does dust settle on the moving blades of a fan?
As the air flows across the blades of a fan, the dust particles in it occasionally pierce through the airflow and hit the blades. The same sort of process occurs when a bug hits the windshield of a car; the bug would normally follow the airflow but its inertia prevents it from moving out of the way quickly enough and it hit. Once a dust particle hits the fan blades, there isn't much to remove it. The air moves remarkably slowly right at the surface of the fan because that surface layer of air experiences lots of viscous drag. Even though the air is moving swiftly only a few millimeters away, the air right on the fan blade is almost stationary. Thus the dust can cling to the blade indefinitely.

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?
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?
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?
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.)


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