How Everything Works
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MLA Citation: Bloomfield, Louis A. "How Everything Works" How Everything Works 19 Jun 2018. Page 132 of 160. 19 Jun 2018 <http://www.howeverythingworks.org/prints.php?topic=all&page=132>.
1311. Lunar gravity is partly what causes oceanic currents. If we had more than one moon orbiting Earth, what [if anything], would happen to the oceans? — MS, St. Charles, Missouri
While the moon's gravity is the major cause of tides (the sun plays a secondary role), the moon's gravity isn't directly responsible for any true currents. Basically, water on the earth's surface swells up into two bulges: one on the side of the earth nearest the moon and one on the side farthest from the moon. As the earth turns, these bulges move across its surface and this movement is responsible for the tides.

If there were more than one moon, the tidal bulges would become misshapen. That is essentially what happens because of the sun. As the moon and sun adopt different arrangements around the earth, the strengths of the tides vary. The strongest tides (spring tides) occur when the moon and sun are on the same or opposite sides of the earth. The weakest tides (neap tides) occur when the moon and sun are at 90° from one another. Extra moons would probably just complicate this situation so that the strengths of the tides would vary erratically as the moons shifted their positions around the earth. Since the timing of the tides is still basically determined by the earth's rotation, there would still be approximately 2 highs and 2 lows a day.


1312. Is it possible to track a person based on the fact that they are listening to a radio receiver? — BRAR, India
While tracking a radio transmitter is easy—you only need to follow the radio waves back to their source—you might think that tracking a radio receiver is impossible. After all, a radio receiver appears to be a passive device that collects radio waves rather than emitting them. But that's not entirely true. Sophisticated radio receivers often use heterodyne techniques in which the signal from a local radio-frequency oscillator is mixed with the signal coming from the antenna. The mixing process subtracts one frequency from the other so that antenna signals from a particular radio station are shifted downward in frequency into the range the radio uses to create sound. This mixing process allows the radio receiver to be very selective about which station it receives. The receiver can easily distinguish the station that's nearest in frequency to its local oscillator from all the other stations, just as its easy to tell which note on a piano is closest in pitch to a particular tuning fork.

But heterodyne techniques have a side effect: they cause the radio receiver to emit radio waves. These waves originate with the local radio-frequency oscillator, and with other internal mixing frequencies such as the intermediate frequency oscillator present in many sophisticated receivers. Because these oscillators don't use very much power, the waves they emit aren't very strong. Nonetheless, they can be detected, particularly at short range. For example, it's possible for police to detect a radar detector that contains its own local microwave oscillator. Similarly, people who have tried to pirate microwave transmissions have been caught because of the microwaves emitted from their receivers. In WWII, the Japanese were apparently very successful at locating US forces by detecting the 455 kHz intermediate frequency oscillators in their radios—a problem that quickly led to a redesign of the radios to prevent that 455 kHz signal from leaking onto the antennas (thanks to Tom Skinner for pointing this out to me). As you can see, it is possible to track someone who is listening to the right type of radio receiver. However, the radio waves from that receiver are going to be very weak and you won't be able to follow them from a great distance.


1313. What is an event horizon? — KRH
An event horizon is the surface around a black hole from which not even light can escape. But to make it clearer what that statement means, consider first what happens to the light from a flashlight that's resting on the surface of a large planet. Light is affected by gravity—it falls just like everything else. The reason you never notice this fact is that light travels so fast that it doesn't have time to fall very far. But suppose that the gravity on the planet is extremely strong. If the flashlight is aimed horizontally, the light will fall and arc downward just enough that it will hit the surface of the planet before escaping into space. To get the light to leave the planet, the flashlight must be tipped a little above horizontal.

If the planet's gravity is even stronger, the flashlight will have to be tipped even more above horizontal. In fact, if the gravity is sufficiently strong, light can only avoid hitting the planet if the flashlight is aimed almost straight up. And beyond a certain strength of gravity, even pointing the flashlight straight up won't keep the light from hitting the planet's surface.

When that situation occurs, an event horizon forms around the planet and forever separates the planet from the universe around it. Actually, the planet ceases to exist as a complex object and is reduced to its most basic characteristics: mass, electric charge, and angular momentum. The planet becomes a black hole. and light emitted at or within this black hole's event horizon falls inward so strongly that it doesn't escape. Since nothing can move faster than light, nothing else can escape from the black hole's event horizon either.

The nature of space and time at the event horizon are quite complicated and counter-intuitive. For example, an object dropped into a black hole will appear to spread out on the event horizon without ever entering it. That's because, to an outside observer, time slows down in the vicinity of the event horizon. By that, I mean that it takes an infinite amount of our time for an object to fall through that event horizon. But the object itself doesn't experience a change in the flow of time. For it, time passes normally and it zips right through the event horizon.

Finally, event horizons and the black holes that have them aren't truly black—quantum mechanical fluctuations at the event horizon allow black holes to emit particles and radiation. This "Hawking radiation," discovered by Stephen Hawking about 25 years ago, means that black holes aren't truly black. Nonetheless, objects that fall into an event horizon never leave intact.


1314. If you use a heavier racket, will you be able to hit a badminton birdie farther? — J, California
Any time you hit an object with a racket or bat, there's a question about how heavy the racket or bat should be for maximum distance. Actually, it isn't weight that's most important in a racket or bat, it's mass—the measure of the racket or bat's inertia. The more massive a racket or bat is, the more inertia it has and the less it slows down when it collides with something else. A more massive racket will slow less when it hits a birdie. From that observation, you might think that larger mass is always better. But a more massive racket or bat is also harder to swing because of its increased inertia.

So there are trade offs in racket or bat mass. For badminton, the birdie has so little mass that it barely slows the racket when the two collide. Increasing the racket's mass would allow it to hit the birdie slightly farther, but only if you continued to swing the racket as fast as before. Since increasing the racket mass will make it harder to swing, it's probably not worthwhile. In all likelihood, people have experimented with racket masses and have determined that the standard mass is just about optimal for the game.


1315. Why does a badminton birdie have such a large tip? Does making it bigger protect the racket? — J, California
The large, rounded head of a badminton birdie serves at least two purposes: it makes sure that the birdie bounces predictably off the racket's string mesh and it protects the strings and birdie from damage. If the birdie's head were smaller, it would strike at most a small area on one of the racket strings. If it hit that string squarely, the birdie might bounce predictably. But if it hit at a glancing angle, the birdie would bounce off at a sharp angle. By spreading out the contact between the birdie and the string mesh, the large head makes the birdie bounce as though it had hit a solid surface rather than one with holes.

Spreading out the contact also prevents damage to the racket and birdie. If they collided over only a tiny area, the forces they exerted on one another would be concentrated over that area and produce enormous local pressures. These pressures could cut the birdie or break a string. But with the birdie's large head, the pressures involved are mild and nothing breaks.


1316. Is there any equipment that can track people in a large, dense forest? — BRAR, India
To track someone in a forest, he must be emitting or reflecting something toward you and doing it in a way that is different from his surroundings. For example, if he is talking in a quiet forest, you can track him by his sound emissions. Or if he is exposed to sunlight in green surroundings, you can track him by his reflections of light.

But while both of these techniques work fine at short distances, they aren't so good at large distances in a dense forest. A better scheme is to look for his thermal radiation. All objects emit thermal radiation to some extent and the spectral character of this thermal radiation depends principally on the temperatures of the objects. If the person is hotter than his surroundings, as is almost always the case, he will emit a different spectrum of thermal radiation than his surrounds. Light sensors that operate in the deep infrared can detect a person's thermal radiation and distinguish it from that of his cooler surroundings. Still, viewing that thermal radiation requires a direct line-of-sight from the person to the infrared sensor, so if the forest is too dense, the person is untrackable.


1317. How does a dead ball work? — DIY, Lyon, France
A dead ball, a ball that doesn't bounce, is one with enormous internal friction. A bouncy ball stores energy when it collides with a surface and then returns this energy when it rebounds. But no ball is perfectly elastic, so some of the collision energy extracted from the ball and surface when they collide is ultimately converted into heat rather than being returned during the rebound. The deader the ball is, the less of the collision energy is returned as rebound energy. A truly dead ball converts all of the collision energy into heat so that it doesn't rebound at all.

Most of the missing collision energy is lost because of sliding friction within the ball. Molecules move across one another as the ball's surface dents inward and these molecules rub. This rubbing produces heat and diminishes the elastic potential energy stored in the ball. When the ball subsequently undents, there just isn't as much stored energy available for a strong rebound. The classic dead "ball" is a beanbag. When you throw a beanbag at a wall, it doesn't rebound because all of its energy is lost through sliding friction between the beans as the beanbag dents.


1318. Is bouncing related to elasticity or hardness? Can a hard body rebound? — DIY, Lyon, France
Bouncing is related to elasticity. Any object that stores energy when deformed will rebound when it collides with a rigid surface. As long as the object is elastic, it doesn't matter whether it's hard or soft. It will still rebound from a rigid surface. Thus both a rubber ball and a steel marble will rebound strongly when you drop them on a steel anvil.

But hardness does have an important effect on bouncing from a non-rigid surface. When a hard object collides with a non-rigid surface, the surface does some or all of the deforming so that the surface becomes involved in the energy storage and bounce. If the surface is elastic, storing energy well when it deforms, then it will make the object rebound strongly. That's what happens when a steel marble collides with a rubber block. However, if the surface isn't very elastic, then the object will not rebound much. That's what happens when a steel marble collides with a thick woolen carpet.


1319. Why does a roller coaster end on a lower level than where it starts? — L, Staten Island, New York
A roller coaster is a gravity-powered train. Since it has no engine or other means of propulsion, it relies on energy stored in the force of gravity to make it move. This energy, known as "gravitational potential energy," exists because separating the roller coaster from the earth requires work—they have to be pulled apart to separate them. Since energy is a conserved quantity, meaning that it can't be created or destroyed, energy invested in the roller coaster by pulling it away from the earth doesn't disappear. It becomes stored energy: gravitational potential energy. The higher the roller coaster is above the earth's surface, the more gravitational potential energy it has.

Since the top of the first hill is the highest point on the track, it's also the point at which the roller coaster's gravitational potential energy is greatest. Moreover, as the roller coaster passes over the top of the first hill, its total energy is greatest. Most of that total energy is gravitational potential energy but a small amount is kinetic energy, the energy of motion.

From that point on, the roller coaster does two things with its energy. First, it begins to transform that energy from one form to another—from gravitational potential energy to kinetic energy and from kinetic energy to gravitational potential energy, back and forth. Second, it begins to transfer some of its energy to its environment, mostly in the form of heat and sound. Each time the roller coaster goes downhill, its gravitational potential energy decreases and its kinetic energy increases. Each time the roller coaster goes uphill, its kinetic energy decreases and its gravitational potential energy increases. But each transfer of energy isn't complete because some of the energy is lost to heat and sound. Because of this lost energy, the roller coaster can't return to its original height after coasting down hill. That's why each successive hill must be lower than the previous hill. Eventually the roller coaster has lost so much of its original total energy that the ride must end. With so little total energy left, the roller coaster can't have much gravitational potential energy and must be much lower than the top of the first hill.

It's then time for the riders to get off, new riders to board, and for a motor-driven chain to drag the roller coaster back to the top of the hill to start the process again. The chain does work on the roller coaster, investing energy into it so that it can carry its riders along the track at break-neck speed again. Overall, energy enters the roller coaster by way of the chain and leaves the roller coaster as heat and sound. In the interim, it goes back and forth between gravitational potential energy and kinetic energy as the roller coaster goes up and down the hills.


1320. My daughter did a school project in which we placed a thermometer inside cloths of various colors. Black cloth showed the highest temperature, blue next, then red, and finally white. Why is that?
Since light carries energy with it, a cloth that absorbs light also absorbs energy. In most cases, this absorbed energy becomes thermal energy in the cloth. Because of this extra thermal energy, the cloth's temperature rises and it begins to transfer the thermal energy to its surroundings as heat. Its temperature stops rising when the thermal energy it receives from the light is exactly equal to the thermal energy it transfers to its surroundings as heat. This final temperature depends on how much light it absorbs—if it absorbs lots of light, then it will reach a high temperature before the balance of energy flow sets in.

A cloth's color is determined by how it absorbs and emits light. Black cloth absorbs essentially all light that hits it, which is why its temperature rises so much. White cloth absorbs virtually no light, which is why it remains cool. Colored cloths fall somewhere in between black and white. Blue cloth absorbs light in the green and red portions of the spectrum while reflecting the blue portion. Red cloth absorbs light in the blue and green portions of the spectrum while reflecting the red portion. Since most light sources put more energy in the red portion of the spectrum than in the blue portion of the spectrum, the blue cloth absorbs more energy than the red cloth. So the sequence of temperatures you observed is the one you should expect to observe.

One final note: most light sources also emit invisible infrared light, which also carries energy. Most of the light from an incandescent lamp is infrared. You can't tell by looking at a piece of cloth how much infrared light it absorbs and how much it reflects. Nonetheless, infrared light affects the cloth's temperature. A piece of white cloth that absorbs infrared light may become surprisingly hot and a piece of black cloth that reflects infrared light may not become as hot as you would expect.


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