How does a tsunami form and how far does it go when it hits land?

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How does a tsunami form and how far does it go when it hits land? — JM, Berkley, MI

A tsunami is simply a giant surface wave on water. Surface waves have several important characteristics, one of which is wavelength—that is, the distance between one crest and the next. The longer its wavelength, the faster a surface wave moves and also the deeper it extends below the surface of the water. In general a surface wave extends downward about one wavelength, so that if the crests are 100 meters apart, the wave is about 100 meters deep.

The wavelength of a tsunami is enormous—hundreds or even thousands of meters. As a result, a tsunami travels hundreds of kilometers per hour and extends downward deep into the ocean. Because it disturbs so much water, it carries a great deal of energy and it delivers this energy to the shore when it hits. Tsunamis are normally created by earthquakes or volcanic eruptions that sudden shift the supporting surfaces of a large amount of water. The water experiences a sudden impulse when the land or seabed shifts and a wave is emitted. You can launch a similar wave simply by shaking the end of a basin of water. But when a large region of land or seabed moves, the wave that’s launched has a very long wavelength and tremendous energy. This tsunami heads off with enormous speed until it encounters the gradual shallowing of a seashore. There it becomes deformed because the lack of water in front of it causes its crest to become incomplete. Eventually the tsunami breaks in churning surf. The height of this breaking wave crest and the distance it travels onto shore before it stops depends on the total energy of the tsunami, but heights of 10 or 20 meters are not uncommon. Such waves can travel hundreds of meters up a beach or oceanfront if the slope is sufficiently gradual.

Why does white noise cancel out the wide range of frequencies in the real world?…

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Why does white noise cancel out the wide range of frequencies in the real world? What range of frequencies does this technology affect? Can you block out the low thud of a neighbor walking in the unit above yours? — EH, Chelmsford, MA

In the context of sound, a source of white noise emits random, non-repetitive sound waves that have equal acoustic powers at all frequencies. That means that the source emits the same amount of energy each second at each frequency, over the entire audible spectrum. What white noise does is to numb your hearing by creating a featureless, uniform background noise at every frequency you can hear. Since your sensitivity to sound volume is logarithmic, meaning that the acoustic power in a sound has to double before you notice that it’s substantially louder, this uniform background makes it extremely difficult for you to hear small sounds. Regardless of a small sound’s frequencies, the white noise is already exposing your ears to those frequencies and the small sound only makes a small change in the volumes of these frequencies. For an analogy, think about how much more you would notice a small blinking red light in the dark than in bright white sunlight. Similarly, white noise creates the acoustic equivalent of white illumination, making it hard for you to notice small noises that would be very easy to hear against complete silence. If the sounds your neighbor makes are small enough, this numbing effect should make them much less noticeable.

There are also much more sophisticated devices that really cancel noise out. However, these look like earphones and must be worn directly on your ears. These devices use microphones to measure the pressure fluctuations in the sounds and then cause the earphones to create exactly the opposite pressure fluctuations. With these noise cancellation devices properly adjusted, the air pressure fluctuations that are sound never reach your ears at all—they are simply cancelled away to nothing before they arrive.

How do glaciers flow downhill?

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How do glaciers flow downhill? — LO and NB, Bothell, WA

Ice is a rather soft material and its crystals can deform permanently when exposed to sufficient stress. If you squeeze ice hard enough, its crystals will gradually change shape in much the same way that a copper penny will change shape if you squeeze it in a press. Since the pressures at the base of a glacier are enormous, the ice crystals there gradually deform to relieve the stress they’re experiencing. This slow deformation allows the whole structure of the glacier to move gradually downhill. If ice crystals were harder, like those in most rocks, glaciers wouldn’t flow. But they are very soft and so the glacier slowly flows downhill.

How does a three-way light bulb work? – AER

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How does a three-way light bulb work? – AER

A three-way light bulb has two filaments inside it. One filament is smaller than the other, consuming less electricity and emitting less light. At the low light setting, only the smaller filament has current running through it and the bulb emits a dim light. At the medium light setting, only the larger filament has current running through it and the bulb emits a medium light. At the high light setting, both filaments have currents running through them and the bulb emits a bright light. To control the two filaments, the bulb has three electrical connections. The two filaments share one of the connection and each has one additional connection of its own. A complicated switch in the lamp determines whether to deliver current to one filament or the other or both. In each case, current flows toward the filament through one connection and returns from the filament through the other connection.

I’ve heard two explanations for how flight is achieved: (A) through lift generat…

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I’ve heard two explanations for how flight is achieved: (A) through lift generated by differential pressure (Bernoulli’s effect) and (B) through elastic collisions between air molecules and the underside of a wing. Which is correct? How does the fact that planes can fly upside down enter into this picture?

Explanation A is entirely correct and explanation B is partly correct. If you extended explanation B to include all collisions between air molecules and the entire wing, then it would also be correct. Explanation A is the continuous fluid picture of flight and the revised explanation B is the granular fluid picture of flight. To the extent that gases are incompressible fluids (as required for Bernoulli’s equation to be completely valid), these two explanations are essentially equivalent.

The lift experienced by a plane’s wing depends on its shape and on its tilt or “angle of attack” into the wind. In general, wings are airfoils—curved shapes that are designed to obtain significant lift forces while experiencing minimal drag forces. Most airplane wings are more highly curved on their tops than on their bottoms and obtain upward lift forces as a result. These lift forces occur because the stable airflow that forms around such a wing involves faster-moving and thus lower-pressure air above the wing than beneath it. However, some airplane wings are symmetric—they have equal curvatures on top and on bottom. These symmetric wings compensate for their symmetry by attacking the air at an angle. When they are tipped so that their leading edges are higher than their trailing edges, these wings also experience upward lift forces. The air again flows more rapidly over than under the wings and the pressure is lower above the wings than beneath them. Even an inverted non-symmetric wing can adjust its angle of attack to obtain an upward lift force, which is how a plane can fly upside down.

In all of these cases, the forces are really exerted on the plane’s wings by the impacts of countless air molecules. These air molecules hit harder and more often beneath the wings than above them and thus exert a net upward force on the plane. The fact that some wings have more surface area on their highly curved tops doesn’t lead to larger downward forces because many of the collision forces exerted by molecules on the top surface of the wing cancel one another, in the same way that forces exerted on opposite sides of a sheet of paper cancel one another.

Why are there dimples on golf balls? – DM

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Why are there dimples on golf balls? – DM

If there were no turbulence around a golf ball as it moved through the air, there would be regions of slow-moving high-pressure air in front of it and behind it, and regions of fast-moving low-pressure air around its sides. Because of their symmetry, these pressures wouldn’t exert any overall force on the golf ball and it would fly through the air without experiencing any air resistance. But there is turbulence behind a moving golf ball and this turbulence spoils the high-pressure region behind the ball. Since there is less high-pressure behind the golf ball to push it forward, the ball experiences a backward force—the slowing force of pressure drag. The size of this pressure drag force is roughly proportional to the size of the turbulent wake.

The size of the turbulent wake depends on the airflow behind the ball. On a smooth ball, air flowing into the rising pressure behind the ball experiences friction with the ball’s surface and loses energy. This surface air soon reverses its direction of flow, triggering a large turbulent wake. A golf ball’s dimples complicate the airflow very near the ball’s surface so that new, rapidly moving air is able to flow in close to the ball’s rear surface, where it can delay the onset of the flow reversal. The turbulent wake that eventually forms is relatively small, so that the golf ball experiences less pressure drag than a smooth ball. That’s why a golf ball can travel so far before slowing down.

What is the difference between the magnetic and electric ballasts used in fluore…

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What is the difference between the magnetic and electric ballasts used in fluorescent lights?

Fluorescent lights work by sending an electric current through a vapor of mercury atoms in what is known as an electric discharge. Unfortunately, electric discharges are very unstable—they are hard to start and, once started, tend to draw more and more current until they overheat and damage their containers and power sources. Thus a fluorescent light needs some device to control the flow of current through its discharge. Since normal fluorescent lamps are powered by alternating current—that is, the current passing through the discharge stops briefly and then reverses direction 120 times each second in the United States and 100 times each second in many other countries (60 or 50 full cycles of reversal, over and back, each second respectively)—the current control device only needs to keep the current under control for about 1/120 of a second. After that the current will reverse and everything will start over.

Older style fluorescent lights use a magnetic ballast to control the current. This ballast consists essentially of a coil of wire around a core of iron. As current flows through the wire, it magnetizes the iron. Because energy is required to magnetize the iron, the presence of the iron inside the coil of wire slows down the current when it first appears in the wire by drawing energy out of that current. This effect, typical of devices known to scientists and engineers as “inductors”, prevents the current passing through the ballast and then through the discharge from increasing too rapidly once it starts. The magnetic ballast is able to slow the current rise through the fluorescent lamp long enough for the alternating current to begin reversing directions. In fact, as the current in the power line begins to reverse, the ballast begins to get rid of the energy stored in its magnetized core. This energy is used to keep the discharge going longer than it would on its own. The ballast thus smoothes out the discharge so that it stays under control and emits an almost steady amount of light.

Modern electronic ballasts still control the current through the discharge, but they use electronic components to achieve this control. Just as an electronic dimmer switch can control the current through an incandescent light bulb in order to adjust the bulb’s brightness, such electronic devices can control the current passing through the discharge in a fluorescent lamp to keep that current from growing dangerously large.

Can the light from a fluorescent lamp be collimated into a beam of parallel rays…

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Can the light from a fluorescent lamp be collimated into a beam of parallel rays?

While a converging lens or a concave mirror can always direct light from a bright source in a particular direction, the degree of collimation (the extent to which the rays become parallel) depends on how large the light source is. The smaller the light source, the better the collimation. Spotlights and movie projects use extremely bright, very small light sources to create their highly collimated beams. Since fluorescent lamps tend to be rather large and have modest surface brightnesses, I’m afraid that you would be disappointed with the best beam that you could create from that light. The ultimate collimated light source is a laser beam. In effect, the identical photons of light in a laser beam all originate from the same point in space, so that the collimated beam is as close to perfectly collimated as the nature of light waves will allow.

Does light have mass? If so, then how can it travel at the speed of light? Doesn…

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Does light have mass? If so, then how can it travel at the speed of light? Doesn’t the mass of an object (particle) approach infinity as its velocity approaches the speed of light?

Light has precisely zero mass and that makes all the difference. You’re right that taking a massive particle up to the speed of light is impossible because doing so would, in a certain sense, give the particle an infinite mass. But the more important issue here is that doing so would require an infinite amount of energy and momentum.

Most physicists use the word mass to mean a particle’s mass at rest—its rest mass—and as you bring the particle to higher and higher speeds, its rest mass doesn’t change. However, the relationship between the particle’s energy and its momentum does change with speed and the particle’s momentum begins to increase more rapidly than it should according to the older, pre-relativistic mechanical theories. In an effort to explain this anomalous increase in momentum while retaining the old Newtonian laws of motion, people sometimes assign a fictitious “mass” to the particle; one that equals the rest mass when the particle is stationary but that increases as the particle’s speed increases. As a particle approaches the speed of light, its momentum increases without limit and so does its “mass.” Not surprisingly, the limitless rises in energy, momentum, and “mass” prevent the massive particle from ever reaching the speed of light.

As for light, it really does have zero mass and therefore can’t be described by the Newtonian laws of motion. All light has is its momentum and its energy. In fact, light can’t travel slower than the speed of light because that would require it to have a mass! So the world of particles is divided into two groups: massless particles that must travel at the speed of light and massive particles that can never travel at the speed of light.

Who invented the microwave oven and how did he think of it?

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Who invented the microwave oven and how did he think of it?

In 1945, American engineer Percy Le Baron Spencer was working with radar equipment at Raytheon and noticed that some candy he had in his pocket had melted. Radar equipment detects objects by bouncing microwaves from them and Spencer realized that it was these microwaves that had heated the candy (as well as his body…oops!). Raytheon soon realized the potential of Spencer’s discovery and began to produce the first microwave ovens: Radaranges. These early devices were large and expensive and it wasn’t until 1967, when Amana, a subsidiary of Raytheon, produced the first household microwave oven, that microwave ovens became widely available.