. Why is a satellite dish necessary to receive satellite broadcasts? Why doesn't a conventional radio antenna work? — AW, Karachi, Pakistan
Normal television broadcasts use electromagnetic waves with relatively low frequencies and long wavelengths while satellite broadcasts use waves with relatively high frequencies and short wavelengths. The short wavelength waves from a satellite are known as microwaves while the longer wavelength waves from a normal broadcast station are generally known as radio waves. Since the optimal antenna size for receiving a particular electromagnetic wave is proportional to the wavelength of the wave, you need a smaller antenna to receive the microwaves from a satellite than you do the radio waves from a normal television station. However, the microwaves from a satellite are much weaker than the radio waves from a nearby television station and a small microwave antenna isn't likely to absorb enough of them to produce a useable signal.
The solution to this dilemma is to concentrate the microwaves from a satellite with the help of an optical imaging system. Although it may not look like one, a satellite dish is really a carefully shaped mirror telescope. Just as the curved mirror of the Hubble space telescope can bring light from a distant star to a focus on an optical image sensor, so the curved wire mesh of a satellite dish can bring microwaves from a distant satellite to focus on a small microwave antenna. This microwave antenna sits at the focus of the satellite dish and absorbs the microwaves that the dish collects. The dish's imaging behavior also ensures that microwaves from only one satellite are brought to a focus on the microwave antenna. You must redirect the dish or move the antenna in order to switch from one satellite to another.
. What is gravity? We know Newton's formula but he did not answer what the true nature of gravity is. I hear talk about "gravitons" — is this real or just another elegant metaphor? — BC
Newton's gravity has been superceded by Einstein's gravity; the gravity of general relativity. In this understanding of gravity, the accelerations associated with gravity result from a curvature of space/time around concentrations of mass & energy. The gravity of general relativity is responsible for such exotic effects as the bending of light by gravity and the existence of black holes.
But physicists are still not satisfied with the gravity of general relativity. General relativity is what's known as a "classical" theory of interactions—it does not include quantum physics and is thus considered to be incomplete. All the other classical theories of interactions have given way to quantum theories. For example, the classical theory of electromagnetic interactions, dating from the works of Oersted, Ampere, Maxwell and others in the 1800's, was replaced in the 1940's and 50's by quantum electrodynamics, through the works of Feynman, Schwinger, Tomonaga, and others. Each time that a classical theory is replaced by a quantum theory, the responsibility for the interactions themselves shifts from classical fields (e.g., the electric and magnetic fields) to quantized or particulate fields (e.g., photons). These sorts of quantum field theories, theories in which interactions between particles are mediated by the exchanges of other particles (the particles of the quantized fields) are the bases for all modern interaction theories except gravity itself. People are still trying to quantize gravity but so far without real success. The particles that mediate gravitational interactions have been named gravitons, but the full theory in which these particles operate is still uncertain.
. What metals and other substances are used in microwave ovens? Specifically, what is the substance on the inside of the microwave that absorbs all the microwaves? — AD, San Anselmo, CA
The walls of a microwave oven's cooking chamber are made of highly conductive metals so that they reflect the microwaves almost completely. Only a very small fraction of the microwaves inside the oven are absorbed by these metal walls and virtually none of the microwaves escape into the room. However, there is a substance inside the cooking chamber that absorbs the microwaves: water in the food! If you don't put water-containing food inside the microwave oven, there will be nothing to absorb the microwaves and they will reflect back to the magnetron and may damage it. The absence of an absorber in the cooking chamber will also increase any minor leakage of microwaves from the oven because the microwave intensity inside the cooking chamber will be much higher than normal.
. What is the difference in distance that a soccer ball will travel if the air pressure in the ball changes? — AB
A properly inflated soccer ball bounces well when you drop it on a hard floor because the ball stores energy by compressing the air during the bounce and the air returns this energy quite efficiently during the rebound. An under inflated soccer ball doesn't bounce so well because it stores energy by bending its leather surface during the bounce and the leather doesn't return energy very efficiently during the rebound. The same result holds true when you kick a ball rather than dropping it on the floor. Whether a moving ball hits a stationary surface or a stationary ball hits a moving surface, the ball is still bouncing from a surface. When you kick a ball with your foot, the ball is bouncing from your foot and a properly inflated ball will bounce more efficiently from your foot than an under inflated ball. The properly inflated ball will rebound at a higher speed and will travel farther.
. Is it possible to greatly increase the speed of a roller coaster, while retaining some safety, by applying the same theory that is used in Bullet Trains? — JA, Henderson, NV
While roller coasters could be made faster if they used the high performance tracks of bullet trains, smoothing out the tracks would only make the ride less jittery and wouldn't reduce the accelerations needed to complete the turns. The faster the train moves, the faster everything must accelerate as the track bends. Doubling the speed of the roller coaster would double the changes in velocity associated with each bend and would halve the time available to complete that change in velocity. As a result, doubling the roller coaster's speed would quadruple the accelerations it experiences on the same track and thus will quadruple the forces involved during the ride. A roller coaster ride already involves some pretty intense forces and accelerations. If those forces and accelerations were increased by a factor of 4, they would be more than most people could handle. Thus I wouldn't expect many riders on a double-speed bullet train roller coaster.
. What is the definitions of a "Hanning window", a "rectangular window", and a "triangular window"? — CV, Cape Town, South Africa
In the days before digital signal processing, the filters that were available for audio or video systems were very simple. These filters monitored the audio or video signal and produced an output signal that was related to the present input signal and to that signals value's in the recent past. Such simple filters could enhance or diminish certain ranges of frequencies and were able to perform basic tasks such as adjusting the balance between treble, midrange, and bass in an audio system.
But with computers and digital signal processing now commonplace, filtering has become much more sophisticated. Filters can now study an audio or video input signal over a long period of time and can even use data about future values of the input signal when producing an output signal. The filters that you ask about are all digital filters that produce an output signal that is related to the past, present, and future values of the input signal. A rectangular window filter is one that determines the output signal from a certain range of past, present, and future input signal values, all weighted evenly. A triangular or "Parzen" window filter is one that determines the output signal from a certain range of past, present, and future input signal values, with the weighting of values decreasing linearly with increasing time in the past or future. A Hanning window filter is one that determines the output signal from the complete past and future input signal values, with the weighting of values decreasing as the cosine of the time in the past or future (see for example, "Numerical Recipes" by Press, Flannery, Teukolsky, and Vetterling). All three filtering windows and filters are used to keep filters that extract certain frequency ranges from the input signal from affecting other frequency ranges. For that purpose, the Hanning window is better than the Parzen window and both are better than the rectangular window. As an example of the applications of these filters, a digital audio filter that makes good use of the Hanning window can enhance the treble of an audio signal uniformly without coloring the midrange at all. Earlier filters that only used past information always colored the midrange and didn't affect the treble uniformly.
. How does one calculate the pressure of air flowing in a tube? My specific application is air traveling in a 1/2-inch tube at a velocity of 14 inches/second. I know that Bernoulli would have the answer, but I cannot find it myself. — NT, Cambridge, MA
Without more information about the air in your tube, it's not possible to determine its pressure. Bernoulli's equation is frequently misunderstood to say that high-speed air is low-pressure air and that low speed air is high-pressure air—two observations that aren't necessarily true. Just because air is moving rapidly doesn't mean that its pressure is low. For example, the air in an airplane cabin is moving quickly but its pressure is higher than that of the air outside the cabin. Similarly, if you were to throw a tank of compressed air across the room, its pressure would remain high despite its increase in speed.
What Bernoulli's equation really says is that air has three forms for its energy and that as long as that air flows smoothly and without significant friction through a system of stationary obstacles, the sum of those three energies can't change. The three energies are kinetic energy (the energy of motion), gravitational potential energy, and an energy associated with pressure that I call pressure potential energy. The obstacles must remain stationary so that they can't do work on the air and thus change its total energy. Since the sum of those three energies doesn't change as air flows through a stationary environment, its pressure typically falls whenever its speed rises and vice versa. If the air also changes altitude significantly, then gravitational potential energy must be included in these energy exchanges.
So the reason why I can't answer your question about air in a pipe is that I don't know what the air's total energy was before it flowed through the pipe. While I can calculate the air's kinetic energy from its speed and we can neglect gravitational potential energy because the air isn't changing altitudes much in the pipe, I need to know what the air's total energy is in order to determine its pressure potential energy and thus its pressure.
. How do rockets work?
Rockets push stored materials in one direction and experience a thrust force in the opposite direction. They make use of the observation that whenever one object pushes on a second object, the second object exerts an equal but oppositely directed force back on the first object. This statement is the famous "action-reaction" concept that is generally known as Newton's third law. While it seems sensible that when you push on a wall it pushes back on you, this situation is extraordinarily general. For example, if you push a passing car forward, that car will still push backward on you with an equal but oppositely directed force. If you push on your neighbor, your neighbor will push back on you with an equal but oppositely directed force even if your neighbor is asleep! In the case of a rocket, the rocket pushes burning fuel downward and the burning fuel pushes upward on the rocket with an equal but oppositely direct force. If the rocket pushes its fuel downward hard enough, the fuel will push up on the rocket hard enough to overcome the rocket's weight and accelerate it upward into the sky and beyond.
. How does a thermometer work? — DL
A common liquid in glass thermometer takes advantage of the fact that liquids generally expand more than solids as their temperatures increase. The glass envelope of the thermometer contains a fine hollow capillary with a sealed reservoir at its base that's filled with a liquid such as alcohol or mercury. If both the liquid and glass expanded equally as they became warmer, the thermometer would simply change sizes slightly as its temperature increased. But the liquid expands more than the glass and can't simply remain in place. Some of it moves up the capillary. That's why the level of liquid in the thermometer rises as the thermometer's temperature rises.
. When making an electromagnet, why does a hard core stay permanently magnetized while a soft core does not? — CD, Houston, TX
Iron and steel are intrinsically magnetic materials, meaning that at the atomic scale they exhibit magnetic order and have magnetic poles present. Most materials, including copper and aluminum, have no such magnetic order—they are nonmagnetic all the way to the atomic scale. But while it is composed of magnetic atoms, a large piece of iron or steel normally doesn't appear magnetic. That's because a large piece of iron or steel contains many tiny magnetic domains. Although each of these magnetic domains is highly magnetic, with a north pole at one end and a south pole at the other end, the metal appears nonmagnetic at first because these domains point equally in all directions and their magnetizations cancel one another. Before the magnetic character of a piece of iron or steel will become visible, something must align its magnetic domains.
In an electromagnet, an iron or steel core is surrounded by a coil of wire. When you run current through that coil of wire, the magnetic field of the current causes the core's magnetic domains to change sizes—the domains that are aligned with the field grow at the expense of the domains misaligned with the field and the whole piece of iron or steel becomes highly magnetic. When you stop current from flowing through the coil of wire, the domains may return to their original sizes and shapes and the iron or steel may become nonmagnetic again.
The abilities for magnetic domains to change sizes depends on the chemical and physical properties of the metal, particularly its crystalline structure. In some magnetic materials, the domains change size extremely easily. These materials are considered to be "soft"—they magnetize easily in the presence of a magnetic field and demagnetize easily when that field is removed. Most electromagnets are made from such soft magnetic materials because it takes only a small current in a wire coil to magnetize the electromagnet's soft core and that core quickly becomes nonmagnetic when you stop the current from flowing.
But in other magnetic materials, the domains don't change size easily. These materials are considered to be "hard"—they are both difficult to magnetize and difficult to demagnetize. You must put lots of current through the coil of wire around a hard magnetic material in order to magnetize that material. But once you turn off the current, the material will retain its magnetization and it will be a permanent magnet.