. What is the approximate terminal velocity for a spent falling bullet that was fired into the air? Is this velocity sufficient to kill someone? - M
A bullet's terminal velocity is the downward speed at which the upward force of air resistance acting on it balances its downward weight. Once the falling bullet reaches this speed, it coasts downward at a steady rate. Because air resistance depends largely on surface area while weight depends on volume, larger bullets will drop faster than smaller bullets (just as a piece of chalk drops faster than chalk dust). While I am not sure of the exact speed of a dropping bullet, I expect it to be several hundred miles per hour. As to whether or not it can kill someone, the answer is most definitely yes. In fact, a distant cousin of mine was killed several years ago during Mardi Gras when a falling spent bullet pierced her brain. Firing bullets into the air is an extraordinarily foolish and inconsiderate action. In cultures where it's common to fire guns during celebrations, innocent people are frequently killed by these descending "party favors." If you ever see people shooting guns into the air, you should immediately seek cover in a basement. Their bullets will return to earth in less than thirty seconds and will be just as deadly when they arrive as if they had been shot right at you.
. How can heat be trapped? — PR, Brooklyn, NY
You can prevent heat from moving about with the help of insulation. The three principal mechanisms of heat transfer are conduction (the passage of heat through a stationary material), convection (the passage of heat in a moving fluid), and radiation (the passage of heat as electromagnetic waves or light). Good insulation doesn't conduct heat well, doesn't support convection, and blocks radiation. Wool is a good example: its hair and trapped air don't conduct heat well, the trapped air can't really undergo convection well, and thermal radiation can't travel through the wool along a straight path. As a result, wearing a wool sweater keeps you from losing heat quickly—you stay warm. Wool has the added benefit of carrying water away from your skin.
. Which gives off more heat energy, an incandescent light bulb or a fluorescent lamp? Which is more efficient to use in the summer or winter? — TJ, Woodbridge, VA
An incandescent lamp turns its electric power completely into heat. Even the visible light it gives off is actually thermal radiation. A fluorescent lamp tries not to produce heat—the light it produces is non-thermal (it doesn't involve hot materials). While a fluorescent lamp is only partly successful at not producing heat, it's still several times more energy efficient than an incandescent lamps—fluorescents produces several times as much illumination for the same amount of electric power. This statement is true both in summer and winter, although fluorescent bulbs lose some of their energy efficiencies in very cold or very hot weather. Fluorescent lamps work best at temperatures between about 15° C and 40° C.
. How does a transistor amplify an input signal in an audio amplifier? — AR, Pierrefonds, Quebec
The answer depends a little on which type of transistor is used, so I'll consider only an audio amplifier based on MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistors). One of these three-electrode devices allows a tiny electric charge on its gate electrode to control a substantial current flowing between its source and drain electrodes. In a typical amplifier, the current flowing in the input circuit is allowed to deposit or remove electric charge from the gate electrode(s) of one or more MOSFETs. This action dramatically changes how much current flows in a second circuit. This second circuit is ultimately responsible for the current that passes out of the amplifier and through the speakers that reproduce sound. As the current in the input circuit fluctuates to represent a particular musical passage, the charges on the gates of the MOSFETs also fluctuate and the MOSFETs vary the current through the output circuit and the speakers. Because MOSFETs are so sensitive to even a tiny amount of charge, it doesn't take much current in the input circuit to cause large changes in the current of the output circuit.
. What are the names of the subatomic and fundamental particles and what do they do? — BA, Fairbury, IL
Subatomic particles and fundamental particles aren't necessarily the same—some subatomic particles are built from several fundamental particles. That's the case for two of the most important subatomic particles: the proton and the neutron. Each of these particles is built from three fundamental particles known as quarks. The proton contains two "up" quarks and one "down" quark. The neutron contains one "up" quark and two "down" quarks. However, another important subatomic particle is also a fundamental particle: the electron. Virtually all matter is composed of these three subatomic particles: protons, neutrons, and electrons.
The list of fundamental particles—particles that are not known to be composed of other particles—is relatively short. It includes 6 types of quarks, which are given the arbitrary names "up", "down", "charm", "strange", "top", and "bottom". These quarks are never found by themselves but are instead used to build two major classes of subatomic particles: baryons (including protons and neutrons) and mesons. The list of fundamental particles also includes 6 types of leptons, which are given the names "electron", "electron neutrino", "muon", "muon neutrino", "tau", and "tau neutrino". These leptons are found by themselves and aren't used to build any other subatomic particles. These quarks and leptons are described as fermions and each has an associated antiparticle.
In addition to quarks and leptons, there are a number of fundamental particles that allow the fundamental fermions to interact with one another. These interaction particles are described as bosons and include the "photon", "W+ Boson", "W- Boson", "Z Boson", 8 different "gluons", and a particle called "Higgs" (which has not yet been observed but is thought to exist).
The list of subatomic particles that can be formed from the fundamental particles is extremely long and listing it here wouldn't be very enlightening. The only subatomic particles that are common in nature are protons, neutrons, electrons, and photons. Some of the others appear through nuclear or subnuclear processes in radioactive materials, nuclear reactors, particle accelerators, or celestial objects, but most of these exotic subatomic particles haven't been common since moments after the big bang.
. How do record players and their needles work? - JW
As a phonograph record turns, the needle of its playing arm slides through a narrow spiral groove on the record's surface. This groove is cut with a 90° angle at its bottom and both of its sides have undulations in them. As the needle slides through the groove, it rides up and down on these undulations. The needle's movement causes currents to flow in two separate pick-ups that are attached to the needle. One pick-up responds to needle motions caused by the right edge of groove and the other pick-up responds to needle motions caused by the left edge of the groove. The physical mechanism for converting needle motion into electric current depends on the needle cartridge—it can involve moving magnets, moving coils of wire, or squeezed piezoelectric crystals. Since the groove undulations represent air pressure fluctuations at the right and left microphones during recording, the currents from the two pick-ups represent those pressure fluctuations during playback. With the help of amplifiers and speakers, these currents are used to reproduce the sounds that were recorded at the two microphones.
. How does steam work? — SS, Nairobi, Kenya
Steam is the gaseous form of water. When the water molecules in liquid or solid water have enough thermal energy, they can break free of one another and become independent particles. Even at room temperature, the air you are breathing is several percent water molecules. But at higher temperatures, the rate at which water molecules leave the surface of solid or liquid water increases so much that these water molecules can form a dense, high-pressure gas. This gas is called steam.
. Is it possible to mechanically connect two motors of equal speeds and powers to provide twice as much power as a single motor? — EG, Torrance, CA
As long as they're both AC induction motors, I don't see any reason why not. While induction motors would turn synchronously with the power line if they had absolutely no load, they naturally lag slightly behind in normal situations. While a line synchronous AC motor would turn at 1800 or 3600 rpm, depending on how it's wired, a typical induction motor turns at 1725 or 3450 rpm. The more you load an induction motor, the slower it turns and the more torque it exerts on that load. By coupling two induction motors together mechanically, you'll make them turn at the same rate. Since the torque each motor exerts on the load depends on rotation speed, they'll both contribute equally to the task and will together provide twice the power of a single motor.
I wouldn't try this with any kind of motor that doesn't have such a clear relationship between rotational speed and power output. If you join two mismatched motors with one another, one may end up doing all the work and the other motor might effectively become a generator rather than a motor!
. Can an object conduct electricity but be nonmagnetic? Are these independent properties? You said during lecture that copper is nonmagnetic but doesn't it conduct electricity?
Electric conductivity and magnetism are pretty much independent properties. There are good conductors that are magnetic (iron) and good conductors that are nonmagnetic (copper). There are also insulators that are magnetic (iron oxide) and insulators that are nonmagnetic (glass).
. How does a magnetic tape record the difference in timbre or sound quality of the sounds? How does it represent a piano versus an electric guitar? Also, how does more than one tone get recorded (e.g., an entire band or symphony)?
Even a single instrument playing a single note produces a complicated sound. The air pressure fluctuations produced by the instrument aren't as simple and smooth as you might think. While the instrument may produce mostly the fundamental tone—the main pitch associated with the note being played—it also produces other tones that are usually integer multiples of the fundamental tone. These higher pitched "harmonics" contribute to the sound we hear and allow us to determine what instrument is playing that sound. We also hear the temporal shape of the sound—the sound envelope. A piano produces a sound that starts loud and gradually becomes softer while a violin produces a sound that starts soft and gradually becomes louder. An electric guitar offers its player even more control over the pitch and sound envelope. The tape recorder detects the pressure fluctuations associated with all these tones and volume changes and records them all as the magnetization of the tape's surface. When many instruments are playing at once, the pressure fluctuations are even more complicated and they add together to create a complicated pressure pattern at the microphone. Nonetheless, the recorder simply detects the air pressure changes at the microphone and records them on the tape, and that's all it needs to do to keep an accurate record of the sound. When the magnetization of the tape is used to reproduce sound, you again hear all the instruments playing.