"A"dventures in Physics Vocabulary Quiz

Answer: Angular momentum

Angular momentum is analogous in many ways to linear momentum. It's conserved (that is, it remains constant) in the absence of an external torque, and it's related to the mass of a particle and to its angular speed. Represented by the letter L for historical reasons, it is defined by the relation L = r x p (that is, the angular momentum is equal to the cross product of the particle's distance from the origin with its linear momentum). Note that the angular momentum changes depending on how you look at it: you must pick the system's origin intelligently!

Incidentally, that equation gives a clue as to why a doorknob is located on the far side of the door from the hinge. You're applying force to the edge of the door, and the further the distance r from the axis of rotation, the greater the angular momentum and the faster the door swings. Try pushing a door closed from a point near the hinge, and you'll see that it's not quite as efficient!

Answer: Apogee

"Apogee" comes from the ancient Greek words "apo" (meaning "far" or "away") and ge/Gaia, meaning "earth." An orbit's closest approach to Earth is its perigee. These terms only refer to orbits around the Earth, of course! When discussing orbits around the Sun, you might mention aphelion (furthest approach to the Sun) and perihelion (closest approach to the Sun); for orbits around other stars, the terms are apastron and periastron.

Answer: The weight of the atmosphere above that point

Although weather patterns certainly affect local atmospheric pressure, the pressure is dominated by the weight of the atmosphere. This is why atmospheric pressure is noticeably lower at high altitudes, and why things like cooking rice take so much longer: the lower atmospheric pressure lowers the boiling point of water, so that rice in boiling water takes longer to cook.

We don't normally think of air as weighing very much, but when you add it all up it's a big effect! Imagine a square column of air, with a cross section of 1 meter by 1 meter, going from sea level all the way to outer space. That column of air is equivalent to a surface mass of 10.2 metric tonnes! (That's more than 22,000 pounds ...)

Answer: The effects of stellar and planetary motion on human lives

"Astrophysics" comes from the Greek word "astron" (star), which is also the root word of "astronomy." The term was invented in order to emphasize the ability of scientists to explain the physics behind celestial phenomena, after millennia of being limited to observation, but there is no clear demarcation between astronomy and astrophysics.

Astrophysics research, while still concerned with the physics of stars, now includes many of humanity's really big questions, from general relativity to the beginnings of our universe. It does NOT, however, include astrology -- a pseudoscientific belief that the motions of the stars and planets somehow guide the lives of individual people. Astrology is not science, and it does not have predictive power.

Answer: F = m*a

F = m*a, Newton's second law, tells us that the force on an object is equal to that object's mass times its acceleration. This makes a lot of sense when considered in light of Newton's first law. Acceleration is simply the rate at which the object's velocity changes, and we know that things tend to move at a constant speed unless and until a force is applied. (We don't see this behavior in the real world because the frictional force tends to slow objects down.) Thus, force and acceleration are directly related to each other!

Because acceleration and velocity are both vectors, the "rate of change" takes into account both the speed of the motion and its direction. Thus, the edge of a CD spins at a constant speed, but it's also undergoing a constant acceleration: its direction changes every instant!

Of the other equations listed, div A = 0 defines the Coulomb gauge of electrodynamics (a useful tool for calculating electromagnetic fields); A = l*w gives the area of a rectangle; and a = l/(1-e^2) gives the semimajor axis of an elliptical orbit (such as the ones followed by the planets around the Sun).

Answer: -273 degrees Celsius (-460 degrees Fahrenheit)

Materials at absolute zero neither absorb heat nor radiate it. Perfect crystals, with a constant "frozen-in" entropy, can form. Molecules and atoms drop to their lowest possible energy states. All sorts of interesting properties result at lower temperatures; for example, superfluids and superconductors are both low-temperature phenomena. (And when we say low, we mean low: helium-4 becomes a superfluid at 2.17 K, or -271 degrees Celsius!)

Artificially achieving absolute zero (which is 0 on both the Kelvin and Rankine temperature scales) is impossible; you would need a machine with perfect efficiency, and we know from the laws of thermodynamics that this isn't possible. Scientists have succeeded in getting very, very close, however: an MIT team led by Wolfgang Ketterle cooled sodium atoms to 450 picoKelvin. The difference between that and absolute zero is equal to 4.5 degrees Celsius, divided by ten thousand million. Very nippy!

Answer: A given element

For much of the 19th and 20th centuries, physicists were greatly concerned with breaking matter into its smallest constituent parts. Large volumes of gases, liquids and solids could be split into atoms, which could in turn be classified as elements (for which the Periodic Table is an extremely useful tool).

As the Twentieth Century dawned, it was discovered that even atoms are made up of smaller building blocks of matter. Each atom contains a number of electrons orbiting around a nucleus of neutrons and protons (which define which element that atom is).

It was later determined that even neutrons and protons are not "fundamental particles": each is composed of quarks. Atoms are still extremely useful, however: in addition to being tiny quantum physics laboratories, they give us the best framework for understanding chemistry.

Answer: Photon

Antimatter is a lot like ordinary matter. It has the same mass and obeys the same equations, though aspects of its behavior are curiously reversed. For example, a positron -- the antiparticle of an electron -- will bend its path by the same amount in a magnetic field, but in the opposite direction. (It would curve clockwise where the electron would curve counterclockwise, for example.) Even quarks, neutrinos and composite particles (like protons and neutrons) have antiparticles, which reverse not only electric charge but also some other quantum numbers (such as strangeness and lepton number). Physicists have even succeeded in making antihydrogen: a positron going around an antiproton! However, massless gauge bosons (like photons, gluons and gravitons) don't have distinct antiparticles.

Antimatter is also associated with another "A" word: annihilation. A particle and an antiparticle with complementary quantum states will annihilate, ceasing to exist. Naturally, the total energy has to be conserved, so lots of energy is released from an annihilation -- perhaps in the form of light, or in another particle-antiparticle pair. One of the strangest (and luckiest) unsolved mysteries in physics is why the universe began with a slight excess of matter over antimatter. If matter and antimatter particles had been balanced, they would all have annihilated each other, and there would be no massive (anti)particles left to form stars, galaxies, or us!

Answer: A helium nucleus: two protons and two neutrons

Every unstable nucleus has a decay mode, a favored way in which its nucleus emits particles and decays to a lighter nucleus. Alpha particles (named after the first letter of the Greek alphabet, long before their identity as helium nuclei was discovered) tend to be emitted by nuclei heavier than lead. Uranium-238, for example, decays via alpha emission to Thorium-234; along its 14-step decay chain to Lead-206 (a stable isotope), a total of eight alpha particles are emitted. Other common decay modes include beta emission (a beta particle being an electron or a positron) and gamma emission (in which a photon is released).

Alpha particles are heavy (compared with other particles, that is!) and highly charged (thanks to those two protons). This makes them extremely likely to interact with (and be absorbed by) matter. They can't travel more than a few inches in air, and can be stopped by a sheet of paper, by ordinary clothing or by the outermost, dead layer of a person's skin. If they get inside a human body, though (usually through breathing, eating, or drinking), the damage they can wreak on living cells is immense. Ingested alpha particles were responsible for one of the most infamous modern cases of radiation poisoning: the November 2006 murder of Russian dissident and British citizen Alexander Litvinenko, using the alpha emitter Polonium-210.

Answer: Ampere's Law

The curl of the magnetic field B, Maxwell wrote, is equal to the permeability of free space times the current J, plus the permeability of free space times the permittivity of free space times the partial derivative of the electric field E with respect to time. (This looks much simpler written out as an equation, but it requires various letters and symbols that aren't part of the standard character set!)

Andre-Marie Ampere's original form of this equation omitted the second term from the right-hand side. This made it the magnetic equivalent of Gauss's Law, giving the properties of the magnetic field as a function of a steady current J. (Gauss's Law gives the electric field as a function of a steady distribution of electric charge.) Maxwell's addition of the second term accounts for magnetic field generation from a change in electric field, which is pleasantly similar to Faraday's Law (which states that a changing magnetic field induces an electric field). Taking Faraday's Law and Ampere's (corrected) Law together reveals that light itself is an electromagnetic wave. It is impossible to overstate the importance of this realization! Electricity, magnetism, and light are all facets of the same phenomenon; this understanding led to the harnessing of electricity and the shape of the modern world.

Thank you for joining me on this tour of an "a"stonishing and "a"we-inspiring set of physics terms. I hope you've enjoyed the ride!