absorption | the gradual conversion of wave energy into heating of the medium; what happens when light hits matter and gives up some of its energy. |

acceleration | The rate of change of velocity; the slope of the tangent line on a v - t graph. |

alpha decay | the radioactive decay of a nucleus via emission of an alpha particle |

alpha particle | a form of radioactivity consisting of helium nuclei |

ammeter | a device for measuring electrical current |

ampere | the metric unit of current, one coulomb pe second; also "amp" |

amplitude | the amount of vibration, often measured from the center to one side; may have different units depending on the nature of the vibration |

Angular magnification | The factor by which an image's apparent angular size is increased (or decreased). Cf. magnification. |

angular momentum | a measure of rotational motion; a conserved quantity for a closed system |

atom | the basic unit of one of the chemical elements |

atomic mass | the mass of an atom |

atomic number | the number of protons in an atom's nucleus; determines what element it is |

attractive | describes a force that tends to pull the two participating objects together |

axis | An arbitrarily chosen point used in the definition of angular momentum. Any object whose direction changes relative to the axis is considered to have angular momentum. No matter what axis is chosen, the angular momentum of a closed system is conserved. |

beta decay | the radioactive decay of a nucleus via the reaction n → p + e^{-} + ν[bar] or p → n + e^{+} + ν; so called because an electron or antielectron is also known as a beta particle |

beta particle | a form of radioactivity consisting of electrons |

cathode ray | the mysterious ray that emanated from the cathode in a vacuum tube; shown by Thomson to be a stream of particles smaller than atoms |

center of mass | the balance point of an object |

center of mass | the balance point or average position of the mass in a system |

charge | a numerical rating of how strongly an object participates in electrical forces |

circuit | an electrical device in which charge can come back to its starting point and be recycled rather than getting stuck in a dead end |

coherent | A light wave whose parts are all in phase with each other. |

collision | an interaction between moving objects that lasts for a certain time |

component | the part of a velocity, acceleration, or force that would be perceptible to an observer who could only see the universe projected along a certain one-dimensional axis |

concave | Describes a surface that is hollowed out like a cave. |

conic section | a curve formed by the intersection of a plane and an infinite cone |

convex | Describes a surface that bulges outward. |

coulomb (C) | the unit of electrical charge |

current | the rate at which charge crosses a certain boundary |

damping | the dissipation of a vibration's energy into heat energy, or the frictional force that causes the loss of energy |

diffraction | The behavior of a wave when it encounters an obstacle or a nonuniformity in its medium; in general, diffraction causes a wave to bend around obstacles and make patterns of strong and weak waves radiating out beyond the obstacle. |

diffuse reflection | Reflection from a rough surface, in which a single ray of light is divided up into many weaker reflected rays going in many directions. |

Doppler effect | the change in a wave's frequency and wavelength due to the motion of the source or the observer or both |

driving force | an external force that pumps energy into a vibrating system |

elastic collision | one in which no KE is converted into other forms of energy |

electric dipole | an object that has an imbalance between positive charge on one side and negative charge on the other; an object that will experience a torque in an electric field |

electric field | the force per unit charge exerted on a test charge at a given point in space |

electrical force | one of the fundamental forces of nature; a noncontact force that can be either repulsive or attractive |

electron | Thomson's name for the particles of which a cathode ray was made |

ellipse | a flattened circle; one of the conic sections |

energy | A numerical scale used to measure the heat, motion, or other properties that would require fuel or physical effort to put into an object; a scalar quantity with units of joules (J). |

equilibrium | a state in which an object's momentum and angular momentum are constant |

field | a property of a point in space describing the forces that would be exerted on a particle if it was there e |

fission | the radioactive decay of a nucleus by splitting into two parts |

fluid friction | a friction force in which at least one of the object is is a fluid |

fluid | a gas or a liquid |

focal length | A property of a lens or mirror, equal to the distance from the lens or mirror to the image it forms of an object that is infinitely far away. |

focus | one of two special points inside an ellipse: the ellipse consists of all points such that the sum of the distances to the two foci equals a certain number; a hyperbola also has a focus |

frequency | the number of cycles per second, the inverse of the period |

fusion | a nuclear reaction in which two nuclei stick together to form one bigger nucleus |

FWHM | the full width at half-maximum of a probability distribution; a measure of the width of the distribution |

gamma ray | a form of radioactivity consisting of a very high-frequency form of light |

gravitational field | the force per unit mass exerted on a test mass at a given point in space |

gravity | A general term for the phenomenon of attraction between things having mass. The attraction between our planet and a human-sized object causes the object to fall. |

half-life | the amount of time that a radioactive atom will survive with probability 1/2 without decaying |

heat | A form of energy that relates to temperature. Heat is different from temperature because an object with twice as much mass requires twice as much heat to increase its temperature by the same amount. Heat is measured in joules, temperature in degrees. (In standard terminology, there is another, finer distinction between heat and thermal energy, which is discussed in "A Numerical Scale of Energy". In this book, I informally refer to both as heat). |

hyperbola | another conic section; it does not close back on itself |

image | A place where an object appears to be, because the rays diffusely reflected from any given point on the object have been bent so that they come back together and then spread out again from the image point, or spread apart as if they had originated from the image. |

impulse, I, J | the amount of momentum transferred, Δp |

independence | the lack of any relationship between two random events |

Index of refraction | An optical property of matter; the speed of light in a vacuum divided by the speed of light in the substance in question. |

induction | the production of an electric field by a changing magnetic field, or vice-versa |

inelastic collision | one in which some KE is converted to other forms of energy |

inertial frame | a frame of reference that is not accelerating, one in which Newton's first law is true |

ion | an electrically charged atom or molecule |

isotope | one of the possible varieties of atoms of a given element, having a certain number of neutrons |

kinetic energy | The energy an object possesses because of its motion. |

kinetic friction | a friction force between surfaces that are slipping past each other |

light | Anything that can travel from one place to another through empty space and can influence matter, but is not affected by gravity. |

magnetic dipole | an object, such as a current loop, an atom, or a bar magnet, that experiences torques due to magnetic forces; the strength of magnetic dipoles is measured by comparison with a standard dipole consisting of a square loop of wire of a given size and carrying a given amount of current |

magnetic field | a field of force, defined in terms of the torque exerted on a test dipole |

magnification | The factor by which an image's linear size is increased (or decreased). Cf. angular magnification. |

magnitude | the "amount" associated with a vector |

mass number | the number of protons plus the number of neutrons in a nucleus; approximately proportional to its atomic mass |

mass | A numerical measure of how difficult it is to change an object's motion. |

matter | Anything that is affected by gravity. |

medium | a physical substance whose vibrations constitute a wave |

millirem | a unit for measuring a person's exposure to radioactivity |

mks system | The use of metric units based on the meter, kilogram, and second. Example: meters per second is the mks unit of speed, not cm/s or km/hr. |

molecule | a group of atoms stuck together |

moment of inertia, I | the proportionality constant in the equation L = 2πI/T |

momentum | a measure of motion, equal to mv for material objects |

neutron | an uncharged particle, the other types that nuclei are made of |

noninertial frame | an accelerating frame of reference, in which Newton's first law is violated |

nonuniform circular motion | circular motion in which the magnitude of the velocity vector changes |

normal force | the force that keeps two objects from occupying the same space |

normalization | the property of probabilities that the sum of the probabilities of all possible outcomes must equal one |

oblique | describes a force that acts at some other angle, one that is not a direct repulsion or attraction |

ohm | the metric unit of electrical resistance, one volt per ampere |

ohmic | describes a substance in which the flow of current between two points is proportional to the voltage difference between them |

open circuit | a circuit that does not function because it has a gap in it |

operational definition | A definition that states what operations should be carried out to measure the thing being defined. |

parabola | the mathematical curve whose graph has y proportional to x^{2} |

period | the time required for a planet to complete one orbit; more generally, the time for one repetition of some repeating motion |

period | the time required for one cycle of a periodic motion |

periodic motion | motion that repeats itself over and over |

photoelectric effect | the ejection, by a photon, of an electron from the surface of an object |

photon | a particle of light |

potential energy | the energy having to do with the distance between to objects that interact via a noncontact force |

power | The rate of transferring energy; a scalar quantity with units of watts (W). |

probability distribution | a curve that specifies the probabilities of various random values of a variable; areas under the curve correspond to probabilities |

probability | the likelihood that something will happen, expressed as a number between zero and one |

proton | a positively charged particle, one of the types that nuclei are made of |

quality factor | the number of oscillations required for a system's energy to fall off by a factor of 535 due to damping |

quantized | describes quantity such as money or electrical charge, that can only exist in certain amounts |

quantum number | a numerical label used to classify a quantum state |

radial | parallel to the radius of a circle; the in-out direction |

real image | A place where an object appears to be, because the rays diffusely reflected from any given point on the object have been bent so that they come back together and then spread out again from the new point. Cf. virtual image. |

reflection | the bouncing back of part of a wave from a boundary; what happens when light hits matter and bounces off, retaining at least some of its energy. |

refraction | The change in direction that occurs when a wave encounters the interface between two media. |

rem | A unit for measuring a person's exposure to radioactivity; cf millirem. |

repulsive | describes a force that tends to push the two participating objects apart |

resistance | the ratio of the voltage difference to the current in an object made of an ohmic substance |

resonance | the tendency of a vibrating system to respond most strongly to a driving force whose frequency is close to its own natural frequency of vibration |

scalar | a quantity that has no direction in space, only an amount |

short circuit | a circuit that does not function because charge is given a low-resistance "shortcut" path that it can follow, instead of the path that makes it do something useful |

significant figures | Digits that contribute to the accuracy of a measurement. |

simple harmonic motion | motion whose x-t graph is a sine wave |

sink | a point at which field vectors converge |

source | a point from which field vectors diverge; often used more inclusively to refer to points of either convergence or divergence |

specular reflection | Reflection from a smooth surface, in which the light ray leaves at the same angle at which it came in. |

spin | the built-in angular momentum possessed by a particle even when at rest |

spring constant | the constant of proportionality between force and elongation of a spring or other object under strain |

stable equilibrium | one in which a force always acts to bring the object back to a certain point |

standing wave | a wave pattern that stays in one place |

static friction | a friction force between surfaces that are not slipping past each other |

steady state | the behavior of a vibrating system after it has had plenty of time to settle into a steady response to a driving force |

strong nuclear force | the force that holds nuclei together against electrical repulsion |

superposition | the adding together of waves that overlap with each other |

Système International | A fancy name for the metric system. |

tangential | tangent to the circle, perpendicular to the radial direction |

temperature | What a thermometer measures. Objects left in contact with each other tend to reach the same temperature. Cf. heat. As discussed in more detail in section "Heat is Kinetic Energy", temperature is essentially a measure of the average kinetic energy per molecule. |

thermal energy | Careful writers make a distinction between heat and thermal energy, but the distinction is often ignored in casual speech, even among physicists. Properly, thermal energy is used to mean the total amount of energy possessed by an object, while heat indicates the amount of thermal energy transferred in or out. The term heat is used in this book to include both meanings. |

torque | the rate of change of angular momentum; a numerical measure of a force's ability to twist on an object |

transmission | the continuation of part of a wave through a boundary |

uniform circular motion | circular motion in which the magnitude of the velocity vector remains constant |

unstable equilibrium | one in which any deviation of the object from its equilibrium position results in a force pushing it even farther away |

vector | a quantity that has both an amount (magnitude) and a direction in space |

velocity | the rate of change of position; the slope of the tangent line on an x - t graph. |

virtual image | Like a real image, but the rays don't actually cross again; they only appear to have come from the point on the image. Cf. real image. |

volt | the metric unit of voltage, one joule per coulomb |

voltage | electrical potential energy per unit charge that will be possessed by a charged particle at a certain point in space |

voltmeter | a device for measuring voltage differences |

wave-particle duality | the idea that light is both a wave and a particle |

wavefunction | the numerical measure of an electron wave, or in general of the wave corresponding to any quantum mechanical particle |

wavelength | the distance in space between repetitions of a periodic wave |

weak nuclear force | the force responsible for beta decay |

weight | the force of gravity on an object, equal to mg |

work | the amount of energy transferred into or out of a system, excluding energy transferred by heat conduction |