M–P
20.1 A–D
 
20.2 E–H
 
20.3 I–L
 
 
20.4 M–P
 
20.5 Q–T
 
20.6 U–Z
 
M–P
M
Magnetic flux
The dot product of the area and the magnetic field passing through it. Graphically, it is a measure of the number and length of magnetic field lines passing through that area. It is measured in Webers (Wb).
Magnification
The ratio of the size of the image produced by a mirror or lens to the size of the original object. This number is negative if the image is upside-down.
Magnitude
A property common to both vectors and scalars. In the graphical representation of a vector, the vector’s magnitude is equal to the length of the arrow.
Margin of error
The amount of error that’s possible in a given measurement.
Mass
A measurement of a body’s inertia, or resistance to being accelerated.
Mass defect
The mass difference between a nucleus and the sum of the masses of the constituent protons and neutrons.
Mass number
The mass number, A, is the sum of the number of protons and neutrons in a nucleus. It is very close to the weight of that nucleus in atomic mass units.
Maxima
In an interference or diffraction pattern, the places where there is the most light.
Mechanical energy
The sum of a system’s potential and kinetic energy. In many systems, including projectiles, pulleys, pendulums, and motion on frictionless surfaces, mechanical energy is conserved. One important type of problem in which mechanical energy is not conserved is the class of problems involving friction.
Medium
The substance that is displaced as a wave propagates through it. Air is the medium for sound waves, the string is the medium of transverse waves on a string, and water is the medium for ocean waves. Note that even if the waves in a given medium travel great distances, the medium itself remains more or less in the same place.
Melting point
The temperature at which a material will change phase from solid to liquid or liquid to solid.
Meson
A class of elementary particle whose mass is between that of a proton and that of an electron. A common kind of meson is the pion.
Michelson-Morley experiment
An experiment in 1879 that showed that the speed of light is constant to all observers. Einstein used the results of this experiment as support for his theory of special relativity.
Minima
In an interference or diffraction pattern, the places where there is the least light.
Mole
The number of hydrogen atoms in one gram of hydrogen, equal to . When counting the number of molecules in a gas, it is often convenient to count them in moles.
Moment of inertia
A rigid body’s resistance to being rotated. The moment of inertia for a single particle is MR2, where M is the mass of the rigid body and R is the distance to the rotation axis. For rigid bodies, calculating the moment of inertia is more complicated, but it generally takes the form of a constant multiplied by MR2.
Momentum
Linear momentum, p, commonly called “momentum” for short, is a vector quantity defined as the product of an object’s mass, m, and its velocity, v.
Motional emf
The emf created by the motion of a charge through a magnetic field.
Mutual Induction
The property by which a changing current in one coil of wire induces an emf in another.
N
Neutrino
An almost massless particle of neutral charge that is released along with a beta particle in beta decay.
Neutron
A neutrally charged particle that, along with protons, constitutes the nucleus of an atom.
Neutron number
The number, N, of neutrons in an atomic nucleus.
Newton
A unit of force: 1 N is equivalent to a 1 kg · m/s2.
Newton’s First Law
An object at rest remains at rest, unless acted upon by a net force. An object in motion remains in motion, unless acted upon by a net force.
Newton’s Law of Universal Gravitation
The force of gravity, F, between two particles of mass and , separated by a distance r, has a magnitude of , where G is the gravitational constant. The force is directed along the line joining the two particles.
Newton’s Second Law
F = ma. The net force, F, acting on an object causes the object to accelerate, a. The magnitude of the acceleration is directly proportional to the net force on the object and inversely proportional to the mass, m, of the object.
Newton’s Third Law
To every action, there is an equal and opposite reaction. If an object A exerts a force on another object B, B will exert on A a force equal in magnitude and opposite in direction to the force exerted by A.
Node
The points on a standing wave where total destructive interference causes the medium to remain fixed at its equilibrium position.
Normal
The line perpendicular to a surface. There is only one normal for any given surface.
Normal force
The reaction force of the ground, a table, etc., when an object is placed upon it. The normal force is a direct consequence of Newton’s Third Law: when an object is placed on the ground, the ground pushes back with the same force that it is pushed upon. As a result, the net force of an object on the ground is zero, and the object does not move.
Nuclear fission
A nuclear reaction in which a high-energy neutron bombards a heavy, unstable atomic nucleus, causing it to split into two smaller nuclei, and releasing some neutrons and a vast amount of energy at the same time.
Nuclear fusion
A nuclear reaction that takes place only at very high temperatures. Two light atoms, often hydrogen, fuse together to form a larger single atom, releasing a vast amount of energy in the process.
Nucleus
The center of an atom, where the protons and neutrons reside. Electrons then orbit this nucleus.
O
Optics
The study of the properties of visible light, i.e., the portion of the electromagnetic spectrum with wavelengths between 360 and 780 nm (1 nm = m/s).
Orbit
When an object is held in circular motion about a massive body, like a planet or a sun, due to the force of gravity, that object is said to be in orbit. Objects in orbit are in perpetual free fall, and so are therefore weightless.
Oscillation
A back-and-forth movement about an equilibrium position. Springs, pendulums, and other oscillators experience harmonic motion.
P
Pascals
The unit for measuring pressure. One Pascal is equal to one Newton per meter squared, 1 Pa = 1 N/m2.
Pendulum
A pendulum consists of a bob connected to a rod or rope. At small angles, a pendulum’s motion approximates simple harmonic motion as it swings back and forth without friction.
Period
The time it takes a system to pass through one cycle of its repetitive motion. The period, T, is the inverse of the motion’s frequency, f = 1/T.
Phase
Two oscillators that have the same frequency and amplitude, but reach their maximum displacements at different times, are said to have different phases. Similarly, two waves are in phase if their crests and troughs line up exactly, and they are out of phase if the crests of one wave line up with the troughs of the other.
Phase change
When a solid, liquid, or gas changes into another phase of matter.
Photoelectric effect
When electromagnetic radiation shines upon a metal, the surface of the metal releases energized electrons. The way in which these electrons are released contradicts classical theories of electromagnetic radiation and supports the quantum view according to which electromagnetic waves are treated as particles.
Photoelectron
The name of an electron released from the surface of a metal due to the photoelectric effect.
Photon
A small particle-like bundle of electromagnetic radiation.
Pitch
Another word for the frequency of a sound wave.
Planck’s constant
A constant, J · s, which is useful in quantum physics. A second constant associated with Planck’s constant is .
Polarization
A process that aligns a wave of light to oscillate in one dimension rather than two.
Potential energy
Energy associated with an object’s position in space, or configuration in relation to other objects. This is a latent form of energy, where the amount of potential energy reflects the amount of energy that potentially could be released as kinetic energy or energy of some other form.
Power
Defined as the rate at which work is done, or the rate at which energy is transformed. P is measured in joules per second (J/s), or watts (W).
Pressure
A measure of force per unit area. Pressure is measured in N/m2 or Pa.
Principal axis
The straight line that runs through the focal point and the vertex of a mirror or lens.
Proton
A positively charged particle that, along with the neutron, occupies the nucleus of the atom.
Pulley
A pulley is a simple machine that consists of a rope that slides around a disk or block.
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