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.