 Mach number (E. Mach)

The ratio of the speed of an object in a given medium to the speed
of sound in that medium.
 Mach's principle (E. Mach; c. 1870)

The inertia of any particular particle or particles of matter is
attributable to the interaction between that piece of matter and
the rest of the Universe. Thus, a body in isolation would have no
inertia.
 magnetic constant

See permeability of free space.
 magnetic monopole

A hypothetical particle which constitutes sources and sinks of the
magnetic field. Magnetic monopoles have never been found, but
would only cause fairly minor modifications to Maxwell's equations.
They also seem to be predicted by some grandunified theories. If
magnetic monopoles do exist, they do not seem to be very common in
our Universe.
 Magnus effect

A rotating cylinder in a moving fluid drags some of the fluid
around with it, in its direction of rotation. This increases the
speed in that region, and thus the pressure is lower.
Consequently, there is a net force on the cylinder in that
direction, perpendicular to the flow of the fluid. This is called
the Magnus effect.
 Malus' law (E.L. Malus)

The light intensity I of a ray with initial intensity I_{0}
travelling through a polarizer at an angle theta between
the polarization of the light ray and the polarization axis of the
polarizer is given by
I = I_{0} cos^{2} theta.
 Maxwell's demon (J.C. Maxwell)

A thought experiment illustrating the concepts of entropy. We
have a container of gas which is partitioned into two equal sides;
each side is in thermal equilibrium with the other. The walls
and the partition of the container are perfect insulators.
Now imagine there is a very small demon who is waiting at the
partition next to a small trap door. He can open and close the
door with negligible work. Let's say he opens the door to allow a
fastmoving molecule to travel from the left side to the right, or
for a slowmoving molecule to travel from the right side to the
left, and keeps it closed for all other molecules. The net effect
would be a flow of heat  from the left side to the right  even
though the container was in thermal equilibrium. This is clearly
a violation of the second law of thermodynamics.
So where did we go wrong? It turns out that information has
to do with entropy as well. In order to sort out the molecules
according to speeds, the demon would be having to keep a memory of
them  and it turns out that increase in entropy of the
maintenance of this simple memory would more than make up for the
decrease in entropy due to the heat flow.
 Maxwell's equations (J.C. Maxwell; 1864)

Four elegant equations which describe classical electromagnetism
in all its splendor. They are:

Gauss' law

The electric flux through a closed surface is
proportional to the algebraic sum of electric charges
contained within that closed surface; in differential
form,
div E = rho,
where rho is the charge density.

Gauss' law for magnetic fields

The magnetic flux through a
closed surface is zero; no magnetic charges exist. In
differential form,
div B = 0.

Faraday's law

The line integral of the electric field around
a closed curve is proportional to the instantaneous time
rate of change of the magnetic flux through a surface
bounded by that closed curve; in differential form,
curl E = dB/dt,
where d/dt here represents partial differentation.
 Ampere's law, modified form

The line integral of the
magnetic field around a closed curve is proportional to the
sum of two terms: first, the algebraic sum of electric
currents flowing through that closed curve; and second,
the instantaneous time rate of change of the electric flux
through a surface bounded by that closed curve; in
differential form,
curl H = J + dD/dt,
where d/dt here represents partial differentiation.
In addition to describing electromagnetism, his equations also
predict that waves can propagate through the electromagnetic
field, and would always propagate at the same speed  these are
electromagnetic waves; the speed can be found by computing
(epsilon_{0} mu_{0})^{1/2}, which is c, the
speed of light in vacuum.
 mediocrity principle

The principle that there is nothing particularly interesting about
our place in space or time, or about ourselves. This principle
probably first made its real appearance in the scientific
community when Shapley discovered that the globular clusters
center around the center of the Galaxy, not around the solar
system. The principle can be considered a stronger form of the
uniformity principle; instead of no place being significantly
different than any other, the mediocrity principle indicates that,
indeed, where you are is not any more special than any other.
 Meissner effect (W. Meissner; 1933)

The decrease of the magnetic flux within a superconducting metal
when it is cooled below the transition temperature. That is,
superconducting materials reflect magnetic fields.
 metre; meter; m

The fundamental SI unit of length, defined as the length of the
path traveled by light in vacuum during a period of 1/299 792 458
s.
 MichelsonMorley experiment (A.A. Michelson, E.W. Morley; 1887)

Possibly the most famous nullexperiment of all time, designed to
verify the existence of the proposed "lumeniferous aether" through
which light waves were thought to propagate. Since the Earth
moves through this aether, a lightbeam fired in the Earth's
direction of motion would lag behind one fired sideways, where no
aether effect would be present. This difference could be detected
with the use of an interferometer.
The experiment showed absolutely no aether shift whatsoever,
where one should have been quite detectable. Thus the aether
concept was discredited as was the idea that one measures the
velocity of light as being added vectorially to the velocity of
the emitter.
See constancy principle.
 Millikan oil drop experiment (R.A. Millikan)

A famous experiment designed to measure the electronic charge.
Drops of oil were carried past a uniform electric field between
charged plates. After charging the drop with xrays, he adjusted
the electric field between the plates so that the oil drop was
exactly balanced against the force of gravity. Then the charge on
the drop would be known. Millikan did this repeatedly and found
that all the charges he measured came in integer multiples only of
a certain smallest value, which is the charge on the electron.
 mole; mol

The fundamental SI unit of substance, defined as the amount of
substance that contains as many elementary units (atoms,
molecules, ions, etc.) as there are atoms in 0.012 kg of
carbon12.
 mu_0

See permeability of free space.
 muon experiment

An experiment which demonstrates verifies the prediction of time
dilation by special relativity. Muons, which are shortlived
subatomic particles, are created with enormous energy in the upper
atmosphere by the interaction of energetic cosmic rays. Muons
have a very short halflife in their own reference frame, about 2.2
us. Since they are travelling very close to c, however, time
dilation effects should become important. A naive calculation
would indicate that, without special relativistic effects, the
muons would travel on the average only about 700 m before
decaying, never reaching the surface of the Earth. Observations
reveal, however, that significant numbers of muons do reach the
Earth. The explanation is that muon is in a moving frame of
reference, and thus time is slowed down for the muons relative to
the Earth, effectively extending the halflife of the muons
relative to the Earth, allowing some of them to reach the surface.

The laws list
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Laws, rules, principles, effects, paradoxes, limits, constants, experiments, & thoughtexperiments in physics.

