
Contributed by ZapperZ and arildno.
This question has popped up many times. So here is an attempt to address it.
...

May2509 10:31 AM
ZapperZ

0 
26,761 
Do Photons Move Slower in a Solid Medium?
Contributed by ZapperZ. Edited and corrected by Gokul43201 and inha
...

Feb206 07:47 PM
ZapperZ

0 
12,013 
WHY DON’T ELECTRONS CRASH INTO THE NUCLEUS IN ATOMS?
Contributed by Marlon and edited by ZapperZ
If one...

Dec2005 01:14 PM
ZapperZ

0 
10,501 
This forum contains simple responses to the most frequentlyasked questions in the Physics subforums. It is a...

Dec2005 01:13 PM
ZapperZ

0 
9,175 
IS LIGHT A WAVE OR A PARTICLE?
Contributed by Marlon and ZapperZ.
In our ordinary world, “wave” and “particle”...

Dec2805 01:43 PM
ZapperZ

0 
8,377 
Contributed by vanesch.
The short answer is: no, there's no violation.
The longer answer can be this: Given the...

May1106 10:35 AM
ZapperZ

0 
7,644 
Written by Hootenanny. Edited by berkeman and Kurdt. Feel free to contact me with any questions, comments or...

Feb308 12:04 PM
Hootenanny

0 
7,609 
Definition/Summary
Work is a measure of change of energy.
The net work done equals the change in Kinetic Energy...

Jul2414 11:39 AM
Greg Bernhardt

0 
459 
Definition/Summary
Stress is force per area, and is a tensor.
It is measured in pascals (Pa), with dimensions of...

Jul2414 11:29 AM
Greg Bernhardt

0 
189 
Definition/Summary
Voltage is electric potential difference, which is potential energy difference per charge: V\ =\...

Jul2414 11:36 AM
Greg Bernhardt

0 
181 
Definition/Summary
The doubleslit (or 'twoslit') experiment clearly demonstrates that individual particles...

Jul2414 11:38 AM
Greg Bernhardt

0 
181 
Definition/Summary
Virtual particles are a mathematical device used in perturbation expansions of the Soperator...

Jul2414 11:35 AM
Greg Bernhardt

0 
171 
Definition/Summary
Take the case of a string with it's one end tied to a wall. The other end is held by you. It...

Jul2414 11:38 AM
Greg Bernhardt

0 
171 
Definition/Summary
One of the most asked questions is concerning how to derive the Heisenberg Uncertainty Relation....

Jul2414 11:34 AM
Greg Bernhardt

0 
170 
Definition/Summary
A Slater determinant is a representation of a manyparticle wave function for a system of...

Jul2414 11:26 AM
Greg Bernhardt

0 
169 
Definition/Summary
Susceptibility is a property of material. In a vacuum it is zero.
Susceptibility is an...

Jul2414 11:29 AM
Greg Bernhardt

0 
168 
Definition/Summary
Vectors (such as velocity or force or momentum) obey the Vector Law of Addition.
That means...

Jul2414 11:35 AM
Greg Bernhardt

0 
165 
Definition/Summary
A wave function is a mathematical function that describes a physical system in quantum...

Jul2414 11:37 AM
Greg Bernhardt

0 
162 
Definition/Summary
This is a derivation of the Uncertainty Principle based on the properties of noncommuting...

Jul2414 11:33 AM
Greg Bernhardt

0 
161 
Definition/Summary
Snell's Law relates the angles of incidence and refraction, for a light ray that passes between...

Jul2414 11:27 AM
Greg Bernhardt

0 
155 
Definition/Summary
A spring is a mechanical device that produces a restoring force when stretched or compressed....

Jul2414 11:28 AM
Greg Bernhardt

0 
150 
Definition/Summary
Timeorder means in order of time. A sequence or product is in timeorder if "earlier" items are...

Jul2414 11:31 AM
Greg Bernhardt

0 
150 
Definition/Summary
Torque is the moment of a force about a point.
Torque is a vector (strictly, a pseudovector,...

Jul2414 11:32 AM
Greg Bernhardt

0 
149 
Definition/Summary
Pressure is normal force per area, or work done per volume, or mechanical energy per volume...

Jul2414 11:11 AM
Greg Bernhardt

0 
148 
Definition/Summary
Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting...

Jul2414 11:14 AM
Greg Bernhardt

0 
145 
Definition/Summary
Poincare's integral invariant is the most fundamental invariant in Hamiltonian Dynamics. For any...

Jul2414 11:07 AM
Greg Bernhardt

0 
144 
Definition/Summary
There are three masses of neutrino (they are unknown, but the differences of their squares is...

Jul2414 10:55 AM
Greg Bernhardt

0 
142 
Definition/Summary
For a given material, the resistance of a length of the material is a constant times its length,...

Jul2414 11:17 AM
Greg Bernhardt

0 
141 
Definition/Summary
Moments of area are properties of rigid bodies, they depend only on shape and not on density,...

Jul2414 10:50 AM
Greg Bernhardt

0 
140 
Definition/Summary
Nonlinear optical processes that occur due to the presence of a secondorder nonlinear...

Jul2414 10:58 AM
Greg Bernhardt

0 
139 
Definition/Summary
Light propagating through a vacuum will obey the principle of superposition, however this is not...

Jul2414 10:57 AM
Greg Bernhardt

0 
138 
Definition/Summary
Momentum is mass times velocity. It is a vector.
\mathbf{p} = m\mathbf{v}
Angular momentum...

Jul2414 10:54 AM
Greg Bernhardt

0 
137 
Definition/Summary
Net force = rate of change of momentum (= mass times acceleration of the centre of mass, if the...

Jul2414 10:56 AM
Greg Bernhardt

0 
137 
Definition/Summary
Permittivity, \varepsilon, of a material is its ability to separate charge when a voltage...

Jul2414 11:05 AM
Greg Bernhardt

0 
137 
Definition/Summary
An object (typically a "mass on a spring") which has a position (or the appropriate...

Jul2414 11:24 AM
Greg Bernhardt

0 
133 
Definition/Summary
This entry describes diffraction of a wave when it passes through a single narrow slit.
...

Jul2414 11:24 AM
Greg Bernhardt

0 
133 
Definition/Summary
The moment of a vector is the crossproduct of the position vector with that vector (e.g., a...

Jul2414 10:53 AM
Greg Bernhardt

0 
131 
Definition/Summary
In the Hamiltonian formulation of classical mechanics, equations of motion can be expressed very...

Jul2414 11:08 AM
Greg Bernhardt

0 
131 
Definition/Summary
"Rolling" means moving along a surface without sliding.
The (instantaneous) point of contact...

Jul2414 11:19 AM
Greg Bernhardt

0 
131 
Definition/Summary
Moment of Inertia is a property of rigid bodies.
It relates rotational force (torque) to...

Jul2414 10:51 AM
Greg Bernhardt

0 
130 
Definition/Summary
Potential energy is simply another name for (minus) the work done by a conservative force.
...

Jul2414 11:09 AM
Greg Bernhardt

0 
128 
Definition/Summary
The second law has various forms. I shall give these here then show how they are logically...

Jul2414 11:23 AM
Greg Bernhardt

0 
128 
Definition/Summary
This is the quantummechanical version of the classical harmonic oscillator. Like the classical...

Jul2414 11:13 AM
Greg Bernhardt

0 
126 
Definition/Summary
Why is a sound wave reflected at the open end of a pipe? Why doesn't it just "escape"?
...

Jul2414 11:04 AM
Greg Bernhardt

0 
124 
Definition/Summary
Between two bodies in contact, there is a pair of Reaction Forces.
They are always equal but...

Jul2414 11:16 AM
Greg Bernhardt

0 
124 
Definition/Summary
Often in physics we need to consider frames of reference that are noninertial (the Earth...

Jul2414 11:20 AM
Greg Bernhardt

0 
123 
Definition/Summary
Angular momentum carries over from classical mechanics into quantum mechanics, but...

Jul2414 11:12 AM
Greg Bernhardt

0 
120 