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6 answers
Newton's 3rd law... hitting drywall (which I break) vs hitting a brick (which breaks me)?
According to the Third Newton's law of motion:
For every action there is an equal and opposite reaction.
So, I understand that if I hit a brick wall with $50\, \mathrm{lbs}$ of force, the brick wall also hits me with $50\, \mathrm{lbs}$ of…
Sidney
- 1,066
44
votes
5 answers
What is the use of a Universal-NOT gate?
The universal-NOT gate in quantum computing is an operation which maps every point on the Bloch sphere to its antipodal point (see Buzek et al, Phys. Rev. A 60, R2626–R2629). In general, a single qubit quantum state, $|\phi\rangle = \alpha |0\rangle…
Joe Fitzsimons
- 8,425
44
votes
4 answers
Virtual vs Real image
I'm doing magnification and lens in class currently, and I really don't get why virtual and real images are called what they are.
A virtual image occurs the object is less than the focal length of the lens from the lens, and a real image occurs when…
Jonathan.
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44
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3 answers
An explanation of Hawking Radiation
Could someone please provide an explanation for the origin of Hawking Radiation? (Ideally someone who I have been speaking with on the h-bar)
Any advanced maths beyond basic calculus will most probably leave me at a loss, though I do not mind a…
Noah P
- 1,788
44
votes
8 answers
Why doesn't an electron ever hit (and stick on) a proton?
Imagine there is a proton confined in a box and we put an electron at 10 cm distance:
It gets an acceleration of thousands of meters/second^2 along a straight line joining the two CM's.
One would expect the electron to hit the positive particle in…
user104372
44
votes
3 answers
Why is 7 TeV considered as a big amount of energy?
Considering that $7$ TeV is more or less the same kinetic energy as a mosquito flying, why is it considered to be a great amount of energy at the LHC?
I mean, a giant particle accelerator that can only provide 7 TeV of energy? (14 in the mass…
Les Adieux
- 3,705
44
votes
1 answer
What, to a physicist, are instantons and the Donaldson invariants?
I study gauge theory from a mathematical perspective. To me, one of the most fundamental ideas is the notion of an instanton on a 4-manifold.
To be precise, I have a Riemannian 4-manifold and a principal $G$-bundle $E$ over it (usually $G=SU(2)$,…
user101446
44
votes
5 answers
Can the Lorentz force expression be derived from Maxwell's equations?
The electromagnetic force on a charge $ e $ is
$$ \vec F = e(\vec E + \vec v\times \vec B),$$
the Lorentz force. But, is this a separate assumption added to the full Maxwell's equations? (the result of some empirical evidence?) Or is it somewhere…
quark1245
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44
votes
5 answers
Why do we need coordinate-free descriptions?
I was reading a book on differential geometry in which it said that a problem early physicists such as Einstein faced was coordinates and they realized that physics does not obey man's coordinate systems.
And why not? When I am walking from school…
Shamisen Expert
- 703
44
votes
4 answers
Why doesn't light, which travels faster than sound, produce a sonic boom?
I know that when an object exceeds the speed of sound ($340$ m/s) a sonic boom is produced.
Light which travels at $300,000,000$ m/s, much more than the speed of sound but doesn't produce a sonic boom, right?
Why?
A.R.K
- 1,423
44
votes
13 answers
Mechanics around a rail tank wagon
Some time ago I came across a problem which might be of interest to the physics.se, I think. The problem sounds like a homework problem, but I think it is not trivial (i am still thinking about it):
Consider a rail tank wagon filled with liquid, say…
Martin Gales
- 2,687
44
votes
7 answers
Does a photon in vacuum have a rest frame?
Quite a few of the questions given on this site mention a photon in vacuum having a rest frame such as it having a zero mass in its rest frame. I find this contradictory since photons must travel at the speed of light in all frames according to…
Physiks lover
- 3,098
44
votes
5 answers
Have we figured out how to analyze turbulent fluids?
I was surprised to read that we don't know how to analyze turbulent fluids. On page 3-9 of The Feynman Lectures on Physics (Volume One), Feynman writes:
Finally, there is a physical problem that is common to many fields, that is very old, and that…
44
votes
4 answers
Cooling a satellite
Satellites are isolated systems, the only way for it to transfer body heat to outer space is thermal radiation. There are solar panels, so there is continuous energy flow to inner system. No airflow to transfer the accumulated heat outer space…
inninaro
- 563
44
votes
3 answers
History of interpretation of Newton's first law
Nowadays it seems to be popular among physics educators to present Newton's first law as a definition of inertial frames and/or a statement that such frames exist. This is clearly a modern overlay. Here is Newton's original statement of the law…
user4552