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1500 questions
40
votes
6 answers
Newton's cradle
Why, when one releases 2 balls in Newton's cradle, two balls on the opposite side bounce out at approximately the same speed as the 1st pair, rather than one ball at higher speed, or 3 balls at lower speed?
jack mosevich
- 401
40
votes
4 answers
Can light exist in $2+1$ or $1+1$ spacetime dimensions?
Spacetime of special relativity is frequently illustrated with its spatial part reduced to one or two spatial dimension (with light sector or cone, respectively). Taken literally, is it possible for $2+1$ or $1+1$ (flat) spacetime dimensions to…
Leos Ondra
- 2,133
40
votes
2 answers
Why does laser light not affect glowing materials?
I have this childrens rubber ball which glows in the dark after it's exposed to light. I "charge" it with a flash light then play with my dogs at night. I thought to try a very intense green laser, and see how the ball reacted.
The laser light had…
Michael
- 780
40
votes
3 answers
Why does my ID card oscillate sideways when walking?
When I was going to my school with my ID card hanging around my neck, it started doing oscillations like a pendulum. I was moving forward and it was oscillating left to right and right to left. What forces are at play here?
Sarthak Sharma
- 875
40
votes
5 answers
What is the answer to Feynman's Disc Paradox?
[This question is Certified Higgs Free!]
Richard Feynman in Lectures on Physics Vol. II Sec. 17-4, "A paradox," describes a problem in electromagnetic induction that did not originate with him, but which has nonetheless become known as "Feynman's…
Terry Bollinger
- 21,997
40
votes
10 answers
Condensation of Water. Classroom Controversy
In our test there was a question that went like so:
Question 4 You have a glass of iced water on an unshaded picnic table and went for a walk for 30 minutes. When you return you noticed
the glass has water on the outside of it.
a. In terms of…
class_question
- 401
40
votes
2 answers
Isn't D'Alembert's wave equation enough to see that Galilean transformations are wrong?
The D'Alembert equation for mechanical waves was written in 1750:
$$\frac{\partial^2u}{\partial x^2}=\dfrac{1}{v^2}\dfrac{\partial^2u}{\partial t^2}$$
(in 1D, $v$ being the propagation speed of the wave)
It is not invariant under a Galilean…
Tropilio
- 640
40
votes
3 answers
Why wouldn't the part of the Earth facing the Sun a half year before be facing away from it now at noon?
The Earth takes 24 hours to spin around its own axis and 365 days to spin around the Sun. So in approximately half a year the Earth will have spun around its axis 182.5 times.
Now take a look at the following picture:
Assuming that the Earth is in…
O S
- 555
40
votes
2 answers
Is our definition of entropy unique?
Suppose we don't know anything about statistical mechanics, not even the existence of atoms.
Why is entropy defined as
$$\delta S=\frac{\delta Q}{T}$$
instead of, say,
$$\delta S=\frac{\delta Q}{T^2}$$
or any other function that will conserve the…
oamer
- 565
40
votes
3 answers
Are there fields corresponding to the composite particles (e.g. hydrogen atom field)?
In classical physics, particles and fields are completely different stuff. However, when a field is quantized, the particles appear as its excitations (e.g. photon appears as a field excitation in the quantization of electromagnetic field). In fact,…
Annera
- 653
40
votes
8 answers
Why is there this relationship between quaternions and Pauli matrices?
I've just started studying quantum mechanics, and I've come across this correlation between Pauli matrices ($\sigma_i$) and quaternions which I can't grasp: namely, that $i\sigma_1$, $i\sigma_2$ and $i\sigma_3$, along with the 2x2 identity matrix…
dahemar
- 2,403
40
votes
6 answers
Is there a theorem that says that QFT reduces to QM in a suitable limit? A theorem similar to Ehrenfest's theorem?
Is there a theorem that says that QFT reduces to QM in a suitable limit?
Of course, it should be, as QFT is relativisitc quantum mechanics.
But, is there a more manifest one? such as Ehrenfest's theorem which dictates the transition from QM to…
Youngsub
40
votes
4 answers
Why is the efficiency of human cells less than the efficiency of an Otto engine?
I always used to think (I don’t know why!) that the efficiency of human (and animal and plant) cells should be equal to or near the efficiency of a Carnot engine or at least should be the highest efficiency among all practical engines. But I…
lucas
- 3,161
- 4
- 22
- 53
40
votes
7 answers
Supergun Launching of Satellites
I should say first that I don't believe this is a feasible launch method, otherwise NASA and other space agencies would be using it by now.
It's based on this BBC news story Saddam Hussein's Supergun but, luckily this monstrosity was never…
user108787
40
votes
3 answers
What is meant by the term "completeness relation"
From my humble (physicist) mathematics training, I have a vague notion of what a Hilbert space actually is mathematically, i.e. an inner product space that is complete, with completeness in this sense heuristically meaning that all possible…
user35305
- 3,167