Ken G
  • Member for 5 years, 4 months
  • Last seen more than 2 years ago
Equation of continuity for stones
Accepted answer
20 votes

The answer to the first question is that the rate of flow involves not just the velocity and cross sectional area, but also density. With stones in a pipe, the area stays the same, so when the ...

View answer
Why does a full moon seem uniformly bright from earth, shouldn't it be dimmer at the "border"?
20 votes

I've wondered the same thing, why the edges of the full Moon don't look darker, and why the terminator for a quarter-Moon phase doesn't look dimmer than the point at the edge of the Moon that is ...

View answer
What evidence do we have that CMB is the result of the Big bang?
10 votes

I know this isn't quite what you are asking, but it is important because otherwise the question cannot be answered. I would say this question exhibits a subtle but scientifically important error in ...

View answer
Does Gauss law depend on the fact that electric force law is inverse square law?
6 votes

The key point is that Gauss' law works for forces that emanate like lines of force out of the sources, whatever is their symmetry. To say that we have an inverse square force is to assume each ...

View answer
Is it heat that causes vibrations on particles or is it the vibrations that causes the heat?
5 votes

In addition to those nice answers, I would point out that the words "heat" and "heat up" are examples where the term "heat" is used in very different ways, so one must be careful. Generally ...

View answer
So is quantum entanglement actually FTL "communication" or is it mundane pre-determination?
4 votes

"How on earth is there a third possibility to this?" That is exactly the $64,000 question of entanglement, but observations show that there is indeed a third possibility. It is possible for ...

View answer
$dW=P\,dV$ or $dW=-P\,dV$?
4 votes

Physics equations always have well-defined sign conventions, but it is not always easy to remember them. Hence, in practice, it is generally advisable to manually apply the signs that make sense. ...

View answer
Super-luminal universal collapse
Accepted answer
3 votes

If the universal dynamics obeys the cosmological principle, then the answer is we could tell if it was contracting right now. The cosmological principle is the key simplification of modern ...

View answer
A More Intuitive Solution for the Kinematics of the Pulley-Rope System
3 votes

There's an easier way to get your solution, just call the length $OA$ equal to $1$ (it's constant, so we can scale all lengths to it), and call length AB equal to $z$. Then $u$ equals the negative ...

View answer
How would this system behave?
3 votes

I think you are missing that the system must have two stable equilibria, one on each side, even though you have made the central equilibrium unstable. Or, thinking in 3D, the stable equilibrium is a ...

View answer
Electron-atom collision problem, Bohr's model of atom
Accepted answer
3 votes

You conserve momentum by making the atom recoil as needed, when the photon is emitted. The recoil will involve negligible kinetic energy, but you can always solve for it self-consistently if you want. ...

View answer
How Quantum-entangled particles communicate with each other
3 votes

There isn't necessarily any need for entangled particles to communicate at all, that requirement stems from a way of thinking called "local realism" (that each particle has its own attributes that it ...

View answer
If we were on the Moon would Earth appear to be in motion or at rest?
3 votes

You are correct, though there would be some small movement due to the fact that the Moon's orbit is not a perfect circle so the Moon is not in perfect synch.

View answer
Does friction oppose rotational or translational motion?
3 votes

The book is assuming that the ball is rolling without sliding, so the direction of rotation is fixed by that constraint. Also, if there is no sliding, the problem is completely time reversible. When ...

View answer
Difference between the meanings of the equations Stress=Young's modulus x Strain and F=-kx
Accepted answer
3 votes

The Young's modulus equation is a more detailed version of Hooke's law. In Hooke's law, you just subsume the details of the spring into its spring constant k, but in Young's approach, you generalize ...

View answer
Are living organisms deterministic?
3 votes

Note that systems are not deterministic, dynamical theories are. So you can look at a dynamical theory, and see if "deterministic" is an attribute of that theory or not, but all you can do with a ...

View answer
Why is $0K$ impossible to reach if the heat capacity goes to $0$ as $T$ approaches $0 K$?
3 votes

You can't see Nernst's theorem by looking at the heat transport, or you get just the problem you are asking about. The key to Nernst's theorem is that you can't just force a system to give up heat-- ...

View answer
Circumference of the earth
3 votes

It's not possible, those are two very different circumferences, are they not?

View answer
First resonant frequency of a ring
Accepted answer
2 votes

The key feature for getting resonance is coherence, so all you need is for the phase to return to the start as you go completely around. So that's one full wavelength per circumference. If you only ...

View answer
In which direction does the friction force due to incline act on a car driving in circles on an inclined speedway?
Accepted answer
2 votes

Friction is always along the surface, and there are not two frictional forces here-- just one. The friction here is called "static friction" (believe it or not, even though the wheels are turning the ...

View answer
Why a pair of electrons with $E<E_F$ is a bound state
2 votes

It just means that to put those two electrons into the state of two free electrons, they'd each need an energy above the Fermi level, so the total energy would need to end up above twice that. Since ...

View answer
What is the difference between Quasi static and Dynamic simulation?
Accepted answer
2 votes

In physics, statics is the subset of physical systems for whom all forces balance, i.e., systems that do not accelerate. Dynamics is the calculation of acceleration. When you stick the prefix "...

View answer
Temperature required for ionisation
2 votes

You need to know the electron density as well as the ionization energy X. You might think you'd get ionization when kT ~ X, but actually you tend to get ionization when kT << X. For example, ...

View answer
Gravitational waves and negative pressure
2 votes

I would be surprised if gravitational waves represented negative pressure. The usual explanation for negative pressure is an energy that increases when volume increases. But I wouldn't have thought ...

View answer
Why is the work done by a battery associated to a potential energy even if the processes are not reversible/conservative?
Accepted answer
2 votes

You always have conservation of energy, so the only difference with "conservative" potentials is that they are reversible and path-independent. But if you burn gasoline and it releases energy, and ...

View answer
Pumping (going faster and faster) on a swing
2 votes

Even if we use energy stored in our bodies, we cannot produce a net momentum in a system that has only internal forces on it. So you know the swing must have an external force on it. It can't be ...

View answer
Is diffraction encoded in the wave equation?
2 votes

Sure, the wave equation has everything except the boundary conditions, and that you always have to do manually anyway. So to do a single or double slit, you need some way to tell the wave equation ...

View answer
I heard light was its own antiparticle. What happens if two photons exactly cancel each other out?
2 votes

It sounds like you are talking about when a photon pair collides and makes an electron-positron pair (for example), which we might call annihilation of the photons (just like any particle/antiparticle)...

View answer
Question about mass measurement and Archimedes law
Accepted answer
2 votes

It's not correct, there's no "2" in the first expression, if I understand the setup. It sounds like you are saying that you put a beaker filled to the top on a scale, and zero the scale. Then you ...

View answer
When sound is cancelled in an acoustic duct, where does the energy go?
2 votes

Generally speaking, when you use one wave to get cancellation of another at some point, it means you get constructive interference somewhere else. So the sound goes somewhere else. There are ways to ...

View answer
1
2 3 4 5 6