# All Questions

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### What can currently be deduced about empty hyperspace?

Michio Kaku's explanation of universes in hyperspace in this youtube video gives a metaphor of our universe being the surface of a hyperspace bubble that's currently becoming larger. He also says ...
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### How do you add temperatures?

This will probably be considered very simple, but I am just a beginner: I'm developing a software application where temperatures need to be added and subtracted. Some temperatures are in Celsius, ...
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### Foucault pendulum initial velocity

This is a basic question I can't solve and it seems it is not addressed on the web. Sorry for my lame painting skills. Assume we have a foucault pendulum suspended in the north pole. the pivot is ...
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### Why is momentum conserved in an inelastic collision and kinetic energy is not conserved?

We know that in an inelastic collision that total momentum of the system before collision equals the total momentum after collision. But total kinetic energy before collision is not equal to total ...
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### Penetration of a bullet into a wall

In which of the following situation will the bullet of the pistol penetrate deeper in the wall? Why? Alternatively will the penetration in both cases be the same? Can you explain the concepts behind ...
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### Resistance and amperage

my problem is to solve the following exercise. First if R1 = R2 then I1 = 1,0 A (ampere). Now I have to find out I1 if R2 = 2*R1. If R1 = R2 then I have got for the total resistance 1 ohm. Is this ...
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Any object can emit and absorb radiation and the power of emission can be represented by the Stefan-Boltzmann law: $$P=A\epsilon\sigma T^4$$ In many texts the net power radiated is the difference ...
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### Geodesic deviation on a 2-sphere - is this the right track?

Apologies if I have this completely wrong (and for the general long-windedness). I've searched online but can't find anything helpful/relevant. I'm trying to use the geodesic equation ...
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### Length of the missing part [on hold]

I have got a very interesting question. A car is driving this way on the picture down. In the looping is a missing part. The radius of the looping is r = 40cm. The car starts at the height h = 3r. If ...
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### Why does product of Moduli and Diff x Weyl Variation vanish?

According to equation 5.2.5 in Polchinski :- $$\int d^2 \sigma~ \delta^{'}g_{ab} \times [-2(P_1 \delta \sigma)_{ab} +(2\delta w - \Delta \cdot \delta \sigma)g^{ab}]=0$$ The assumption here is that " ...
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### Reach every point in phase space

I think that found that the potential $$V(x):=V_0 (1- |x|)^2$$ meets every point in phase space for $x \in [-1,1]$, which means that it fulfills the necessary condition to be ergodic. Is this ...
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### A beam of light, traveling around a planet

Some time ago, I happened to have found one problem: The refractive index of the planet's atmosphere decreases with height above the surface under the following law: $$n=n_0-\alpha h$$ ...
17 views

### A ball rolling in a bowl [on hold]

There is a small ball with a radius of 2 mm inside of a bowl with a radius of 22 mm. If the bowl is tilted 1 degree in any direction, what does the slope of the bowl need to be in order to get the ...
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### What moves an object? Momentum or Kinetic Energy?

What exactly makes an object move? Momentum is $\vec p= mv$ where $m$ is mass of an object and $v$ is the velocity of an object, and as a result of this when I fire a bullet the bullet may travel much ...
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### Microscope Objective / NA, F/#, Exit aperture - when are they non-intuitive?

NA = sin(theta) where theta is the half-angle (see: http://en.wikipedia.org/wiki/Numerical_aperture) F/# = (focal length)/(lens diameter) (same reference as above) An infinity corrected microscope ...
12 views

### What happens to light at sharp points?

At the tip of the sharp point shown, what will happen to light incident on it. This curiosity was invoked by a friend and also my childhood of watching shiny pointed swords in cartoons. Original ...
66 views

### Understanding the Charge Conjugation Operator

I am trying to understand the charge conjugation operator. http://en.wikipedia.org/wiki/C_parity Because the operator is Hermitian, this seems to imply that there is a (possibly spontaneous?) ...
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### Why does frequency increase as the length of an open air column shortens?

I am curious as to why the frequency of a wavelength increases as an open air column becomes smaller in length. Is it because an open air column will always contain half a wavelength, therefore if the ...
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### Center of mass two particles with one moving

Two small particles of mass $m_1$ and mass $m_2$ attract each other with a force that varies with the inverse cube of their separation. At time $t_0$ , $m_1$ has velocity v directed towards $m_2$ , ...
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### How was the energy formula found and who found it?

This is mostly a history of Physics question: Who found the formulas for energy, for instance that the kinetic energy of a system state is $\frac{1}{2}mv^{2}$, and how was this actually discovered? ...
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### Lens parameters / Principal Planes vs. Entrance and Exit Pupil

Under what conditions are the Principal Planes not the entrance and exit pupil? For the definition of Principal Planes see: http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/priplan.html ...
252 views

### Positivity of Total Gravitational Energy in GR

I read the following statement in the introduction to an article: Over the last 30 years, one of the greatest achievements in classical general relativity has certainly been the proof of the ...
313 views
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### Is there a quantity measured in kilogram seconds?

I'm trying to get a full grasp on the relationship between many of the units that are used in kinetic physics. I've found that it is possible to make a venn-diagram that shows the factors of many of ...
43 views

### Magnification of an astronomical telescope not in normal adjustment?

I am stuck on this question: A telescope consists of two thin converging lenses of focal lengths 100cm and 10cm respectively. It is used to view an object 2000cm from the objective. What is the ...
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### Is gravity not a force?

When I jump out of a plane and begin to fall and accelerate, I feel no force whatsoever. When I stand on the earth, I feel only the force of the Earth pushing me outwards. I contend the accelerating ...
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### Faraday's law and “infinite induction”

I may have confused after thinking too much about Faraday's law. If an emf is induced in a circuit due to some changing magnetic field, the induced current will be in a direction such that the ...
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### What forces of stress really represent?

I'm studying fluid mechanics and the book says the following: For any continuum, forces acting on a piece of material are of two types. First there are forces of stress, whereby the piece of ...
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### Why do we fall down when the bicycle slows down? [duplicate]

My question is: It is easy to balance a bicycle when it is moving at a fairly high velocity, say 7 m/s or 25 km/hr. But when a bicycle slows down, it is hard to keep it upright, and the person riding ...
5k views

### Does a current carrying wire produce electric field outside?

In the modern texts of electromagnetism in the presence of stationary currents the electric field is assumed conservative $\nabla \times E =0$. Using this we get $E_{||}^{out}=E_{||}^{in}$ which ...
878 views

### A force opposing Gravity

Every Action Has An Equal and Opposite Reaction (Newton's Third Law.) If this is the case, does gravity have an equal-opposing force? From asking around I still haven't got a very clear answer; ...
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### Why there is a time gap between the news presenter and the field reporter?

This one has been in my mind for years, but I simply can not find any reason for this event. The Situation Imagine yourself sitting in the living room while watching television. It's 9 p.m and the ...
42 views

### Can human beings be the best possible design physically? [on hold]

by the stature and design of the human body is it possible that the best size and structure physically goes to a human being .. if not so then what could be the improvements and flaws in it ? is there ...
58 views

### Frequency difference of absorbed and emitted photons?

In a transparent medium, how does the frequency of incoming light compare to the exiting light? I thought it was the same, but doesn't there have to be an energy difference in two electron states in ...
19 views

### Inductor Charging and Discharging

Why does the inductor discharges in the same path ( same direction of current ) as that of its charging unlike a capacitor which discharges in the direction opposite to its charging(current direction ...
19 views

### Superfluid $\Leftrightarrow$ Bose-Einstein condensate?

On the Wikipedia page on superfluidity one can find the sentence not all Bose-Einstein condensates can be regarded as superfluids, and not all superfluids are Bose–Einstein condensates. So I was ...
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### What would a laser beam's E-M waves actually look like if you zoomed in?

I am trying to understand how a real world beam of laser actually reflects the physics description of oscillating electromagnetic waves. So say we are looking side on at a vertically polarized laser ...
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### How many measurements of a physical quantity should you do to get a representative value / distribution?

When we measure a physical quantity multiple times, we can calculate several values which should help us understand the whole measurement better: Standard deviation, variance, median, average, and so ...
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### Heat balance: how to obtain T(t) with energy in the form u(T(t))

I have the heat balance equation for a cooling case in the form: $$\frac{dU(T(t))}{dt}=-J$$ with $U$: energy (J) $T$: temperature (K) $t$: time (s) $J$: heat flux leaving the system (W) I have ...
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### kitaev-honeycomb : can't get wilson loop squared to yield +1

I'm new here, loving this website and I'm having some difficulty with the wilson-loop operator in kitaev's honeycomb model. problem statement The Kitaev model (Kitaev, 2006 is the original paper) ...
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### How does a photon travel through glass?

This was discussed in an answer to a related question but I think that it deserves a separate and, hopefully, more clear answer. Consider a single photon ($\lambda$=532 nm) traveling through a plate ...
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### Origin of phases in amplitudes in QFT

Amplitudes in QFT are typically real. I'd like to understand the physical meaning of an amplitude having a phase. I know of three ways that amplitudes can get a phase: If the couplings have an ...
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### Double slit experiment from first principles of QM

I have read many descriptions of electron double slit experiment but I could not find the description from the first principles of quantum mechanics. Most of the descriptions makes comparison with ...
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### How to find a vector normal to a cylinder in cylindric coordinates? [migrated]

I'm trying to solve a problem which demands to multiply a vector M and vector normal to a cylinder's surface in cylindric coordinates. Height of the cylinder is infinite and its radius is R. So how do ...