# All Questions

175 views

### Proving $t=(1+\sqrt{1+2hg/v^2 } ) (v/g)$ for a thrown ball

If we throw a ball from the hight point $h$ from the earth, with initial velocity $v’$, how to prove that the time it takes the ball to reach the earth is given by: ...
214 views

### How is the singularity in Newtonian gravity resolved?

In Who's afraid of a Black Hole? at time ~20:38, Michio Kaku makes a claim that $1/r$ when $r=0$ equals $\infty$ and when physicists see the $\infty$ they see a monstrosity. Coincidentally, Newton's ...
955 views

### How did Einstein derive general relativity?

How did Einstein derive general relativity (GR)? Did he use: the equivalence principle? The principle of least action? Anything else? Note, I'm not looking for a full mathematical derivation of GR! ...
206 views

### An accelerating and shrinking train in special relativity

Suppose when a train is at rest, it has a length of $L$. Let the position of the back of the train at any time be $A$, and let the position of the front of the train at any time be $B$. Now assume ...
79 views

### Where to go to minimize tidal forces?

Suppose you design an experiment where you need to minimize the effects of tidal forces. Where would you go? There are a few possibilities, and the choice depends on how much effort you are willing to ...
379 views

### What kind of cooling mechanism could be used in outer space?

This question arises out of this question on Quora - Apollo 11: 1969 Moon Landing: Did Neil Armstrong really land on the moon? I'm convinced with most of the explanations provided in the first ...
152 views

### What is the simplest system that has both, discontinous and continous phase transitions?

I am looking the simplest system that has both discontinous phase transition and a continous phase transition between the same phases (you can change one parameter). discontinous transition: first ...
1k views

### Is it possible to create only a magnetic wave?

I want to know if its possible to create only a magnetic field,which i can direct to a certain chosen object?
167 views

### How is there a connection between energy and information?

http://physicsworld.com/cws/article/news/2010/nov/19/information-converted-to-energy In the last part of the third paragraph of the above article it says: "In fact, Szilárd formulated an equivalence ...
207 views

### Black hole entropy

Bekenstein and Hawking derived the expression for black hole entropy as, $$S_{BH}={c^3 A\over 4 G \hbar}.$$ We know from the hindsight that entropy has statistical interpretation. It is a measure ...
320 views

### Meaning of subscript in $V=\frac{1}{2}\left(\frac{d^2 V}{{dq_i}{dq_j}}\right)_0$

This is probably a simple question, but what does the subscript $0$ mean in the following expression? $$V=\frac{1}{2}\left(\frac{d^2 V}{{dq_i}{dq_j}}\right)_0$$
74 views

### Am I making the right assumption about a jump discontinuity in the acceleration?

A train heads from Station A to Station B, 4 km away. If the train begins at rest and ends at rest, and its maximum acceleration is 1.5 m/s^2 and maximum deceleration is -6 m/s^2, what's the least ...
51 views

### If a people happen to be fall into the ocean from 50m high, then would the diving pose in the olympiad make less percussion

If a people happen to be fall into the ocean from 50m high, then would the diving pose in the olympiad make less percussion then the leg going first into the water so that the body receive less ...
189 views

### Does the mass of the damper affect the transfer function in a vertical mass-spring-damper?

Usually in system dynamics, I dealt with horizontal mass spring dampers. Now in my advanced class I am dealing with vertical mass spring dampers. So a spring is hanging from the ceiling with a mass ...
73 views

### How Lyman transition (to the ground state from higher excited) happens ? The dipole selection rule is +/- 1?

How are the lyman series observed when the dipole selection rule is +/-1 in l change for hydrogen atom ?
619 views

### Is the principle of least action a boundary value or initial condition problem?

Here is a question that's been bothering me since I was a sophomore in university, and should have probably asked before graduating: In analytic (Lagrangian) mechanics, the derivation of the ...
435 views

### Overcoming Friction

Let's say a box is moved by attaching a rope to it and pulling with an applied force at a certain angle. Now, let's say that this is being done on an inclined ramp. How would I find the acceleration? ...
263 views

### Why is there a factor of 1/2 in the interaction energy of an induced dipole with the field that induces it?

In this paper, there's the following sentence: ...and the factor 1/2 takes into account that the dipole moment is an induced, not a permanent one. Without any further explanation. I looked ...
438 views

### Magnitude of New Comet C/2012 S1 (ISON)

A new comet (magnitude 18.8) has been discovered beyond the orbit of Jupiter. Comet ISON will get within 0.012 AU of the Sun by the end of November 2013 and ~0.4 AU from of Earth early in January ...
367 views

### propeller flying physics for the layman

I'm starting a (quad?)copter build, and i can find plenty of knowledge about stabilizing the craft and things related to gyroscopy. but there's absolutely zero information on things that help me ...
99 views

### How is explained the lower ionization potential for atoms compared to closed shell nobel atoms?

Why adding just one electron changes tremendously the ionization potential from any of the nobel atoms ? If it is the screening why adding a second electron increases the ionization potential ? ...
127 views

### What is the reference spectrum?

What is the reference spectrum? I need to know how to calculate the reference spectrum of a wavelength 500nm.
395 views

### Why 2s state is lower in energy that 2p state in atoms?

The s orbital have higher probability to be closer to the core and feels larger attraction than the p orbital and on average is further away and in addition p has repulsive potentilal l(l+1)h^2/2mr^2. ...
91 views

### Decomposition of elastic constants of a crystalline material

I have performed a calculation the tensor of elastic constants (or stiffness tensor) for a given crystalline material. From there, I calculated various elastic properties, such as Young’s modulus, ...
61 views

### A physical quantity that is a real combination and commutability

Suppose that a matrix $$A ~=~ x_1 B + x_2 C$$ is a linear combination of two self-adjoint matrices $B$ and $C$. I'm interested in when $A$ represents a physical quantity. When the linear ...
1k views

### Where does the equation $E_q = \frac{kq}{r^2}$ come from in this problem?

A point charge is placed at each corner of a square with side length $a$. The charges all have the same magnitude $q$. Two of the charges are positive and two are negative, as shown in the ...
143 views

### How closely may an artificial satellite orbit Earth without decaying appreciably each orbit?

Just what the title states. An artificial satellite will orbit Earth with some defined angular momentum. An aircraft,on the other hand, relies upon it's engines to provide continuous forward velocity ...
387 views

### Velocity using spring contraction

I'm trying to determine the maximum final velocity of a body with which it contracts a spring until it touches the wall. It happens only with the presence of maximum energy. So, I've taken the kinetic ...
144 views

### Does a rotating magnetic monopole have electric and magnetic moment in classical view?

Would a rotating sphere of magnetic monopole charge have electric moment ? In a duality transformation E->B.c etc. how is the magnetic moment translated m = I.S -> ? Mel = d/dt(-Qmag/c).S ? A more ...
460 views

### What's the difference between electron movement and charge movement in electricity? [duplicate]

Possible Duplicate: how does electricity propagate in a conductor? I have read that in an electrical wire electrons movement is very slow while the energy or charge is very fast. What's the ...
215 views

### Why is fractional statistics and non-Abelian common for fractional charges?

Why non integer spins obey Fermi statistics? Why is fractional statistics and non-Abelian common for fractional charges?
100 views

### How is retrograde motion explained in Tycho's model?

Tycho proposed a model of the solar system where all planets but Earth move around the sun while the sun and the moon move around the earth. I wonder how this model could explain the retrograde ...
668 views

### Do we say that phonon has effective mass through its dispersion relation?

The effective mass is proportional to the second derivative of the dispersion relation d2k/dE2. Do we say that phonon have effective mass through it ? Spin wave have.
281 views

### What is the difference between “in-plane shear modulus” and “shear modulus”

In the wiki it is article only about shear modulus and it is written that it's unit is Pascal. At the same time in all researches about Red Blood Cells shear stresses, authors write "in-plane shear ...
284 views

### What is the variation of Gauss-Bonnet term a total derivative of?

What is the variation of Gauss-Bonnet term total derivative of? i.e. Variation of Gauss-Bonnet combination $= \nabla_{\mu} C^{\mu}$. What's $C^{\mu}$ in 4-dimensions?
173 views

### The radius of the universe and time

Ok - fair warning - Non-physicist asking dumb-assed questions here again. I've been reading a lot of Einstein, Feynmann, Ferris etc. I'm just loving this stuff. But I suddenly found myself looking at ...
657 views

### Uniform Circular Motion

Why does the object not go inward, into the circle if the acceleration is inward? I think its because the velocity to outward? So they sort of cancel each other out? But if the speed is kept ...
56 views

### how is the reference for the star maps in google determined?

If I go to google sky - http://www.google.com/sky/ Ihere is a longitude and latitude measure at the bottom of the page. Iow are these coordinates determined and who sets it? Do those coordinates ...
1k views

### Lagrangian to Hamiltonian in Quantum Field Theory

While deriving Hamiltonian from Lagrangian density, we use the formula $$\mathcal{H} ~=~ \pi \dot{\phi} - \mathcal{L}.$$ But since we are considering space and time as parameters, why the formula ...
74 views

### Is this phase right?

Hello at physics lectures we wrote a phase of a sine wave like this: $$\phi = kx - \omega t$$ Is this right? As I recall the phase of a wave should be written like this: $$\phi = \omega t - kx$$ ...
271 views

### Number of unique 2-electron integrals

Consider 2-electron integrals over real basis functions of the form (\mu\nu|\lambda\sigma) = \int d\vec{r}_{1}d\vec{r}_{2} \phi_{\mu}(\vec{r}_{1}) \phi_{\nu}(\vec{r}_{1}) r_{12}^{-1} ...
154 views

### Breaking of conformal symmetry

I am wondering something about the breaking of conformal symmetry: I know that it can be broken at the quantum level, anomalously, but I never encountered or heard about a model where it is broken "à ...
3k views

### Why is the solar noon time different every day?

If you check the local time for solar noon is different every day. Why is it so? Is it because Earth doesn't make a complete rotation in exactly 24 hours?
316 views

### Extracting acceleration vector from rotated aircraft

Suppose we have an aircraft with accelerometer measuring accelerations along each axis. It is mounted in a way so it is perpendicular to the plane in all axes (that should be obvious). We also have ...
156 views

### Matrix representing the quantity - why can some matrices not be physical quantity?

In Heisenberg picture, my textbook says that the following matrix $A = \frac{5}{3}\Sigma_1 + i\frac{4}{3}\Sigma_2$ cannot represent physical quantity. the book says this is because ...
123 views

### How is charge measured and mass of the electron at the same time?

There are few constants that usually come together, $e/m$ also $h/e$. How are they decoupled? If the speed of light is "derived" as Wikipedia states how meter defined and time?
131 views

### How can the properties of the Earth be determined?

How can the properties of the Earth be determined? And how is the gravitational constant then determined from them? A satellite can determine the G.M.. But, how they are individually determined ...
143 views

### Does a microwave resonantly excite the rotational levels when cooking?

Wikipedia states there is no resonance absorption, but says at the same time that the molecules are oscillating like dipoles, which is kind of the same if you are exciting the rotational levels ? The ...