# All Questions

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### Non-discoveries by the Kepler space telescope: exomoons, co-orbital planets, trojans

I am just reading the review article Advances in exoplanet science from Kepler (arxiv preprint: http://arxiv.org/abs/1409.1595), and I found a remarkable paragraph (last paragraph in section "...
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### Two boxes connected by a rope

Two boxes, $A$ and $B$, are connected to each end of a light vertical rope. A constant force of $70.0N$ is applied to box $A$. Starting from rest, box $B$ descends $11.6m$ in $4.40s$ The tension force ...
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### What does it mean when physical theories are inconsistent?

I am hoping that someone can explain in layman terms why Newtonian mechanics and Maxwell's equations are inconsistent. Wikipedia says that this inconsistency is what led to the development of ...
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### Finding the acceleration given only an angle

This is one of my homework problems for a College Physics I course: Dana has a sports medal suspended by a long ribbon from her rearview mirror. As she accelerates onto the highway, she notices ...
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### How can fusion within the sun be possible if there is no such thing as helium-2 (2 protons, no neutrons)

As stated in the question where does the sun(or other star) get the necessary neutron in order to produce the Helium atom? and how does this process occur (explain how the neutron incorporates).
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### What does kinetic energy at infinity mean in terms of supernovae?

I have noticed that in some paper that the term "kinetic energy at infinity" is used. I understand what potential energy with reference to infinity mean, but what does the term kinetic energy of ...
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### Projections in Polar coordinate system

I really understand what projections in Cartesian coordinate system, I can imagine this, but I absolutely do not understand projection in polar system. For example, I have a speed, $U$, and I must ...
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### Philosophical Interpretation of String Theory [closed]

I want to know whether string theory is supposed to describe the world exactly, or whether it's just an approximation of some more fundamental theory. Is it similar to how the wave-equation ...
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### Density of States vs Dispersion

I have a rather naive question regarding DOS and dispersion. We showed the existence of a band gap in class for a small, periodic perturbation in class last week. When drawing this, the professor ...
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### The elusive difference between impulse and momentum

1) In classical mechanics, impulse is the product of a force, F, and the time, t, for which it acts. The impulse of a force acting for a given time interval is equal to the change in linear ...
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### Absorbative polarisers

Absorbative polarisers are one way of getting linearly polarised light from an unpolarised beam. The key idea is that these materials are manufactured such that light can be absorbed in all ...
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### The forces acting on 2 boxes [closed]

Consider the following figure: The surface is not friction-less. When the force $F$ is applied on mass A: Is the force the mass A applies on mass B larger, smaller or equal then the Force that B ...
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### Are there any difference between gravitational potential of different types of black holes?

There are four possible types of black holes that could exist in the theory of gravitation (called general relativity). Are there any difference between the value of gravitational potential of these ...
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### Definition respective derivation of angular momentum formula

I am reading An Introduction to Mechanics by Kleppner and Kolenkow (2014). On page 241 is the definition of the angular momentum: Here is the formal definition of the angular momentum $\vec{L}$ ...
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### Modeling a wine cooler heat loss to ambient

I'm trying to model the steady state heat loss to ambient, in W, for a wine cooler similar to the following: For the modeling, I will need the following variables/constants: $T_a$ [K]: Ambient ...
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### Approximation to the number of seconds in a year?

Is there any mathematical formula which shows that there are approximately $\pi \times 10^7$ seconds in one year. I understand that the pi is probably due to the earth's circular orbit, but am not ...
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### Choice of units when truncating Taylor series for physical quantities

It is common practice in physics to truncate Taylor series of (possibly) very complicated functions to obtain a good approximation of the relevant physical behaviour; for example, the Coulomb ...
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### Thermal emission cathode electron velocity distribution

I can't find any experimental data (or theoretical expression) on what is the velocity (or energy) distribution of thermal emission cathode electrons emmited from the cathode at approximately 2000 K (...
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### Can be this configuration used to faster than light communication?

I know from some popular science articles or books that is possible to make special pairs of particles which are quantum entangled. Then each of entangled particles can be moved to different places ...
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### What is the definition of inertial mass?

What is the definition of inertial mass? I can see two options, either it's the coefficient associated with the object being accelerated in Newton's 2nd Law, or it's the coefficient relating momentum ...
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### Why is the Ohm's law $j=\sigma E$ accurate? [closed]

Ohm's law $j=\sigma\cdot E=\frac{Q}{A}\cdot \frac{F}{Q}=...?$ $$j=\rho\cdot v \\ =\frac{q}{V}\cdot \frac{s}{t}=\frac{1}{A}\cdot I$$ So why is $j=\sigma E$?
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### Reduce density matrix for given eigenfunction [closed]

My question is about how to find reduce density matrix for partition of given eigenfunction. Full question is just in image.
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### Is relativistic event horizon half of Newtonian event horizon?

Is Relativistic event horizon half of Newtonian event horizon? relativistic escape velocity formula (from $m\phi=E-E_0$) is $v_e=\sqrt{2\phi-(\frac{\phi}{c})^2}$ and the Newtonian version of the ...
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### Sympletic transformation and Hamiltonian function

Let's say that $x:=(p,q)$ is a trajectory in phase space and $$x'(t) = J \nabla H(x(t))$$ are Hamilton's equation of motion. Now I transform $F: M \rightarrow N, x \mapsto y(x)$ diffeomorphic to some ...
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### How to explain centrifugal force from frame of reference of Earth?

Suppose we have a circular table. We have made a straight line groove in the table extending from the center to the circumference. Now we place a block at some distance from the center in the groove ...
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### Derivation of the average Velocity formula with constant acceleration (using calculus)

I have been looking at $$v_{avg} = \frac{v_{i} + v_{f}}{2},$$ when the acceleration is constant, where $v_i$ is equal to the initial velocity and $v_f$ is equal to the final velocity. How can ...
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### What is zero impedance in AC circuit?

If a capacitor is connected with an inductor, then because $$Z=\frac{1}{j\omega C}+j\omega L,$$ the Z may be zero. Does that mean when I apply a voltage, the current will be infinite large? What's ...
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### How to calculate the ship resistance caused by water viscosity

If there is a ship going in the sea at 50km/h, and a length of 5m, width of 2m, how do I calculate the ship resistance caused by water viscosity? in other words I want to calculate the drag Force that ...
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### How can I calculate the speed of an object knowing its horizontal and vertical velocity components?

Let's say a ball is thrown and it experiences typical projectile motion (moves in a parabolic arc etc.) and the only information we know are the equations for the horizontal and vertical components of ...
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### Anti-neutrons, anti-quarks, isospin: What is observed and what is derived?

I would be a little more restrained with the existence of antineutrons. First at all - if I understood right - the existence of antiquarks is hypothetical. If one not agree with this please refer to ...
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### Is angular frequency dependent on time in damped harmonic motion?

I have a doubt regarding the angular frequency of a harmonic oscillator when there is damping involved. The frequency of the oscillation changes with time in the case of damping, but I haven't seen ...
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### If a truck collides with a car, can the truck experience a larger force?

I am confused, here is a question: A large truck and a mini bus both have same velocity V and they collide and stop. The collision lasts for 1 second. A) Which one of the two will experience ...
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### Matter and antimatter differences?

I've heard (and after googling for a while, found) that the only difference between matter and anti-matter is simply charge. This bothers me when it comes to the neutron. Matter and anti-matter ...
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### If a body is floating in a static fluid, then the volume of the displaced fluid equal to the volume of the inmerse part of the object (proof)

Suppose an arbitrary body is floating in a static fluid, either totally or partially immersed in it, then the volume of the displaced fluid equal to the volume of the immersed fraction of the object. ...
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### Why is the slippage constraint for one moving cylinder and one fixed cylinder $r(\phi - \theta)=R \theta$? [closed]

Why is the slippage constraint for one moving cylinder and one fixed cylinder $r(\phi - \theta)=R \theta$? Every time I write it down on paper I get the result $r\phi = R \theta$. I am not sure if I ...
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### Free fall and projectile motion

I'm wondering if something is falling say from a roof, would the distance it falls be the final $y$ position? Also would all the $y$ components (velocity, displacement) be negative?
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### How to design windy area - what variables?

When you're walking around malls or parking structures, or a building corner, you'll sometimes notice areas with consistent windiness. I was wondering what positions, angles, wind direction, pressure,...
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### Finding power in series circuit

A resistor of resistance 12 ohms is connected in series with a cell of negligible internal resistance. The power dissipated in the resistor is P. The resistor is replaced with a resistor of resistance ...
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### Particle acceleration at magnetized shocks by convective electric fields?

Let us assume we have a flow of charged particles in a quasi-neutral state (i.e., a plasma) convecting at some speed, $\mathbf{V}_{sw}$ = V$_{sw}$ $\hat{x}$, and the particles have species-dependent ...
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### What is the functional shape assumed by a flexible rod?

Be L a flexible rod. Say that it is very difficult to significantly stretch it, so that we can uniquely identify a point on it by a parameter $l \in [0, L]$ where $L$ is its length. Be $C$ a set of ...
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### Tensor notation

I'm trying to understand the Maxwell Stress tensor notation. I'm given that each element in the tensor is given by T_{ij}=\epsilon_{0}(E_{i}E_{j}-\frac{1}{2}\delta_{ij}E^2)+\frac{1}{\mu_{0}}(B_{...
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### Material for moisture moderation

Do you know of any material that can moderate the level of moisture of its local environment? Example: if the desired moisture level is 30% and the local environment of the material is at 6% the ...
This is a question from Altland and Simons book "Condensed Matter Field Theory". In the second exercise on page 64, the book claims that if we define $\hat P_s, \hat P_d$ to be the operators that ...