# All Questions

72 views

### 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 ...
518 views

### 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 ...
78 views

### 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 ...
88 views

### 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 ...
66 views

### 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 ...
66 views

### 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 ...
304 views

### Why is the Ohm's law $j=\sigma E$ accurate?

Ohm's law $j=\sigma*E=\frac{Q}{A}*\frac{F}{Q}=...?$ $j=\rho*v$= $\frac{q}{V}*\frac{s}{t}=\frac{1}{A}*I$ So why is $j=\sigma$E
41 views

### 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.
56 views

### Isn't it impossible to make a theoretical model to describe the behavior and the statistics of a turbulent flow? [closed]

Why mathematicians are interested to smooth solutions of Navier Stokes equations? For the three-dimensional system of equations, and given some initial conditions, mathematicians have not yet proved ...
232 views

### 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 ...
44 views

### 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 ...
99 views

### 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 ...
257 views

### 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 ...
237 views

### 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 ...
154 views

### 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 ...
450 views

### 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 ...
543 views

### 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 ...
281 views

### 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 ...
650 views

### 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 ...
329 views

### 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 ...
100 views

### 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. ...
36 views

### Torque between Nematic Disclinations

I am looking at a liquid crystal system in 2 dimensions well in the nematic phase. Say the system has been quenched rapidly or is confined on some geometry due to which there are disclinations ...
62 views

### 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 ...
338 views

### 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?
66 views

### 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, ...
82 views

### 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 ...
60 views

### 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 ...
45 views

### 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 ...
89 views

### Tensor notation

I'm trying to understand the Maxwell Stress tensor notation. I'm given that each element in the tensor is given by ...
18 views

### 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 ...
132 views

### Projection Method in Hubbard model

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 ...
87 views

### Force applied at the moment of hit

I have been thinking of this problem quite long time, but couldn't find the solution: Assume that we can measure only the velocity, v, of an object with mass, m, when it hits a rigid and stationary ...
660 views

### How fast would the Earth need to spin for us to feel weightless?

It's a classic question with many answers all over the Internet, but none here so I figured I'd ask it: How fast would the Earth need to spin for a person (or anything for that matter) to feel ...
149 views