# All Questions

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### Plausibility of the distance estimation in a cycle ergometer gym exercise

A friend of mine pedaled for an hour on a cycle ergometer, a stationary bicycle in a gym. At the end of the ride the cycle computer reported a distance of $40\:\mathrm{km}$ and an average power of ...
174 views

### Why sphere minimizes surface area for a given volume?

I was studying surface tension recently. Rain drops or bubbles of any kind which form are always of a spherical shape. This is because the liquid tries to minimize the surface area as the molecules ...
41 views

### Looking at past events by observing sky? [duplicate]

Since solar system (and our galaxy) is constantly in motion at a great speed, is it possible that we can look at our own image as it was sometime ago? Since the light of that age takes sometime to ...
18 views

### Can I control distance of surfaces with a magnet? [closed]

Does the magnetic force on a surface changes when the electric current in the surface is changed?
51 views

### Need to know height of wave generated by spherical object falling into water [duplicate]

Working with an architect at present.I need to know the height of wave/ripple that would be created by a spherical object weighing approximately 10 tonnes and measuring 42 metres in diameter falling ...
39 views

### How gravitational force works for higher mass body with distance

$$F= \frac{G(m_1m_2)}{r^2}$$ This is the newton's universal gravitation law. Here, I want to ask how does a very very higher mass body affect the smaller body, whereas they are far away from each ...
315 views

### How to prove the average velocity formula without calculus

I often see the formula: $$\overline v = \frac {v + v_0}{2}$$ in physics textbooks, to describe the average velocity for an object moving with constant acceleration. This formula is often ...
86 views

### Can we find a vector if its dot product and cross product with another vector is given?

If I have two vectors $\vec{b}$ and $\vec{v}$, and I know that $$\vec{b} \times \vec{v} = \vec{c}$$ and $$\vec{b}\cdot\vec{v} = \lambda$$ can I find the $\vec{v}$ vector in terms of the $\vec{c}$ ...
4k views

### intensifying a light source

When trying to intensify a beam of light by refracting it through a lens, (as in a lighthouse fresnel system or similar railroad style switch lamp from years past), is the beam intensity increased by ...
68 views

### Measuring spin with Stern-Gerlach apparatus

I have a beam of electrons prepared in the state $|\Psi\rangle = cos(\theta/2)|+\rangle_z + sin(\theta/2)|-\rangle_z$ passing through a Stern-Gerlach apparatus which is free to rotate along the ...
38 views

### Infrared light and heat

I know that mid wave IR, long wave IR, and far IR are all infrared light that we feel as heat. Often, even when it is cold and windy, if I face the sun in a sunny spot it feels like it is 80 degrees F ...
27 views

### A book containing a large subset of known exact solutions to the EFEs

I am looking for a good book which contains the known exact solutions to the EFEs. A "library" of sorts. I am aware that it's not really possible to get all of them in a single book, but at least one ...
20 views

### Methods for Obtaining Which Path Information in a Quantum Eraser Experiment

In all of the quantum eraser experiments I have heard of, polarization is used as a means for identifying the path of a photon. Is this the only method, or do other methods exist for obtaining which ...
4k views

### Is time travel possible? Is it possible to go back in time?

I read somewhere that according to relativity, black holes and other space related stuff it is possible to jump into past. Is it possible for anything to go back in time either continuously or by ...
117 views

### Traveling through time, reaching the past - POSSIBLE? [closed]

I was obsessed in studying the "faith" and I've made several great discoveries. I don't want to publish anything yet, but I have a semi-proven (proven partially) / theories / so I am curious on bones. ...
53 views

### The entanglement of a two dimensional particle and the one of two one dimensional particles

Case 1: Usually, entanglement is a correlation between more than one particles. For example, the momentum $p_1$ of the particle $1$ and the one $p_2$ of the particle $2$ have the relation that ...
5k views

### Mathematically-oriented Treatment of General Relativity

Can someone suggest a textbook that treats general relativity from a rigorous mathematical perspective? Ideally, such a book would Prove all theorems used. Use modern "mathematical notation" as ...
225 views

### Entropy and Gibbs Free Energy

I've been struggling with the notion of entropy and gibbs free energy for almost three days now. Different sources on and off the internet say different things about entropy. Gibbs Free Energy is ...
22 views

### Vacuum solutions to the EFEs

I was wondering if there are any papers in which authors have compared cosmological observations to those phenomena predicted by vacuum solutions to the EFEs? Obviously there is matter in the ...
88 views

### Can light have zero wavelength?

As you increase the energy of a photon it's wavelength shortens. Is it theoretically posible for light to not have a wavelength? Like a still pond?
351 views

### What exactly is temperature in the Gibbs free energy?

In equation of Gibbs free energy change $(\Delta G) = (\Delta H)-T(\Delta S)$, is $T$ the system's or surrounding's temperature? Edit:Oh sorry I am not clear earlier now I get clear question.I know ...
43 views

### Can the use of a magnetic compass be inconsistent with Galileo's Relativity Hypothesis?

I just read Galileo's Relativity Hypothesis from this website: http://physics.ucr.edu/~wudka/Physics7/Notes_www/node47.html It states that - "any two observers moving at constant speed and direction ...
45 views

### Why trajectories approach to origin tangent to the slower direction?

I am reading non-linear dynamics from Strogartz. Suppose, I have two solutions of a non linear system: $x(t) = x_0e^{at}$ and $y(t) = y_0e^{-t}$, where $a\in \mathbb{R}$. Now it is clear that,for ...