# All Questions

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### Calculating change in gravitational potential energy [closed]

A satellite mass $500\ kg$ is elevated from altitude $10000\ km$ to altitude $20000\ km$. Radius of earth is $6378\ km$. Mass of earth is $5.97 \cdot 10^{24}\ kg$. Gravity is given to be $9.81\ m/s$ ...
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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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### 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 ...
327 views

### Does angular speed of the earth around the sun remain constant?

I presume that angular speed is nothing but angular velocity without direction. So, since the earth always takes nearly 365 days to complete one revolution around the sun, can't we conclude that it's ...
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### Simplifying Friedmann's Equation

So we have one of Friedmann's equation: $$\rho_c = \frac{3H^2}{8\pi G}$$ Using This website, resources where gathered for specific times in the universe. The resources being the Hubble constant at ...
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### AC circuit analysis: Why does current have the same frequency as the voltage?

The above example we have a 50hz AC voltage source. Why is that when we solve for I, I must have frequency of 50 Hz? Is there any phyiscal (non-mathematical) reason to explain this? Thank you
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### What is the difference between a vortex ring and a circular line vortex?

Can someone explain the difference between a vortex ring and a circular line vortex? Or are they the same?
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### How do objects even move due to gravity?

I am an newbie general relativistic learner and I learnt that gravity is bending of space-time and since objects move in straight-lines but since its curved they follow curved movement through space ...
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### Finding resultant and direction of resultant

In this question- A motorboat is racing towards north at 25km/h and the water current in that region is 10km/hr in the direction of 60 degree east of south. Find the resultant velocity of the ...
111 views

### Time dilation confusion

I'm just starting to learn about special relativity, and I'm a little bit confused about something. Take the example of an observer in $S$ on the ground observing a train move at constant velocity $v$ ...
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### Deriving Time from Acceleration, Displacement, and Initial Velocity

I know the equation for displacement as a function of time is $$\vec{s} = \vec{v_i} \Delta{t}+\frac{1}{2}\vec{a}\,(\Delta t)^2$$ I need to solve for $\Delta t$ I'm having problems rearranging to do ...
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### Question about electromagnetic Spectrum [closed]

I have question related to electromagnetic Spectrum. If energy of photon is $E=m_e c^2$, to which part of the electromagnetic spectrum does it belong ?
136 views

### From Paris to … London [closed]

(Excuse the pun in the title, couldn't resist) Paris and London are connected by a straight underground tunnel, as shown in the diagram below. A train travels between the two cities powered only by ...
161 views

### Coherent sources for interference

Regular interference pattern can be observed if the two wave sources are coherent. Two sources are coherent if they have the same frequency and constant phase difference. Could someone please ...
133 views

### If you spin a magnet in space, far from any magnet field, would it spin forever, being able to produce electricity forever?

This is of course a purely theoretical question and it would take energy to get the magnet moving in the first place but once it's moving in the vacuum of space, with no gravity or magnetic field ...
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### The study of the physical universe [closed]

If physics is the study of the physical universe, wouldn't that mean chemistry, biology, neuroscience, zoology, history, economics, sociology, etc, wouldn't they all be subfields of physics? I believe ...
76 views

### The Equivalence Principle approaching the speed of light [duplicate]

If, in local system, an accelerated system is completely physically equivalent to a system inside a gravitational field. Acceleration results in an increase in speed. Right? What happens when the ...
165 views

### Why does time slow down the closer you are to a mass?

When ever i look this up all I get is sites saying how its because general relativity says "-" why does it do it though? it is because there is more motion near gravity than further away? Or is it ...
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### Can a super conducting magnet & gravity create a cheap source of energy?

I have a question about whether this would produce a cheap source of energy. If you created two superconductive magnets configured in a figure of eight layout, in a cold enough environment so you ...
125 views

### Minkowski spacetime vs Euclidian space

Two questions marked in bold: What is the magnitude of a Minkowski spacetime four velocity? I'm deducing that it is c for all observers, but I'd like it confirmed. In Euclidian space, a velocity ...
148 views

### Microscopic understanding of Brewster angle

The textbook explanation is that, at Brewster's angle, the electric dipoles excited by the incident light can't reradiate to the reflection direction since it's aligned with the dipole moments. Is ...
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### How do I write the units for this answer? [closed]

If the force (at 90∘ to the lever arm) applied is 15 N and it is applied at 2.0 m from the pivot point (point of rotation), what is the torque on the lever? Is it 30.0 N/m?
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### Electric Field and Calculus: What is the physical significance of infinitesimal $dA$ in the equation of Gauss's Theorem?

In many equations we see infinitesimals $dA$, $dS$, $dx$ and so on. What is is the physical significance of these? Someone told me it signifies a small entity. For example,in case of $dA$ it signifies ...
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### What does “coherent wave function of a macroscopic body” mean?

What is meant by the "coherent wave function of a macroscopic body"? I found this phrase in a paper on QM, but am unfamiliar with the terminology.
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### How to model an apartment's airconditioning?

Background: This reminds situation reminds be of an episode from the The Big Bang Theory, although it is quite different from it. My roommate and I have similar temperature preferences and also share ...
557 views

### Does the coefficient of restitution matter in a collision?

Suppose two particles are colliding. Let initial velocities be $u_1$ and $u_2$ .Their final velocities are $v_1$ and $v_2$. Now $u_1$ and $u_2$ are known. By intuition I can understand that $v_1$ and ...
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### Solar stills with Wine [closed]

Is it possible to obtain more concentrated alcohol by using wine in a solar still instead of seawater? If yes, as I suspected, how does the concentration (of alcohol) of the distillate vary with ...
2k views

### Energy & Mass of a Photon [duplicate]

$$\text{Please read the whole question before answering}$$ Before I ask my question, I would like to say that "Yes, I do know a photon has no mass." I was helping someone here on P.SE with the ...
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### Direction of current in concentric cylinders

Example 7.2 in David Griffiths E & M book (3rd edition) has a side view of 2 concentric cylinders, with smaller radius $a$ and larger radius $b$. The region in between $a$ and $b$ has ...
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### Can someone show me how Green's function would apply for this simple case?

I'm reading up on some stuff on basic electrostatic here: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/laplace.html Can someone use Green's function to show me the form of $V$? Update: I ...
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### What is Absorption Grating

I came across the word "absorption grating" in a review article. I googled it tried to find out what it means but couldn't. Could you explain it to me?
256 views

### Topple a mass standing still

Consider the following situation: This is taken from an example from my book with a solution. The box weight is $W$ and the coefficient of friction between the floor and the object is $\mu$. What ...
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### What is considered 100% efficiency when talking about photovoltaic cells?

I was reading this article in Popular Science and figured that I actually don't know what is considered 100% efficiency. Is it when every photon that hits the cell kicks out an electron that is ...
55 views

### Trapping an electron

Imagine that one could theoretically trap a single electron in a small box, with walls that somehow prevent the electron from passing through and out of the box. Now, the box begins to move in on ...
131 views

### Is there a difference in handwritten nabla $\vec{\nabla}$ with an overset arrow and typeset nabla $\nabla$?

According to some physicist at KIT it is usual to write the following when using pen and paper: whereas in typeset texts you write $\nabla$. Is that true? Are there sources for this convention?
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### What is entropy of the universe? [duplicate]

What is entropy of the universe? What is the change in entropy of our universe? Is this change increases or decreases? If the entropy of universe keeps on increasing then what are the consequences? Is ...
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### CPT theorem and annihilation of matter and antimatter after the big bang

Is the hypothesis that antimatter is moving backwards in time compatible with the hypothesis of annihilation of matter and antimatter after the big bang? It is said that the big bang should have ...
140 views

### Does pulling or pushing take more effort? [duplicate]

In which case is more effort needed: when you push or when you pull? We have a common experience that man tends to first push anything. How it is justified and what are the reasons behind this?
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### Distance between Newton's Rings fringes is does not seem linear

On the outer edges of Newton Ring patterns, the fringes are really close together, and much more uneven. Also, the spacing does not seem to decrease linearly at all as you move from the center. Why is ...
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### Understanding ket notation and the unitary matrix, what does a/the unitary matrix represent?

I am reading over this paper http://www.scottaaronson.com/thesis.pdf, specifically page 24 under "Quantum computing cheat sheet". The author is explaining ket notation and how it relates to ...
162 views

### Rotating uniform rod [closed]

A uniform rod of mass 1.2 kg and length 1.8 m is pivoted in the horizontal position as shown (black point). The rod is at rest and then released. The acceleration due to gravity is \$g = 9.8 ...
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### What's the point of NASA's impossible space engine? Wouldn't a mirror work better?

My understanding (which is very little) is that the point of NASA's recent virtual-particle engine is to convert solar energy into momentum. That's fine, but what's the point? If the spacecraft is ...
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### Quantum uncertainty of a singularity: A Black Hole Problem

A black hole in GR seems to have a sharply defined radius. Of zero, that is since all the mass has been compressed into the infinitely dense center point. However, shouldn't the fuzziness of quantum ...
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### Size of black hole

I am wondering about size of black hole. How is it possible that we have black holes of different sizes? As I know the singularity is point which is infinite small and is infinite dense. So my ...
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### Black Hole gravity pull [duplicate]

Please I want to Know that if the radius of the black hole = 0 then how it have its surface and how it absorbs things? \begin{align} g &= \frac{Gm}{0^2} \\ &= \infty \end{align} then ...
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### Can a vector be defined by invariance of some algebraic operation to translations rather than rotations?

Every physics book I've come across defines a vector as an n-tuple of numbers that can be combined via an inner product that's invariant to rotations. Is it possible to instead define a vector via ...
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### Peak at zero in one device and not the other

I was wondering if anyone could shed some light on this problem. I have placed two accelerometers on an animal one sampling at 50 Hz the other at 100 Hz. They were placed in the same position. I then ...
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### A question about special relativity theory

We have two sets: set No.1 and set No.2 as in this picture: The observer is fixed to set No.1 . He sees set No.1 motionless and observes set No. 2 approaching with velocity 100,000 m/s. Each set ...
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### Why do objects of a given color appear white under light of that color?

Under light from the sun, a red object will scatter the red component of light, and absorb all others. Hence, the human eye perceives it as "red". White objects scatter all wavelengths of light. ...
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### speed of gas molecules when they escape to space

I wonder if the escape velocity of gas molecules is the same as the one of rockets, which is 7.8km/s. And do oxygen escape at all or not? Are they too heavy?

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