# Tagged Questions

Energy is a quantity which gives an overview of the amount of work doable by the system.

4answers
2k views

### Why doesn't light kill me?

I was attending my philosophy class and in the middle of student presentations, I found myself mentally wondering off and thinking about light. After a few minutes of trying to piece together how the ...
2answers
56 views

### Energy dependent Maxwell-Boltzmann distribution

I'm having a bit of a problem figuring out the energy dependent Maxwell-Boltzmann distribution. According to my book (Ashcroft & Mermin) they write the velocity dependent distribution as: ...
2answers
56 views

### What is the derivation for the exponential energy relation and where does it apply?

Very often when people state a relaxation time $\tau_\text{kin-kin}, \tau_\text{rot-kin}$,, etc. they think of a context where the energy relaxation goes as $\propto\text e^{-t/\tau}$. Related is an ...
2answers
157 views

### First law of thermodynamics?

The first law says that the change in internal energy is equal to the work done on the system (W) minus the work done by the system (Q). However, can $Q$ be any kind of work, such as mechanical work? ...
1answer
110 views

### Why do wind power plants have just 3 blades? [duplicate]

Why do wind power plants have just 3 blades? It seems that adding more blades would increase the area that interacts with the wind and gather more energy. (Image from Wikipedia.)
1answer
60 views

### Do electrons need specific energies to excite electrons

Photons need specific energy levels, equal to the difference between two energy levels to excite an electron in an atom. Is this the same case with electrons that collide with atoms?
2answers
58 views

### Why is potential energy negative when orbiting in a gravitational field?

I had to do a problem, and part of it was to find the mechanical energy of satellite orbiting around mars, and I had all of the information I needed. I thought the total mechanical energy would be the ...
0answers
45 views

### Work And Energy Question [closed]

$H = 3\text{ m}$,$m=2\text{ kg}$ The right side is rough. I want to figure: what is the coefficient of friction $\mu$? How high and exceed the maximum return on the plane right body? I know ...
4answers
83 views

### The Preference for Low Energy States

The idea that systems will achieve the lowest energy state they can because they are more "stable" is clear enough. My question is, what causes this tendency? I've researched the question and been ...
2answers
100 views

### Negative potential energy of gravity

Does the negative potential energy in the gravitational field have to be considered in calculating the total mass of the system in question (because of $E=mc^2$)? If so it seems to me that the ...
3answers
69 views

### How do you determine the heat transfer from a P-V diagram?

I doubt this question has been addressed properly before, but if there are similar answers, do direct them to me. I am currently studying the First Law of Thermodynamics, which includes the p-V ...
2answers
78 views

### What happens to the energy not absorbed by a radio?

If a radio tunes to a specific frequency, where does the excess energy go? If one continues to hit the resonant frequency, shouldn't the wire begin to melt at some point from too much energy?
0answers
36 views

### A space vehicle travelling at a velocity of 1296 (km/h) separates by a controlled explosion into two sections of mass 7276 (kg) and 236 (kg) [closed]

A space vehicle traveling at a velocity of 1296 (km/h) separated by a controlled explosion into two sections of mass 7276 (kg) and 236 (kg). the two parts continue in the same direction with the ...
0answers
27 views

### Energy or Work done to pull an Iron cyclinder into a Solenoid

RadiI have been following the calcuations from these lecture slides here (slide 11). Where the slides attempt to approximate how much a solenoid pulls, by working out the energy required to pull an ...
1answer
58 views

### Why does compressing a piston increase the internal energy?

When we compress a piston, its total internal energy increases, however I don't understand why. As the piston compresses, the temperature should change, as the total energy density increases. As a ...
2answers
129 views

### What does it mean if a body has kinetic energy?

What does it mean if a body has kinetic energy? Does it mean that the momentum vectors of each particle of that body has the same direction? What about angular momentum?
1answer
34 views

### What lifting mechanism is likely to have the best energy recovery ratio?

Suppose I was designing an apparatus which needed to lift 250kg 5cm high, hold it there for a few seconds, and then lower the object back to the original height. Such a process would need to be ...
1answer
60 views

### Why are electrons in one atom attracted to negative electrons in another atom? [closed]

Why or for what purpose is this attraction purposeful or what is the reasoning behind why these energy particles are drawn to each other? Any proven results, or proposed theories of your own, or ...
0answers
35 views

### What is the total work done in this problem? [closed]

A 1800 kg trick airplane is 450 m in the air. At this point the plane takes a dive with an initial speed of 42 m/s and accelerates to 64 m/s, dropping a total distance of 120 m. (a) Using the ground ...
2answers
141 views

### Perpendicular Elastic Collision (different masses, different velocities)

I'm stuck on a mechanics problem and I can't make any headway past momentum and kinetic energy being conserved. Here is the problem: Two hover cars are approaching an intersection from ...
1answer
36 views

### Pendulum system: how is derived the output as Energy?

Good day to everyone, I want to understand in which way the "Energy equation" is been implemented to this pendulum system. $x_1(t)$: The angular position of the mass $x_2(t)$: The angular velocity ...
1answer
51 views

### Maximum separation when 2 unlike charges shot apart

I want to compute the maximum separation when 2 unlike charges are shot apart from each other ...
3answers
130 views

### Integrating factor $1/T$ in 2nd Law of Thermodynamics

How would you prove that $1/T$ is the most suitable integrating factor to transform $\delta Q$ to an exact differential in the second law of thermodynamics: $$dS = \frac{\delta Q}{T}$$ Where $dS$ is ...
2answers
648 views

### Why can't Humans run any faster?

If you wanted to at least semi-realistically model the key components of Human running, what are the factors that determine the top running speed of an individual? The primary things to consider would ...
3answers
83 views

### Why doesn't a stationary electron lose energy by radiating electric field (as per coulomb's law)?

If an electron in a universe constantly generates an electric field why does it not get annihilated ? I am confused because I read that an accelerating charge radiates and loses energy. So, why won't ...
1answer
37 views

### Composition of solar spectrum

I read some where that there are three types of UV and infrared rays namely UV-A, UV-B, UV-C and near infrared, mid infrared and far infrared. Which is the most abundant among the the three in ...
1answer
45 views

### Total energy is extremal for the static solutions of equation of motions

In physics total energy is extremal for the static solutions of equation of motions. Can anyone explain this sentence to me?
1answer
80 views

### Energy stored in fields

How to intuitively think of energy stored in a magnetic/electric field? Kindly answer in a bit simple terms without referring to mathematics.
1answer
88 views

### Energy Functional

I am a graduate student in pure mathematics, during my study on Ricci Flow I faced some functional known as energy functional. For example Einstein-Hilbert functional is called an energy functional, ...
1answer
79 views

### Energy needed to lift and bring down an object

A mass of 0.5 Kg needs to be moved from point A to another point (B) which is 1 meters above point A. The time for this movement should be 0.2 seconds, then the mass is kept at position B for another ...
1answer
62 views

### Basic energy calculation for N identical spin system

We have a system that has N identical spins $n_i$, and each spin can be in state 1 or 0. The overall energy for the system is $\epsilon\sum_{i=1}^{N}n_i$. My understanding: There is only one ...
1answer
52 views

### Fundamental properties of motion

The first paragraph of the Wikipedia article on the angular momentum operator states that In both classical and quantum mechanical systems, angular momentum (together with linear momentum and ...
0answers
79 views

### General physics question involving Heisenberg Uncertainty Principle

Question: An unstable particle produced in a high-energy collision is measured to have an energy of $483\ \mathrm{MeV}$ and an uncertainty in energy of $84\ \mathrm{keV}$. Use the Heisenberg ...
0answers
47 views

### How to convert Richter magnitude scale to approximate TNT?

I know the Richter magnitude scale is often used for measuring the strength of earthquakes. At the same time, explosive/destructive releases of energy are often quoted in equivalent amounts of TNT. Is ...
1answer
46 views

### What energy changes take place when you operate a jet-ski?

What energy changes take place when you operate a jet-ski?
9answers
3k views

### What makes running so much less energy-efficient than bicycling?

Most people can ride 10 km on their bike. However, running 10 km is a lot harder to do. Why? According to the law of conservation of energy, bicycling should be more intensive because you have to ...
1answer
55 views

### Determine KE of electron given momentum & mass [closed]

Some info: wavelength of electron: $2.78 \times 10^{-10}$ momentum of electron: $2.38 \times 10^{-24}$ Determine KE of electron. In a provided hint: $KE = \frac{p^2}{2m}$. So I have: KE = ...
1answer
74 views

### Simple harmonic oscillator system and changes in its total energy

Suppose I have a body of mass $M$ connected to a spring (which is connected to a vertical wall) with a stiffness coefficient of $k$ on some frictionless surface. The body oscillates from point $C$ to ...
2answers
66 views

### Is it possible to “add cold” or to “add heat” to systems?

Amanda just poured herself a cup of hot coffee to get her day started. She took her first sip and nearly burned her tongue. Since she didn't have much time to sit and wait for it to cool down, ...
1answer
47 views

### Liquid oxygen how do they use it as fuel?

Rockets are said to be using liquid oxygen as fuel. How do they use liquid oxygen since it's just oxygen, it only helps in the combustion process. How can it be a fuel on its own?
2answers
132 views

### Energy of particle in electric field

I'm taking a physics class and the professor teaches us really basic things in lecture and then gives homework way beyond what he taught in lecture. Obviously I need to find some resource other than ...
1answer
75 views

### Mass-energy equivalence and Newton's Second Law of motion

According to Einstein's Mass-energy equivalence, $E = mc^2$ OR $m = \frac E{c^2}$..... (1) and According to Newton's Second Law of motion, $F = ma$ OR $m = \frac Fa$ ..... (2) If we compare eq. ...
0answers
36 views

### Time reversed laser

Recently, I read an article on time reversed laser. I don't know why they call it a time reversed. I have a doubt that why they use two laser in the device. And what is an anti-laser? The device ...
3answers
124 views

### Universe to energy

Would it violate any known laws of physics to construct a universe containing no mass, only energy?
1answer
76 views

### Calculate/estimate power of a fission bomb

I have some questions about the released energy and power of a nuclear fission bomb. What are the key dependencies of the power of a fission bomb? Is it true that the power of a fission bomb depends ...
1answer
53 views

### How to find work done due to friction [closed]

The force F=40N is applied on a 10kg block at an angle of 36 with the horizontal. The block moves a distance of 15m. If the surface is frictionless. If the coefficient of kinetic friction is 0.25, ...
2answers
106 views

### What particles carry various forms of energy?

If I didn't get this wrong, light or heat energy consists of photons and they in turn effect electrons' behavior and thus responsible for chemical and electrical energy. What kind of similar particle ...
1answer
71 views

### calculating work done by friction

I want to calculate the work done by friction if the length $L$ of uniform rope on the table slides off. There is friction between the cord and the table with coefficient of kinetic friction $\mu_k$. ...
1answer
27 views

### Is lattice enthalpy positive or negative?

I've learnt that the lattice enthalpy (defined as the energy change from a solid ionic lattice to separate gaseous ions) is always positive, obviously. However, I've seen it explained as the opposite ...
5answers
172 views

### Why is momentum conserved (or rather what makes an object carry on moving infinitely)?

I know this is an incredibly simple question, but I am trying to find a very simple explanation to this other than the simple logic that energy is conserved when two items impact and bounce off each ...
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