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

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5answers
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+100

Is the total energy of the universe constant?

If total energy is conserved just transformed and never newly created, is there a sum of all energies that is constant? Why is it probably not that easy?
9
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6answers
365 views

Is time travel impossible because it implies total energy in the universe is non-constant over time?

I have always argued with my friends regarding Time Travel that it is impossible. My argument has been that it will destroy the theory that all the energy in the universe is constant since when one ...
0
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0answers
41 views

How is energy converted within the body? [on hold]

I guess that this question is going to be on hold but: how does the energy in food is transmitted to our cells and we gain energy? what is there in the food which is transmitted as the energy to our ...
1
vote
3answers
705 views

Where do planets get energy to revolve around sun? [duplicate]

We know that every planet in our solar system revolve's around the sun in a particular orbit. But were to they get the energy to revolve around the sun. And why do they not drop into the sun there is ...
3
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4answers
2k 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 ...
2
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1answer
211 views

Question about conservation of energy and force

Feynman mentions in his book, The Feynman Lectures on physics Let us now illustrate the energy principle with a more complicated problem, the screw jack shown in Fig. 4-5. A handle 20 inches ...
0
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2answers
98 views

Energy required to start and then stop rotating an object, when we have gravity

Imagine an arbitrary object with center of mass at the red dot in the picture, rotational inertia from the point of the hanging: I, hanging from a fixed point, what is the energy required to start ...
2
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10answers
335 views
+50

How far can light go?

When a ray of light is projected, (say) from the surface of Earth to outside in space. The condition is that, there is no obstruction to it till infinity (it travels only in vaccum). My question is ...
-5
votes
0answers
56 views

Energy manipulation [on hold]

Haha, I don't want to offend anybody, but I want to make a superhero that can control all forms of energy. From what I've heard you can do a lot like, manipulate time and space, open blackholes, ...
4
votes
2answers
94 views

Do virtual photons have a frequency?

Real photons do have frequencies, which is directly related to its energy. So, can virtual photons that take part in EM interactions have frequencies too? When my hand is pressed up against a glass ...
1
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1answer
52 views

Highest background gamma ray energy?

What is the highest naturally occurring gamma ray energy that you would see in background? And what is it is source?
2
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3answers
37 views

Domain of different types of collision?

Can anyone tell me why elastic collision occur between atomic particles? inelastic collision occur between ordinary objects? perfectly inelastic collision occur during shooting? ...
1
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3answers
98 views

Calculate the rise in temperature from directed radiation [on hold]

For example I have a 1 watt laser and direct it to a sheet of metal (copper), if I were to direct it for say a time interval of 1 minute what would be the change in temperature? I can predict that it ...
2
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2answers
151 views

When do we see particles to be in a superposition of energy states?

I have two doubts: Exactly when does this happen? and If we are in a superposition of states (lets say E1 and E2) and the particle absorbs a photon, what will happen? If E3-E1 = hf, will it go to E3? ...
1
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0answers
25 views

Marvin the Martian vs. the Death Star: how much energy will they actually need to disintegrate the Earth? [duplicate]

Marvin the Martian vs. the Death Star: how much energy will they actually need to disintegrate the Earth? According to a detailed analysis by Dave Typinski, Marvin the Martian’s Illudium Q-36 ...
0
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1answer
84 views

Euler-Lagrange equation for Lagrangian $L= \frac{1}{2}(-f(x)\dot{t}^2 + g(x)\dot{x}^2 + 2l(x)\dot{x}\dot{t})$ [on hold]

If metric is $$ds^2 = -f(x)dt^2 + g(x)dx^2 + 2l(x)dxdt $$ Then we have this Lagrangian: $$L= \frac{1}{2}(-f(x)\dot{t}^2 + g(x)\dot{x}^2 + 2l(x)\dot{x}\dot{t}).$$ The Euler-Lagrange equation for $t$ ...
3
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0answers
63 views

QFT and violation of Heisenberg uncertainty principle

In some QFT books is said that a free electron can emit a virtual photon as long as it reabsorbs the photon and returns to its original state within a time: $$\Delta t<\dfrac{\hbar}{2\Delta E}$$ ...
4
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2answers
53 views

Conservation of kinetic energy in collision

Why is kinetic energy conserved in collision between glass balls while it is not conserved in collision between a ball and floor?
1
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0answers
113 views

Marvin the Martian vs. the Death Star: how much energy will they actually need to disintegrate the Earth?

According to a detailed analysis by Dave Typinski, Marvin the Martian’s Illudium Q-36 Explosive Space Modulator will require $1.711 \cdot 10^{32}~\text{J}$ to shatter the Earth into a gravitationally ...
3
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2answers
22 views

Factors on which Coefficient of restitution depend

What are the factors on which coefficient of restitution depend? What is the reason for more coefficient of restitution of two glass balls (0.95) than for two lead balls (0.20)?
1
vote
1answer
28 views

Energy efficiency for common fuels

If we take matter-antimatter annihilation as being 100% energy efficient, what would be the energy efficiency of: Uranium fission, Fusion of Hydrogen to Helium, and Combustion of Kerosene. I ...
7
votes
6answers
802 views

Why does a conservative force return the work done against it by a body to that body?

Newton's 3rd law of motion: Newton's third law of motion or the law of action and reaction implies that there is no isolated force in nature. Whenever there is any force at all , there must be ...
1
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1answer
28 views

Relationship between momentum and energy for rocket-powered aircraft

I'm trying to write a problem for my students in an algebra-based physics class. We have a flight simulator and I've got a glider in the simulator that has a couple rocket boosters attached. Students ...
0
votes
1answer
81 views

Thermodynamic Internal Energy Reference States

The internal energy, $U$, of a given thermodynamic state, $\boldsymbol{R}$, as introduced in thermodynamics textbooks like Callen seems to be defined as: $U (\boldsymbol{R}) = \Delta U ...
2
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2answers
131 views

Solar spectrum units

Why is intensity $I$ on a graph of the solar spectrum always showed in units of $[\mathrm{W/m^2/nm}]$ instead of simply $[\mathrm{W/m^2}]$? (The y-axis on the graph.) It is apparently shown as ...
1
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2answers
53 views

Can antimatter-matter collisions generate heat?

We have a project at school to develop an idea about how we can generate electricity. My question is: do collisions between matter and antimatter particles generate heat, so that one could harvest it ...
2
votes
2answers
71 views

Understanding relationship between work and energy

I've read over 10 books about work and energy, and I just simply can't understand it. First of all, they go ahead and randomly define that work is force times distance: $$W=F X \cos\theta$$ Okay, ...
0
votes
0answers
16 views

Solving Rotational Kinematics friction with energy [closed]

So i've got the next schematics: So that: R is given. [m] Mass of the body: m [kg]. Friction of the circular part: $$f=\frac {b\cdot\theta^3}{\pi^4}$$ What ...
1
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1answer
69 views

Is energy affected by forces?

Curious, is any certain form of energy, such as heat, affected by a force? Can you push energy from one place to another?
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3answers
90 views

Is the kinetic energy of an electron always $1.6 \cdot 10^{-19}~\text{J}$?

I was doing the following problem. For $d$. They bizarrely added an extra step. They calculated the momentum of the electron to find the De Broglie wavelength using the kinetic energy relationship. ...
1
vote
2answers
60 views

Do I need to convert units to be compatible with constants?

I want to calculate the wavelength of radiation given its energy. I know I need to use $E=h f$ and $f = c / \lambda$. All I'm given is $E = 20 \text{ keV}$, now my true question is: Do I use $E = ...
5
votes
7answers
3k views

Ball flying towards me or me flying towards ball

Suppose a ball is flying towards me at a speed of 10m/s and that, on impact, I feel "x" amount of pain. If, instead, it was me flying towards the ball at the same speed, with all other conditions ...
1
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2answers
3k views

Is turquoise closer to blue or green?

I am having a discussion with my coworkers. Does anyone know what is the wavelength of turquoise color and whether it is closer to green or blue when comparing their wavelengths ?
2
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2answers
48 views

Why is there conservation of kinetic energy in elastic collision and not in inelastic collision?

Why is there conservation of kinetic energy in elastic collision and not in inelastic collision? What would be the difference that lead to conservation of kinetic energy in elastic collision and not ...
0
votes
1answer
52 views

Why does the water rise upwards when the bottle fall down with accleretion $>>g$?

When a medium-sized bottle filled with water quarterly falls from a height with acceleration $g$, the water remains calm. But when it falls with acceleration $>>g$, the water abruptly rises up ...
0
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2answers
46 views

Why are ripples formed in water round? [duplicate]

Despite whatever be the shape of object that you drop in water, it be 1D, 2D, 3D, they all produce ripples in a circular pattern, is this pattern followed under water as well (in terms of density ...
7
votes
3answers
573 views

Energy needed for Superman to take off and fly at the speed of sound

I just watched "Man of Steel", and I'm wondering if my logic is correct. Let's assume Superman is 80 kg. The energy required to take off from the rest to reach the speed of sound in air (if I neglect ...
1
vote
1answer
24 views

How can kinetic energy be conserved in an elastic collision

How can kinetic energy be conserved in an elastic collision as collision is said to occur between two bodies if they physically collide against each other or if the path of one of then is affected by ...
0
votes
1answer
239 views

Integers, Energy levels, and wavenumbers for a particle in a 2D box

(This question is not about coding) I have built a little code in Python that allows the user to plot the energy vs the wave number of particle in a 2D box, depending on what values for the integers ...
0
votes
3answers
48 views

Change in energy ideal gas

I am supposed to calculate the change in energy upon changing both the temperature from $T_1$ to $T_2$ and the volume from $V_1$ to $V_2$. Now I was wondering whether this solution is correct: We ...
0
votes
1answer
17 views

Battery charging using a windmill?

i want to charge 4*(12V/100AH battery) using a 3*( 12V/5AMP GENERATOR ) which is drawing its power from a windmill. these battery have to heat 500 litre of water from 30 celcius to 50 celcius how ...
0
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1answer
32 views

Too many antennas around a single energy transmitting antenna?

I am not a physicist. So I am not even sure, if I am phrasing this question properly. Lets say there is a wireless transmitter transmitting energy. Let us say I place $N$ number of antennas around it ...
0
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2answers
44 views

Can energy be “subtracted” from vacuum?

It probably sounds absurd, but I heard such expressions as "subtracting a particle from vacuum", or from squeezed vacuum, in the context of quantum experiments, or quantum optics, (not black holes). ...
4
votes
4answers
206 views

Why does a spring fixed to a wall have PE and not KE?

As many books,including mine (Principles of Physics by Resnick and Halliday) introduce, I also will present my example like this: Let there be a spring of negligible mass attached on one side ...
0
votes
0answers
21 views

what factors cause the “energy gap” to change between different materials?

To my understanding, every electron wants to be "excited" its next energy level. Depending on the material, the electron will need a different amount of energy to reach the next energy level. This ...
0
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4answers
41 views

Frequency of a photon

in classical physics frequency represents how many cycles in one unit time, but I do not know how we define a frequency for a particle? what does it mean for a particle to have a frequency, the book I ...
0
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0answers
20 views

Different ways of converting solar energy to other forms of energy

Okay so after receiving our Physics papers today I decided to tell the teacher why I lost marks on the last question: "Mention the energy conversions undergone here Sun --> Coal-fired boiler --> ...
0
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0answers
23 views

Is my representation of $\varphi$ Work function correct?

I am a middle-school so my understanding of physics may not be as solid as you professional physicists but never the less thought its worth a try to learn more. I read about photo-electric effect by ...
6
votes
3answers
345 views

Why must a body lose energy to an opposing force?

A body must do work against an opposing force to continue motion. I have found this statement many times. But what is the reason behind it? Suppose $F_1$ is acting on a body to accelerate it (to ...
0
votes
2answers
78 views

Energy of damped harmonic oscillator begins to increase with very large Q in numerical integration

I have numerically integrated the (reduced) homogeneous equation of a damped harmonic oscillator in order to see how the error propagates. $$\frac{d^2 X}{d\phi^2} + ...