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

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Why is it mandatory to 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 ...
2
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3answers
96 views
+50

Why do the agents of conservative force return work done against them to the body that worked on them?

Let a body moves & a conservative force opposes its motion by applying an opposing force on it. Therefore, by Newton's third law of motion, the body will also exert reactive force on the agent of ...
3
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2answers
178 views

1D Infinite Square Box Discrete Energy levels but Continous Momenta?

In the 1d particle in the box the energy of the particle should be completely determined by the momentum of the particle that you observe correct? So how can you have discrete energy levels and a ...
3
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2answers
213 views

Does relativistic mass exhibit gravitiational effects?

Groundhog Day Update, 2014 The simple and dumb way to ask my main question is this: If something like a neutron start goes sailing by at very close to the speed of light, say fast enough to double ...
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1answer
101 views

Gravitational interactions by energy or by mass?

The mass-energy equivalence, first established by Einstein is an important and highly discussed phenomenon in physics. Without claiming much knowledge about high-end discussions on this topic, I would ...
3
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1answer
352 views

How quickly does gravity extend from created mass? [duplicate]

I apologize in advance if this is a stupid or off-topic question. Since Energy can be converted into matter according to $E=mc^2$, how fast does it take for the gravity of that new matter to extend ...
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3answers
52 views

Reconciling Units in Classical System Analogies: Why Does Torque Have Units of Energy?

In classical physics we often cast an analogy between translational and rotational systems Force < > Torque Energy < > Rotational Energy Momentum < > Angular Momentum and considering SI ...
0
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0answers
14 views

A car collides with a net [on hold]

A little car of 500 grams with a speed of 2 m/s collides with a net with a spring constant of 400 N/m. If the force is bigger then 40 N, the net breaks. 1) Prove that the net brakes 2) What is the ...
5
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2answers
72 views

Hydrogen energy levels and energy-time uncertainty principle

Some hydrogen atom exists in some excited quantum state, and after some time $\Delta t$ it's de-excited, emitting a photon carrying the energy difference. It is claimed that this photon will carry ...
2
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0answers
27 views

Sum of energy for 2 solids in rotation

I would like to compute the sum of energy of the following case: Two solids are turning (disks). Yellow solid is turning at $-w1 rd/s$ around its center of gravity and blue solid is turning at ...
4
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4answers
6k views

What is the pure energy in matter antimatter annihilation made of?

I used to read the term "pure energy" in the context of matter antimatter annihilation. Is the "pure energy" spoken of photons? Is it some form of heat? Some kind of particles with mass ? ...
0
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2answers
45 views

Is it possible for electrons to carry more than one charge?

Sorry if this has been asked before. Could also be a really basic question (new to electrical study). I am a bit confused about the relationship between electrons and charges. So what I understand is ...
2
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4answers
117 views

Kinetic energy vs. momentum?

As simple as this question might seem, I failed to intuitively answer it. Let's assume there is a $10,000$ $kg$ truck moving at $1$ $m/s$, so its momentum and KE are: $p=10,000$ $kg.m/s$ and ...
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2answers
87 views

What causes damage, kinetic energy or momentum?

Let's assume we have 2 bullets. The first bullet has $450$ $J$ of KE and $3$ $kg.m/s$ of momentum, while the second bullet has $250$ $J$ of KE and $5$ $kg.m/s$. Now if they are both shot at ballistic ...
0
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1answer
29 views

What happens when I reverse an electrical generator?

I am trying to find out whether I could run a generator in reverse as well as forward. It will be a wind generator converted to run from a different power source. This is a new invention to which I ...
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3answers
4k views

Why is Energy = Voltage x Charge, and how to prove that?

As you know the equation $\mathbf{E=V\times Q}$. Where: $\mathbf E$ is the energy measured in joules, $\mathbf V$ is potential difference (Voltage), $\mathbf Q$ is the charge. So my qustion is: ...
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2answers
56 views

What is the percentage of energy recovery in Kinetic Energy Recovery Systems(KERS) in cars?

Kinetic Energy Recovery Systems (KERS) use flywheels to recover energy from the kinetic motion of cars. They use a rotating flywheel that generates energy as it rotates- this generates the electric ...
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5answers
120 views

A force is exerted on a body, kinetic energy increases but no work is done by the force. Why?

After reading the article, I was totally perplexed . I was reading the External forces and internal energy transfers in Principles of Physics by Resnick,Halliday,Walker. It goes like that An ...
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0answers
21 views

Energy and momentum [on hold]

In a good wind location offers a modern wind turbine with a maximum output of 1 MW over 2 million kWh per year. a) How many m ^ 3 of oil is equivalent to that energy content if 1 liter of oil ...
9
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3answers
2k views

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?
2
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2answers
124 views

Will a water-cooled system heat up a room less than an air-cooled system?

Recently - motivated by the recent heat where we live - an acquaintance argued that a water cooled computer will not heat up the room as much as an air cooled computer will. His reasoning was that the ...
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3answers
54 views

Can Thermal Energy be converted into usable energy?

I'm going to go out on a limb and ask a question that will probably be a bit humiliating: Can Thermal Energy be converted into usable energy? I've been reading about conservation of energy, and I know ...
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1answer
42 views

Determining zero energy from $k=0$?

I'm curious as to the equations necessary for finding a total energy of 0 (or, I suppose, the energy density of empty space due to quantum fluctuations) in a flat Friedmann universe such as ours. The ...
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0answers
19 views

What is the energy-momentum four vector of the particle? [closed]

An particle with mass $0.511~\text{MeV/c^2}$ is travelling $0.4c$ in the x-direction. What is the energy-momentum four vector of the particle (E/c, $p_x$, $p_y$, $p_z$)? A ship moving in the ...
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1answer
78 views

What happens to gas molecules after ionization?

I know that gas molecules conduct electricity after they get ionized but what will happen if we keep increasing the voltage even after ionization? Will it explode? If it will then how much energy ...
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0answers
15 views

Inertial Frame Missing [closed]

HELP ! I've lost my inertial frame, but my energy equations all still seem to work. Do we really need an inertial frame concept? Surely we just take the potential function (which will be well ...
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1answer
1k views

Does light lose energy in transit?

Consider a photon is an energetic particle and therefore has a gravitational field. When a photon passes a molecule or particle of dust in space it will pull the dust towards it and deform the dust. ...
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2answers
71 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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0answers
40 views

Perpetual Mobile and Gravitation

I have fundamental question about what is called the “law of conservation of energy”. We all hear about the tidal power stations which using the tidal power. The source of the tidal power came from ...
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2answers
85 views

Conservation of energy in transformers

How does energy remain conserved in a transformer if emf is increasing, or decreasing? Does the current decreases to accomodate? Does Ohm's law still hold here? Although we know, Ohm's law is not ...
4
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2answers
1k views

Is the E-cat by Andrea Rossi et al. for real?

Does this thing really do what they say? http://www.youtube.com/watch?v=EhvD4KuAEmo If it does, it looks like this will probably be the biggest breakthrough in science ever :)
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1answer
30 views

Big jump or small jumps?

To travel a certain distance s, is it more energy efficient to use one massive jump, or several small jumps? (First approach deleted, didn't make sense) This approach is probably a lot better. ...
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2answers
46 views

Friction between a block resting on top of another pushed block [closed]

A block of mass $m$ is is on top of a block of mass $M$ on a table. A constant horizontal force $F$ acts on the lower block. What is the kinetic friction between the bigger block and a surface? Find ...
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2answers
3k views

What is the relationship between Energy, Entropy, and Information?

What is the relationship between Energy, Entropy, and Information? I read this - What Is Energy? Where did it come from? - and the top answer says that 'energy' is an abstract number that is a ...
0
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3answers
67 views

Where does the energy required to stop a moving body come from and go?

For a long time, I have been thinking about this: In order to stop a moving body, I have to apply impulse to change the existing momentum to $0$. For the impulse I have to impart force on the moving ...
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1answer
129 views

How to calculate the amout of time for oil to get out from a pneumatic cylinder?

I have a system. You can assume it is just a 1" diameter steel pneumatic cylinder (1 foot long). Assume we ignore the bore size issue at this point. One chamber is filled up by oil (6" of volume), ...
3
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2answers
59 views

How can water be liquid and gas in the same environment?

Think about it. If there's water in a gaseous form, how can there also be water in a liquid form? The gaseous water would have to have more energy than the liquid, but if it's the same temperature, ...
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1answer
60 views

what property of steam makes it the preferable motive fluid in jet ejectors? why not air or nitrogen?

I want to know what properties of steam make it the fluid of choice in steam turbines jet ejectors. I want to understand the mechanism of energy conversion in these equipment and hence understand ...
11
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4answers
6k views

Are Fresnel lenses widely used for solar electricity? If not, why not?

I was just wondering why Fresnel Lenses are not widely used in the production of solar electricity. Their use there would mean that you could produce heat within a fraction of a second, up to a few ...
2
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2answers
66 views

Food calories vs gravitational potential energy

Imagine you could use all of the energy you have ever extracted from your food as a source of propulsion. Is the energy contained in the food you’ve eaten over the course of your lifetime enough to ...
1
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1answer
71 views

Energy Stored in a Capacitor with and without Dielectric

I have seen the equation $V = \frac {V_0}{K}$, but also the equation $V=\frac{1}{2}CV^2$. The values of C and V increase in the same linear ration with K (because $C=KC_0$). However, as the energy ...
10
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6answers
2k views

How can energy be useful when it is 'abstract'?

The topic which haunted me for two years until I gave up on it. But now I am doing engineering and this topic suddenly popped out of my textbook from nowhere. I seriously need to understand this topic ...
0
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1answer
53 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 ...
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2answers
7k views

What happens to the absorbed light energy?

When light comes across with a solid material, some of it is reflected, some of it passes through and some of it is absorbed. I understand the reflection and passing through, but I don't understand ...
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1answer
552 views

Why doesn't the potential energy of any object equal 0

Consider a particle on the ground. This particle is raised by a force of magnitude $mg$ to a height $h$ above the ground. At this point, the work done on the particle by the force is $mgh$, which is ...
0
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4answers
25 views

High power short duration from low energy source?

1MW, can 1000Joules source of energy deliver 1M of power in 1 millisecond? $Energy =$ $P$ $x$ $t$ , so does it make sense to have small amounts of energy in large pulses of power? In an extremely ...
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2answers
103 views

Particle in a Box: Energy Less than the Potential Energy

I am reading quantum mechanics from Shankar's Principles of Quantum Mechanics. On page 157 he defines the box potential $V(x)$ as $$ V(x) = \left\{ \begin{array}{rl} 0 &\mbox{ if $|x|< L/2$} ...
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1answer
54 views

Energy Stored In A Capacitor (Slowly Moving Parallel Plates Together)

I think this is a canonical problem with capacitors, but I'm having difficulty understanding the paradox. In Purcell's Electricity And Magnetism, problem 3.16 reads: Calculate the electrical ...
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0answers
20 views

What is the best way to heat air? [closed]

I want to know which way we could create hot air most efficient, how can we convert all energy input into air temperature and decrease air density as much as possible? Is the plasma (such as fire) is ...
1
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1answer
24 views

How to find the maximum height with a loss of 5 J of energy due to air drag [closed]

Can someone please help me to figure out how to use the 5J of energy into my equation for the maximum height. I know that the maximum height is given by, $$h_{max}={(v_f-v_i)^2 \over 2g}$$ but I'm ...