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

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Making a plastic scintillator glow?

I have a plastic scintillator I had purchased off eBay, which is believed to be BC408 http://www.phys.ufl.edu/courses/phy4803L/group_I/muon/bicron_bc400-416.pdf Upon shining a small UV LED (10-40mA, ...
3
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1answer
44 views

How much of the chemical energy in a burning log is released as sound?

Firewood has energy content of about 15 MJ/kg for hardwoods. You throw one of these hardwood logs onto a fire hot enough that it burns with, say, 50% efficiency. How much of that energy will be ...
0
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2answers
30 views

Energy density of magnetostatic fields

Consider that we are given magnetostatic field $B=B_1$ and $\frac{dB}{dt}=0$ from the beginning of time to the end of time. We are interested in the potential energy contained in the field. We know ...
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4answers
421 views

Is energy expended when a force is exerted on a object? [duplicate]

Energy is expended when a person exerts force on a object. 1) From the equation Work Done = Force x Distance, if a force is exerted, but no distance moved, then no work is done. Hence, the statement ...
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4answers
3k views

How does lowering your thermostat save energy?

Say it is winter and the outside temperature is 0 degrees f. I set my thermostat to 74 degrees. When the temperature inside my home reaches 72 degrees, the furnace will kick on and warm the house to ...
0
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1answer
84 views

What is matter made of in the light of Quantum Mechanics? [closed]

I've always wondered what matter (particles, force particles, etc.) was actually made of considering the fact that quantum mechanics has shown us that particles can actually act as a probabilistic ...
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4answers
403 views

Is there a fundamental reason not to define the work vice-versa

My question arises from something which has never been really clear: in continuum mechanics, why is strain energy defined as: $$W=\int_\Omega ...
0
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1answer
52 views

The Hamiltonian and Energy

if anyone can give assistance on this question it is much appreciated! Suppose I have a Hamiltonian $$H=\frac{p^{2}}{2m}+V(r)+F(r,t) $$ where $$F(r,t) = Q(r)\Re{(e^{{iT(t)}})}:\quad Q(r), T(t),T'(t) ...
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1answer
67 views

If all energy is removed from an object, what will it stop moving relative to?

If you somehow removed all energy from an object, what would it stop moving relative to? According to relativity, an object's position is not relative to the universe, just reference points, and, as ...
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5answers
305 views

Is this expression for the kinetic energy of a spinning disk revolving about a second axis correct?

My question is motivated from a question from another user. You can see the configuration of the rotating system here: One disk/ring in double rotation and sum of energy. I am not interested in all ...
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4answers
293 views

Is $E^2=(mc^2)^2+(pc)^2$ correct, or is $E=mc^2$ the correct one?

I have been having trouble distinguishing these two equations and figuring out which one is correct. I have watched a video that says that $E^2=(mc^2)^2+(pc)^2$ is correct, but I do not know why. It ...
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1answer
32 views

Solving kinetic energy exercise through electrical work formula instead of voltage formula?

While studying electric potential, I run into an issue on exercise 43 of Chapter 24 of Fundamentals of Physics 8th ed. Vol. 3 (Halliday et al.). The exercise states the following, paraphrased: A ...
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2answers
54 views

Is the coefficient of restitution frame independent and energy conservation?

In this question I am ignoring relativistic effects. The following statements I think are true: change in Kinetic energy is not invariant under change of frame Force is invariant under change in ...
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1answer
69 views

A question regarding collisions

Let us consider a system of 2 identical spherical bodies connected by a massless string that is taut. If one body is placed at the origin then the other is placed at some coordinate $(x,y)$. The ...
0
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1answer
53 views

Energy without temperature

If you know the entropy $S$ of your system. Is there a general way to calculate the internal energy $U$ of your system? So the entropy $S$ is the only thing I know of my system. I have no information ...
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0answers
24 views

What is the physical interpretation of time-energy uncertainty? [duplicate]

I have a question. What is the physical interpretation of time-energy uncertainty?
5
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3answers
204 views

Is any energy required to deflect an asteroid, with force always perpendicular to its trajectory?

Let's assume there is an asteroid traveling on a straight line (far from any gravitational source), and we need to deflect it from its actual trajectory, so we build a rocket motor on the surface and ...
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1answer
44 views

Can we extract chemical energy remotely?

To state directly. Can we use any electromagnetic method to extract energy from fat? Humanity store useless fat inside their body. And to work out mechanically leads to frustration of tireness and ...
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1answer
71 views

Quantum mechanics: Finite square well problem

What will happen if the potential is less than 0, for instance $V(x)=-10eV$. Is this means there will be no bound states? Since solution to the time independent Schrodinger equation (those discrete ...
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2answers
61 views

Can we harvest energy from constant gripping?

Gripping things hard needs energy even though nothing really moves. So I think we could harvest energy from this action, but I know that any type of energy harvesting requires moving parts. So can ...
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2answers
2k views

Where does the energy from a parachute go?

When a parachute slows the velocity of an object where does the energy go? If it's a falling object the acceleration from gravity is roughly constant. How does air drag "dissipate" the extra energy?
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1answer
58 views

Energy Imparted by a Magnetic Field

As I understand it, when you switch on an electromagnet it generates a magnetic field that pervades all space, imparting potential/kinetic energy to every object with which it interacts. Where does ...
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5answers
106 views

Relation between the conservation of Energy and the conservation of Momentum?

I just learn about the momentum and it conservative attribute in a closed and isolate system and there is something I don't understand when I relate it to the conservation of the energy Considering ...
3
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1answer
384 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
67 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 ...
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2answers
245 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 ...
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0answers
170 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 ...
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2answers
49 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 ...
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1answer
50 views

What happens when I reverse an electrical generator? [closed]

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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6answers
718 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 ...
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347 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
110 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 ...
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5answers
277 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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1answer
111 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 ...
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3answers
70 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 ...
0
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1answer
50 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 ...
4
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3answers
289 views

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 ...
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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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1answer
47 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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3answers
93 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 ...
3
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2answers
84 views

Food calories vs gravitational potential energy [closed]

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 ...
0
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1answer
71 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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4answers
28 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 ...
3
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2answers
71 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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0answers
22 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 ...
0
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2answers
137 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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6answers
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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 ...
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1answer
30 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 ...
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1answer
37 views

how to use the energy work kinetic theorem [closed]

A single force acts on a $3.4~\text{kg}$ particle-like object in such a way that the position of the object as a function of time is given by $x = 4.2t - 2.1t^2 + 2.5t^3$, with $x$ in meters & $t$ ...
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1answer
36 views

Relevance of this equation

The book 'The variational princples of mechanics' by Lanczos was recommended to me and I am reading it now. So far it is an enjoyable journey, but I don't see the significance of this equation: $t = ...