The law of conservation of energy, which states that the amount of energy in a system is constant. For questions about Earth's environment, see the climate-science tag instead.

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1answer
43 views

What is rotational potential energy?

I don't understand the concept of rotational potential energy. During linear motion, when a force is applied, the work it does gets converted to kinetic energy and there is no change in the ...
-3
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0answers
19 views

Loop the loop question: A marble of mass $m$ and radius $r$ rolls along the looped track in the image [on hold]

A marble of mass m and radius r rolls along the looped track in the image: What is the minimum value of H if the marble is to reach the highest point of the loop without leaving the track?
-4
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1answer
28 views

Oscillation of a simple pendulum [on hold]

What is maximum possible time period of oscillation of a simple pendulum on earth? Please elaborate your answers.
2
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2answers
90 views

Can energy conservation be derived from Newton's Laws?

Can the idea of energy conservation be derived from Newtons's laws? From inspection of his laws you can vaguely discern a relationship but I want to know of you can manipulate his laws to prove it. ...
3
votes
1answer
57 views

Conservation of energy in interference of light

In interference of light, I know that energy is conserved globally but how the energy disappeared at minima appears at maxima? Is there any path by which energy flowed or is it just energy couldn't ...
0
votes
1answer
36 views

Acceleration in full-swing pendulum

Consider a $360^{0}$ swing pendulum with a massless rod. If I start it from the position of unstable balance, at the very top of the circumference, according to the conservation of energy, its ...
1
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1answer
54 views

Simple Conservation of Energy Question

Deduce that the speed $V$ with which the bullet strikes the plasticine is about $160 \ m/s$ Using the conservation of energy, I equated the kinetic energy of the bullet to the potential energy of ...
0
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2answers
61 views

Find the speed of a mass $m$ as it slides down a slope at an angle $θ$ starting from a vertical height $h$ using energy conservation [closed]

Note: The object starts from rest. I'm confused as to what formula for energy I should be using to solve this problem. Any ideas?
2
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2answers
493 views

Escape velocity at an angle

Comparing the work required for a mass to escape the Earth's gravity to the necessary initial kinetic energy gives us the escape velocity from the surface of the Earth of around $11\;\mathrm{km\cdot s^...
0
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1answer
73 views

Why isn't mass still being created in the universe? [on hold]

I was watching a lecture by Alan Guth and wished I was in the classroom to be able to ask a question. He explained that the total energy in the universe could be very close to zero since mass and ...
-3
votes
1answer
64 views

Conservation of energy in the form of y=kx +b?

I am doing an experiment on method of mixtures and specific heat capacities. During my experiment I have to use the graph as a means of finding the SHC of the subject. So I did the experiment with ...
-3
votes
0answers
33 views

Maximum compression Distance of spring? [closed]

So I found the final velocity of the block to be .633 m/s using the conservation of momentum: Pinitial, block + Pinitial, bullet = Pfinal, block + Pfinal, bullet Then I found the kinetic energy of ...
0
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0answers
28 views

Joule's “Work-Heat Equivalent” Experiment

I have been reading about the Joule's experiment that is supposed to have shown that work is the equivalent of heat. I can't really find the original paper and I am finding trouble understanding how ...
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0answers
39 views

Energy Conservation in Changing Potential Well

If you prepare a particle in a basis state, $|n\rangle$ of an infinite potential well of length $L$, the energy of that state will be $\langle E\rangle = E_n$, with zero variance. If you then ...
6
votes
4answers
277 views

How to solve this problem without using energy conservation

I believe most of you probably solved the following problem using energy conservation as shown here. It states a block of mass $m$ sliding on a sphere of radius $R$. I've be trying to solve this ...
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2answers
58 views

Lagrangian of an effective potential

If there is a system, described by an Lagrangian $\mathcal{L}$ of the form $$\mathcal{L} = T-V = \frac{m}{2}\left(\dot{r}^2+r^2\dot{\phi}^2\right) + \frac{k}{r},\tag{1}$$ where $T$ is the kinetic ...
1
vote
1answer
58 views

Why do particles moving in circular motion emit em radiation? [duplicate]

For example particles moving in a synchrotron . Which energy of the particle is converted to the energy of the radiation ? What is the relation between this energy and the change in direction of the ...
0
votes
1answer
30 views

Where does gravity get it's energy to cause tides? [duplicate]

The moon's orbital radius is fixed as a function of its velocity (I know it is actually drifting..). Since there is nothing in space to resist this velocity, it will continue orbiting "forever". Now ...
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1answer
26 views

Calculating the work done by friction on a rigid body due to the piston

I have a uniform rod of length 0.5m and mass 0.1kg, and on this rod are two balls of mass 0.03kg both at a distance of 9.997cm away from the center of the rod. There is a pin about the center of the ...
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4answers
3k views

LED conversion efficiency exceed 100%

I have read this article, Which says that conversion efficiency of the LED have exceeded 100%. The results are published in Physical Review Letters In their experiments, the researchers reduced ...
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0answers
61 views

Energy to winding or unroll a tube with a gaz under pressure inside

I take a tube with a gas under pressure inside it. The volume of the tube is not constant. When I winding the tube its volume decreases. So, the pressure increases in the same time so I need an energy ...
0
votes
1answer
33 views

Feynman's argument on machine limits: how are the machines reset?

Feynman makes an argument in his Lectures that loses me on one point. His argument (abbreviated): We suppose that there is a reversible machine—which lowers one unit of weight by one unit of ...
2
votes
1answer
123 views

Physics Chain Problem

A chain of length d lies on a table, $d/4$ of it hangs loose on the side of it. Friction coefficient is 0.2. It is released and begins to slide. What is the final velocity with which it falls of the ...
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2answers
79 views

Prove energy conservation using Noether's theorem

I wonder how you prove that energy is conserved under a time translation using Noether's theorem. I've tried myself but without success. What I've come up with so far is that I start by inducing the ...
2
votes
2answers
93 views

Newton's Cradle: why does it stay symmetric? [duplicate]

How is it that always the same number of balls leave at the other end in Newton's cradle. I understand that the momentum needs to be conserved, but as momentum is defined as p=m*v couldn't you have a ...
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2answers
68 views

Work done according to Newton's Principia

Actually I was going through the principia, reading Newton's derivations of the properties of ellipses. Suddenly I had this question which got stuck in my mind - Throughout Principia Newton uses the ...
-1
votes
2answers
112 views

Do conservation laws contradict quantum mechanics? [closed]

Take for example the double-slit experiment interpreted in the Copenhagen sense. The particle leaves as an object with mass, yet passes through the slits as a massless wave, only to collapse again as ...
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0answers
32 views

Feynman's paradox : “conservation of angular momentum or Lenz's law ?” [duplicate]

When I was in high school, out of my interest I read electro-magnetism topic from Feynman's lecture on physics. There was a Paradox question in that topic, answer of which i am seeking here... Below ...
0
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1answer
41 views

Velocity Related To Gravitation [closed]

Which Equation is correct $v^2=\large{\frac{2GM}r}$ OR $v^2=\large{\frac{GM}r}$? I am getting confused here.
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3answers
175 views

Are mechanical energy of an element of a rope and energy density constant in the case of mechanical waves?

I'm confused about energy driven by a wave. Consider a sinousoidal wave moving in a rope. In my view each element $dm$ of the rope follows a simple harmonic motion in time. That means that the ...
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vote
3answers
67 views

Is conservation of energy only for isolated systems?

I am thinking in the mechanical context. Everywhere I research (e.g. Wikipedia) the law of conservation of energy is stated only for the special case of an isolated system. I am wondering if ...
0
votes
3answers
40 views

Finding an object's final velocity with distance dependant force [duplicate]

I am (just for fun) trying to calculate the velocity with which the moon would impact the earth if it stopped orbiting and simply headed towards earth. I know the force is $\frac{G m_1 m_2}{r^2}$. ...
-7
votes
1answer
46 views

Can we produce energy from pure substance having considerable mass? [closed]

According to Einstein's formula, $E=mc^2$. Any substance which has mass can be converted into energy or energy is also existing in the form of mass. So I am very much interested to build a machine ...
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3answers
53 views

Change in linear momentum [closed]

Let A and B be to rigid blocks. A is stationary while B is moving with a velocity $u$. So if B collides with A. (These blocks are on a frictionless surface.) What would happen after B collides with ...
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votes
1answer
35 views

In this energy conservation problem, why are the answers different with different units? [closed]

Really basic question, but basically I'm given a change in height in centimeters (that's how I measured it). From that, I'm supposed to find the initial kinetic energy. $KE_i = PE_f$ After doing ...
5
votes
1answer
56 views

Understanding potential energy

I am self-studying the classical mechanics using the book by Taylor, and I have a question about the potential energy. The book (pg 111) says: If all forces on an object are conservative, we can ...
2
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0answers
45 views

What are the conditions for the string of a pendulum to become slack? [closed]

Suppose the bob of a stationary pendulum is given a sharp hit to impart it some horizontal speed. Now what are the conditions for the string to become slack after rotating a particular angle? ...
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2answers
38 views

Why do people say that 'power flows' into or out of a component in an electric circuit?

Currently, I am learning about electric circuits, and how to apply the basic concepts of physics, like work and energy. But I keep running into the same phrase--on Wikipedia, my textbooks, everywhere--...
0
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1answer
35 views

Are all heaters exactly as energy efficient as each other?

I many applications, efficiency is measured in terms of how much of the input energy is wasted generating heat instead of the actual type of energy you want. For example, lightbulbs generating heat ...
1
vote
1answer
46 views

Free fall in non-uniform field

Imagine I'm a space-diver, with mass $m_1 $, 500 miles above the Earth's surface at $x_i$. I want to calculate my position, velocity, and acceleration as a function of time, accounting for the Earth's ...
2
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1answer
50 views

Energy conservation around a black hole

In the Schwarzschild black hole, the Killing vector "time translation" $k^a$, so that the following quantity is conserved along a geodesic: $$E = -g_{ab}k^au^b = (1 - \frac{2GM}{r})\frac{dt}{d\tau}.$$...
3
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1answer
104 views

Is Phone-Charging Plant Product Even Possible?

Bioo, a new company making the rounds in the press, claims to produce "electricity from plants' photosynthesis." Specifically, their website claims that a single potted plant with their integrated ...
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0answers
15 views

Derivation of Poynting theorem in matter

In most textbooks I have read they derive the Poynting theorem using the Maxwell's Equation in vacuum and the fact that the force density $f=\pmb{E} \cdot \pmb{J}$. Then they just generalize it ...
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1answer
49 views

What are the assumptions behind the Lagrangian derivation of energy?

What are the assumptions behind the Lagrangian derivation of energy? I understand that we're searching for a function $L$ that describes a set of physics so that solving the energy minimization ...
2
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0answers
35 views

Is expectation value of the Hamiltonian always the energy? [duplicate]

There are time dependent & space dependent systems (magnetic fields) and time independent (particle in a box or harmonic oscillator). In the latter the expectation value is the 'average' energy ...
0
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1answer
40 views

Conservation of energy and realm of possibility

The law of conservation of energy states that energy cannot be created or destroyed. Based on this principle, you can safely conclude that any effect resulting from a cause must somehow keep all ...
0
votes
1answer
38 views

Expansion of the Universe and Conservation of Energy [duplicate]

How the expansion of the universe doesn't violate the principle of the conservation of energy? If we would put a spring between two objects, the distance would get bigger between them, so would the ...
0
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0answers
26 views

Collision time of two particles [duplicate]

The question I am trying to answer is the following. Consider two particles of mass $m_1,m_2$, initially at rest at a distance $2a$ apart. They are attracted to each other via Newton's law with ...
4
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3answers
80 views

Where does the energy of a photon trying to escape a black hole go?

I've heard "light cannot escape a black hole" explained several ways. One is that if a photon inside the event horizon tries to escape a black hole it loses energy to gravity. As it loses energy its ...
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1answer
82 views

How does the uncertainty principle relate to quantum fluctuations?

I found a webpage that just kind of mentions the uncertainty principle lightly but doesn't really go into detail as to why we need it in the first place when considering quantum fluctuations and ...