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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3answers
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When we catch a falling object, where does its kinetic energy go?

Some of it is transformed into heat and noise, but where does the rest go? Further details: The object does not bounce off (e.g. a sack of rice marble ball). The hand remains completely static ...
-1
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1answer
31 views

Projectile at an angle. Finding the angle using conservation of energy [on hold]

Ball is thrown at an angle α and at the highest point it's potential and kinetic energy are the same. Find the angle. Use the law of energy conservation. What the professor wanted $mV^2/2 = 2* ...
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0answers
38 views

Time travel, could it destroy the Universe? [on hold]

One Universe, One mass. In order for "Mass" to be created, (not matter, not energy, but MASS) a creation point where new mass is Incorporated must exist. No "creation" point has ever been observed. ...
24
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2answers
2k views

How does light speed up after coming out of a glass slab?

As I learned today in School, my teacher told me that when light enters a Glass Slab it slows down due to the change in density and it speeds up as it goes out of the Glass Slab. This causes a lateral ...
0
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2answers
48 views

Whats the point in conserving electricity if the AC current produced cannot be stored?

If we cannot save the ac current produced in a battery or some device like the DC current there is no point in conserving it because current once produced should be consumed somehow!!
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 ...
6
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5answers
796 views

Classical Mechanics contradicts Conservation of energy?

Imagine a Stanford torus rotating with 1 rpm so that centripetal/reactive centrifugal acceleration provides about 1.0g of artificial gravitational acceleration inside the ring. The picture below shows ...
4
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2answers
54 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?
2
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2answers
49 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 ...
1
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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 ...
1
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1answer
23 views

Finding the energy lost due to nonconservative forces

I stomped across this question and would very much appreciate any form of clarification. A otter 75kg slides down a hill starting from rest. Hyp = 8.8, height = 6.5, final speed of otter = 9.2 m/s. ...
2
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3answers
58 views

How is energy conserved in magnet?

When an object (i.e.magnetic) is attracted towards the magnet, work is done so that the energy is used but from where this energy comes from?
0
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0answers
17 views

What happens when an object attached to a spring is stretched and then let go?

This is on a horizontal surface. I've just been wondering what exactly happens...when you stretch it and let go, I know it goes back to equilibrium, but does it go past equilibrium, compress itself a ...
0
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1answer
28 views

Energy loss through gravity and the law of conservation of energy

Since every object exerts a force of gravity on all objects around it, how can it lose so much energy at all times without ever stopping? If an object always loses energy from gravity, how does it ...
10
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2answers
1k views

Calculating the force required to lift a weight with a screw

I am trying to learn physics by myself as I do not have a good teacher at school. I've been reading Feynman Lectures on Physics and I can't figure out how he found out this number. Here is an excerpt ...
2
votes
2answers
86 views

A student of mine wants to build a Gyroscopic Inertial Thruster

I work as a tutor with students in their first three semesters. One of my more eager students is obsessed with this thing: A Gyroscopic Inertial Thruster Here is a Wikipedia Article on that. I was ...
0
votes
0answers
18 views

frequency shift in background radiation, where is the missing energy? [duplicate]

It is known that the dilatation of the universe cause a shift in frequency in the background radiation toward lower frequencies. It is also known that photons with lower frequency have lower energy ...
2
votes
1answer
50 views

Why is the Carnot engine the most efficient?

It seems that the only condition used in proving that the Carnot engine is the most efficient is that it is reversible. More specifically, the Carnot engine can be run in reverse as a refrigerator. ...
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2answers
24 views

Understanding a graph of energy conservation with bounded and unbounded motions?

This graph is from the physics undergraduate text "Classical Mechanics by Douglas Gregory". Above this graph was the statement: What I didn't understand is- as stated in the under [*paragraph], ...
1
vote
1answer
38 views

Calculating speed of spheres in an asymmetric Newton's cradle?

If we assume there is an asymmetric Newton's cradle with 3 spheres of different masses and the first one is initialized with some certain speed can we calculate the outcome speed of the all the three ...
6
votes
2answers
126 views

Was the Big Bang actually cold?

As I understand, from watching the Discovery Channel, the total amount of energy in the universe is zero. As such, people like Hawking explain that the universe can be created out of nothing ...
0
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0answers
17 views

Muscular energy transformations with regards to gravity

Maybe this has more to do with biology than I know, but I have been wondering just what kind of energy muscles use and convert when lifting a weight off the ground or lowering a weight back to the ...
0
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1answer
26 views

What happens to K.E. in matter antimatter annihilation?

If I have two matter and antimatter particles, say an electron and a positron, each moving towards each other with a certain speed, they annihilate after the collision. Does the energy of the photons ...
0
votes
1answer
45 views

What happens to the energy of photons when two light waves with plane wavefront interferes destructively? [duplicate]

When I began learning about optical interference, I came to know about destructive interference in which light waves cancel each other. How the energy is still conserved ? I found that the ...
1
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1answer
49 views

Is change in kinetic energy invariant?

Consider the inelastic collision between two bodies. This question follows on from this: Is the coefficient of restitution frame independent and energy conservation?. One of the answers to this ...
1
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0answers
119 views

Friction and work from torque

I would like to understand where is the error in the third case, for that I gave 2 easier cases where I'm able to find the energy from heating is equal to the energy lost by torque. Case 1/ Purple ...
0
votes
1answer
61 views

Violation of conservation of energy… or not?

The work done by surface tension is double the increase in potential energy. This is a clear violation of conservation of energy. What is the flaw here? Work done by surface tension is \begin{align} ...
0
votes
1answer
44 views

Why do we have to use an integral in this scenario to figure out $v_{max}$? [duplicate]

$$\int_0^R mg\frac{r}{2}dr=\frac{m}{2}v_{max}$$ and can't write $$\frac{m}{2}v_{max}=mgR$$ don't we usually equal potential energy to kinetic energy? A stone is falling through a tunnel towards the ...
1
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1answer
138 views

What prevents this rotational perpetuum mobile from working? [closed]

As a newbie in physics I imagine this device and I would like to know why it can't increase its energy. Note I study the sum of energy during one second (or less) after set friction ON. The device: ...
2
votes
1answer
37 views

How many properties like charge, mass etc any quanta or particle must have?

How many properties are required to measure full energy of a fundamental particle? I know $E=mc^2$, but what about charge, spin, etc? Which full equation would give me all parameters of any particle?
0
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1answer
62 views

Can Gauss' and Ampere's Laws be written in terms of the divergence of an energy four-vector?

In the first 20 minutes of this video, Susskind derives the continuity equation for charge conservation: $$\dot{\rho}+\nabla\cdot\vec{J}=0$$ (Where $\vec{J}=\frac{\partial\dot{q}^m}{\partial A^m} ...
1
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1answer
70 views

Heat Equation with In-Depth Radiation Exact Solution

I am looking to solve the heat conduction equation in a semi-infinite solid with in-depth radiation on the domain $-\infty < x < 0$. The governing equation of this problem is: $$\rho c ...
-1
votes
2answers
50 views

Inelastic collision theoretical problem - kinetic energy

I was wondering if there is a possibility where in inelastic collision the final kinetic energy is not less than the initial kinetic energy.
2
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6answers
492 views

Can energy be created and destroyed?

The indroduction of the principle of conservation of mechanical energy has been tremendously useful from the practical point of view. But .. Consider the case in which we shoot an electron up in the ...
0
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2answers
96 views

Do electrons “check in” at the quantized energy radius before they leap?

Quantum jumps inside atoms always have the same energy, at least in a hydrogen atom when jumping from $n=1$ to $n=2$, like from a 1s1 to a 2s1 state. My question is, if an electron can be anywhere in ...
2
votes
1answer
49 views

momentum and energy of the rods

I have two electromagnets and I attach them to two rods(same mass) such that, first one will be attached to centre of first rod and the second one will be to the end of second rod. Now I bring them ...
5
votes
2answers
139 views

I need help figuring out what is wrong in this aspiring perpetuum mobile

Credits: My question is motivated from a question from another user (One disk/ring in double rotation and sum of energy), I just reformulated what I think he tried to ask into, what seem to me, ...
8
votes
2answers
2k views

Does a cooling mass lose mass as it radiates?

Assume we have a red hot cannonball in space. It starts off with mass M at 1000K. Later it has cooled by radiation to 100K. Has the mass decreased?
3
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5answers
117 views

Mass and Energy

Would the mass of burnt firewood be equal to the mass of firewood before burning? Then where does that heat come from? According to Einstein's equation, $E=mc^2$ Shouldn't there be some mass going ...
0
votes
1answer
55 views

Finding the total energy of satellite in orbit with the Earth

I'm given a problem where a satellite with mass $m$ is orbiting the Earth with mass $M$ and radius $R$ ($m<<M$) in a elliptical orbit. The ellipse has semimajor axis $a$ and semiminor axis $b$. ...
0
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0answers
29 views

Where did the energy of the universe come from? [duplicate]

Last week my Physics teacher asked the class this question: If the internal energy of the universe is constant, then where did all the energy come from originally?
2
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4answers
117 views

Conservation of potential energy for a wormhole

So you managed to build a stable traversable wormhole. Somehow you managed to acquire the exotic negative-tension materials with sufficient densities to make it all work. Now you place opening A of ...
3
votes
1answer
100 views

How does conservation of energy manifest itself quantum mechanically?

We know that classically, if we have some theory $\mathcal{L}$ such that the action $\int d^4 x \mathcal{L}$ is invariant under time translation, then we can use Noether's theorem to find that (the ...
2
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1answer
38 views

Conservation of energy and inclines

I have question which I'm unsure of whether or not I am thinking about it right. If an object is sliding down frictionless incline and it then comes to the "foot" of the incline where it encounters ...
0
votes
1answer
74 views

Changing momentum of moving trolley

We have a trolley of mass $m$ and it is moving at a velocity $v$ along a smooth horizontal plane. It is full of water, and is leaking water at a constant rate out of the bottom of the trolley, i.e ...
2
votes
3answers
106 views

The Difference Between voltage and current

I know that this question has been asked many times before, and I have read over several of the threads asking this question, but they do not include the gripe I have with my problem of understanding ...
0
votes
1answer
37 views

A question on a body moving up a ramp [closed]

Let us consider a ramp of vertical height $h$ and an angle $x$. A body at the bottom of the ramp is given a velocity $v$ such that it moves up the ramp length and continues as a body that is ...
0
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2answers
46 views

How is energy converted when massless spring is attached to a rigid support

Say a massless spring of length l is attached to a rigid support. It is extended to length l + x. Now at this position, the ...
2
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0answers
173 views

Sum of energy from torques of several disks in double rotation [closed]

Here the study: An external system (not drawn) give energy for rotate disks around themselves and around green axis. All disks have energy at start, at $t=0$ friction is ON and at $t=0$ external ...
4
votes
1answer
156 views

Energy conservation of Virtual Particles - Quantum Fluctuation?

I (as a middle-school student) was wondering how virtual particles even conserve energy of the entire system? I don't mean just the particle's energy, but conservation with respect to the ...