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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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 ...
15
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5answers
1k views
+500

How was the formula for kinetic energy found, and who found it?

My questions mostly concern the history of physics. Who found the formula for kinetic energy $E_k =\frac{1}{2}mv^{2}$ and how was this formula actually discovered? I've recently watched Leonard ...
-1
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3answers
58 views

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 ...
2
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3answers
360 views

Two dimensional elastic collisions with varying angle of incident

If in an elastic collision I know all initial values and that mass for each object remains constant throughout the collision (but different from one another) how can I determine their final velocity ...
-1
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1answer
30 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* ...
12
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5answers
3k 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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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. ...
5
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3answers
228 views

Is energy conserved in general relativity? Does $\nabla_aT^{ab}=0$ represent the conservation of energy and momentum?

For example, the radiation dominated cosmology, the energy density of radiation is propotional to $a^{-4}$ and the volume is propotional to $a^3$, where $a$ is the scale factor. So the total energy ...
28
votes
6answers
2k views

Why are rockets so big?

I'm curious why rockets are so big in their size. Since both the gravitational potential one need to overcome in order to put thing into orbit, and the chemical energy burned from the fuel, are ...
0
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3answers
458 views

Conservation of Angular Momentum, as related to a flywheel

Trying to work out some pesky flywheel dynamics for a project I'm working on, would love some for your assistance to better understand the underlying concepts. For a given flywheel (thin-walled ...
31
votes
3answers
3k views

When water climbs up a piece of paper, where is the energy coming from?

Take a glass of water and piece of toilet paper. If you keep the paper vertical, and touch the surface of the water with the tip of the paper, you can see the water being absorbed and climbing up the ...
0
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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
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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!!
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?
-10
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2answers
258 views

if the universe is flat does it mean it exists only in our mind as math? [closed]

My dad, who is a retired mathematician, has this attitude, which I think we all have as kids, that ultimately reality is made of stuff. End of story. If you look around yourself in the world, ...
1
vote
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 ...
6
votes
3answers
195 views

Optimal speed for the water wheel

The hydroelectricity plants extract the potential energy of highly deployed massive object (water) as it falls down. Without turbine, all that energy would be converted into speed (kinetic energy) at ...
2
votes
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
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 ...
1
vote
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
vote
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
votes
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 ...
2
votes
1answer
246 views

Sufficient conditions for the energy to be not conserved?

I'm almost embarrased to ask this question because I thought I was by now very confident with classical mechanics. Someone has stated that given a mechanical system with a Lagrangian $L$ s.t. ...
10
votes
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
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2answers
85 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 ...
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2answers
2k views

Finding max height of a mass launched by a spring?

I have the following problem on my homework. The actual problem has numbers, but I figure using symbolic notation will help me understand the concept in the long run. A block of mass $M$ is launched ...
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 ...
0
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3answers
75 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 ...
12
votes
6answers
3k views

Is the energy conserved in a moving frame of reference?

Consider this situation: When the box is at the bottom of the frictionless incline, it will have a velocity of $v_f$. The person is an inertial frame of reference that moves at a constant ...
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
2answers
178 views

Poynting's Theorem simplified?

How can I apply Poynting's theorem to any system that has a magnetic field & electric field, to state conservation of energy? How does Poynting's theorem state conservation of energy in EM? I ...
0
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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 ...
16
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7answers
3k views

Why can't we store light in the form of light?

We can store cold (ice), heat (i.e. hot water bag) and electrical charge (batteries). We can even "store" a magnetic field in a magnet. We can convert light into energy and then, if we want, back to ...
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. ...
1
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1answer
99 views

making an equation dimensionless

I have a balance of energy equation as following (for a spherical particle that colliding with a spherical fluid droplet) Left hand side is for before collision and RHS for after that: ...
2
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1answer
118 views

Proof of the conservation of the energy functional for the Gross-Pitaevskii equation?

From the Gross-Pitaevskii equation \begin{equation}i\hbar\frac{\partial\psi}{\partial t}=\left(-\frac{\hbar^2}{2m}\nabla^2+V+g|\psi|^2\right)\psi\end{equation} using the variational relation ...
1
vote
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], ...
3
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4answers
125 views

Virtual particles/quantum tunneling - conservation of energy?

I'm confused as to how the above phenomena can take place since arent they breaking the law of conservation of energy (even, if temporarily)?
5
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5answers
579 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 ...
3
votes
2answers
110 views

How to model energy loss in a rotating body?

I recently asked a question about modeling instability in a rotating rigid body. I now realize that I was mentally confounding two different effects: The "Dzhanibekov effect" in which a rigid ...
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 ...
0
votes
2answers
110 views

Perspective and changes in kinetic energy

Say you have two planes flying next to each other at the same speed and one decides to pick up speed by burning a tank of rocket fuel. If someone on the ground wanted to know that plane's new speed ...
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 ...
20
votes
6answers
10k views

Newton's cradle

Why, when one releases 2 balls in Newton's cradle, two balls on the opposite side bounce out at approximately the same speed as the 1st pair, rather than one ball at higher speed, or 3 balls at lower ...
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
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0answers
62 views

Adiabatic Expansion with expanding control volume

I have a tube with a length $L$ and diameter $d$ that is open on 1 side. At a certain point (say $x$) from the closed end of the tube, I have a gas with a high pressure. At this point, $x$, there is ...