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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Rolling disk in inclined plane and flat plane? [duplicate]

A disk is rolling (not sliding) in the inclined plane with initial velocity of zero. So there is friction between disk and plane. But when we use the energy conservation rule, we consider as: ...
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1answer
97 views

Could the Sun be partly powered by cosmic rays?

The Earth and the Sun has magnetic fields which shields us from cosmic rays, as a charged cosmic ray particle will loose kinetic energy when its direction is perpendicular to the magnetic field. So ...
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2answers
118 views

Work producing current = energy stored in the magnetic field?

It is stated that "the formula for the energy stored in the magnetic field is: $$E = \left(\frac{1}{2}\right)(LI)^2$$ and the energy stored in the magnetic field is equal to the work done to produce ...
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1answer
1k views

Confusion over the gravitational potential energy inside a sphere

I'm confused over the concept of gravitational potential energy inside a sphere. I understand that the gravitational potential energy inside the sphere is supposed to be a constant $U = ...
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1answer
1k views

How do I show the fractional change in Kinetic Energy in a completely inelastic collision?

Given the formula $$\frac{{\Delta}(KE)}{KE_i}=\frac{(KE_f-KE_i)}{KE_i}=\frac{-M}{(m+M)}$$ Now I know these that the conservation of momentum is always applicable. Also I understand that ...
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2answers
360 views

Energy of quarks and the mass of the proton

We know that energy of quarks inside the proton can not be exactly fixed because if it,the 'proton decay' must not be exist. My question is if the energy of quarks inside the proton is not exactly ...
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2answers
138 views

Angle rotated by a rod when it's hit by a pendulum

Consider a pendulum of length $h$ with a bob of mass $m$ it is held horizontally at and angle of $90^{\circ}$ with the vertical. A rod of mass $M$ and length $h$ is pivoted at its upper end and this ...
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2answers
63 views

Distribution of Kinetic energy after an Impact

How does the huge amount of kinetic energy of some fast object like a meteor or rocket etc. gets distributed into various kinds of energy like kinetic, sound, light etc after making an impact with ...
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3answers
290 views

(Why) would unmaintained water heater use more electrical energy?

I'm specifically thinking about lime/sedimentation at the bottom of water heater, and calcification of heating elements (and not possible thermal insulation deterioration). It is very often claimed ...
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1answer
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Does the uncertainity principle violate the law of conservation of energy?

What is the scientific view of the beginning of universe? Quantum fluctuation seems to contradict with the law of conservation of energy. Uncertainity Principle does seem to violate the Law of ...
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2answers
147 views

Maximum permissible speed while going down a ramp

So, I was playing hill climb racing and I noticed that if we move with high speeds towards a ramp going down we just jump it off. While lower speeds, help us to stay in contact with the ramp. ...
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2answers
352 views

Where does elastic potential energy go after it is released?

Say you have two objects colliding, and there is some elasticity between them. Some of the kinetic energy of the objects will change into elastic potential energy when they collide, but when they ...
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2answers
239 views

What happens to a photon when it enters a black hole?

The photon has a mass of 0, but it has energy because of its motion. When it is sucked into the black hole and becomes a singularity, it loses its energy because it is no longer moving. It is not ...
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1answer
71 views

Where / what is the source of the force that causes refraction?

So we know that as light moves from, say, a glass block to air, it speeds up and refracts, but if there is to be acceleration then surely there must be a resultant force to cause it? So where does ...
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2answers
145 views

Energy of a damped oscillator

$$ E=\frac{1}{2}m\left(\frac{dx}{dt}\right)^2+\frac{1}{2}m\omega_0^2x^2. $$ This is the equation for the energy of a oscillator. The second term is the potential energy. Now, my question is, will ...
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2answers
423 views

What will be the velocity of a comet is falling to the Earth from infinity at the time of impact if Earth had no atmosphere?

What will be the velocity of a comet is falling to the Earth from infinity at the time of impact if Earth had no atmosphere? the comet is falling radially towards the earth. Will this velocity be ...
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1answer
129 views

Where does the potential energy go when mass is lifted out of Earth's gravitational field?

There is a finite amount of energy needed to be spent on a rocket or a rock that is taken out of the Earth's gravitational field. Lets suppose the rock is taken far enough to have no effect of Earth's ...
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0answers
44 views

Determining impact velocity over long distances [duplicate]

Everything I have found concerning impact velocity from a given height deals with constant acceleration due to gravity. I would like to know how to work in varying acceleration into the equation. So, ...
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0answers
39 views

Mechanistic description of leverage

How would you describe how a lever is able to apply a large for to an object from only a small applied force without recourse to conservation of energy? Is there a purely F=ma type derivation? For ...
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2answers
139 views

Blueshift and increase in energy!

Imagine that there is a car and it is not moving but its headlights are on. There is a wall in front of the car but is very far away. Right now energy is being used only in switching on the ...
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2answers
824 views

Experimental proof for Conservation of Total Energy

In physics, the law conservation of energy states that the total energy of an isolated system cannot change—it is said to be conserved over time. (Source : wikipedia(conservation of energy) And ...
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1answer
100 views

How does a MRI use 20.1kW yet creates over 204K BTU/h? [closed]

I've been reading through a manual about MRI operating procedures for a large healthcare provider. The manual (written by Siemens) states that the MRI machines use 9kW in stand-by mode and 20.1kW ...
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4answers
528 views

Does car lose kinetic energy when turning?

I am writing simple car simulation. Assume non friction, then in straight line the car doesn't lose speed. But what if the car is turning, there should be some kinetic energy loses to change the ...
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2answers
188 views

Energy of an inductor

I know that for an inductor having self inductance $L$ energy stored in its steady state when a current $I$ has been established is given by $U = \frac{LI^2}{2}$. But after this current has been ...
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2answers
554 views

Energy transfer in elastic collision [duplicate]

In a given reference frame where object 1 (with known mass and velocity) collides elastically with object 2 (with known mass and velocity), can we identify which object loses kinetic energy? Is it ...
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2answers
4k views

Perfectly centered break of a perfectly aligned pool ball rack

Imagine the beginning of a game of pool, you have 16 balls, 15 of them in a triangle <| and 1 of them being the cue ball off to the left of that triangle. Imagine that the rack (the 15 balls in a ...
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1answer
207 views

How does a flywheel generate electricity at a constant voltage?

I believe I am missing something simple here. My question concerns flywheel energy storage. Say we have stored some amount of energy in a spinning flywheel. The flywheel is attached to a generator. ...
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1answer
85 views

How do I calculate the work done by snow when stopping a person from falling from a plane? [closed]

I have the following data: Assume a man falling from a plane from a height of 370 m ($=h$) high (Straight down). The man falls into a pile of snow and creates a 1.1 m ($=d$) deep crater. The man has a ...
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1answer
133 views

What does the absence of hydrodynamic entrance effects do to the momentum conservation Equation?

I am trying to do a homework problem where we re-write the mass, momentum and energy conservation formulas for downward flow in a vertical pipe and it says "where all hydrodynamic entrance effects ...
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1answer
189 views

Why positronium can annihilate in vacuum?

I thought that the annihilation process of positronium cannot take place without a third-party particle. This can be directly derived from energy & momentum conservation: energy conservation: ...
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2answers
303 views

How energy conservation is not violated here?

If I lift a book of mass $m$ from the ground and put it at height $h$,then initial and final energies with respect to ground are 0 and $mgh$ respectively. So how energy is conserved here?
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1answer
157 views

A variable mass system (cart of sand with a hole)

Imagine we have a frictionless semicircular skateboard ramp and we release a cart filled with sand from one edge of the ramp. The cart has a hole and it loses sand at a constant rate. My question is ...
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1answer
125 views

If an electron can't go faster than light, how is energy conserved here?

I came across a physics question recently about accelerating electrons with potential differences and it got me wondering. Here is the question that brought the concept to mind. First off, imagine ...
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2answers
150 views

Rigid bodies - the wheel

As I've been taught lately in my mechanics course: the wheel has a unique property: at every moment of motion, the touching point between the wheel and the ground is not in movement and ...
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0answers
34 views

James Joule's Conservation of Energy

Is there a manifest or documentation of the original Conservation of Energy as stated by James Joule word for word? Edit 1.1: I found Joule's Memoir on google, skimming through to find any conclusive ...
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3answers
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When is energy conserved in a collision and not momentum?

Consider the following example: A bullet of mass 45g is fired at a speed of 220 m/s into a 5.0 kg sandbag hanging from a string from the ceiling. The sandbag absorbs the bullet and begins to ...
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1answer
111 views

Sign of $dr$ in Schwarzschild geodesics

There is an equation that relates energy $E$, angular momentum $L$ and other constants and variables to find $\left(\frac{dr}{d\tau}\right)^2$ in a plane. ...
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2answers
105 views

What is going on with star's energy?

The sun radiates $3.8 × 10^{26} J$ energy each second, this energy is absorbed by various celestial objects but most of it is reflected or emitted back in the space, similarly there must be energy ...
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1answer
2k views

What is happening to rotational kinetic energy when moment of inertia is changed?

I know this question is asked here a lot, but I just had to ask this to finalise the concept. When a system lets say a rod of length $L$ and mass $M$ is rotating with angular speed $omega_1$ its ...
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3answers
698 views

Where does the kinetic energy go?

A uniform cylinder was placed on a frictionless bearing and set to rotate about its vertical axis. After a cylinder has reached a specific state of rotation it is heated without any mechanical support ...
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4answers
152 views

Solids: is energy expended in some fashion by, say, a table to keep the top from sinking down against gravity?

A friend and I were discussing this, and couldn't decide. Since it takes energy to physically raise something up above the planet's surface, does it continue to require energy in order to keep it ...
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2answers
92 views

Finding the value of velocity $u$ [closed]

A block of mass $m$ is pushed towards a moveable wedge of mass $\eta m$ and height $h$, with a velocity $u$. All surfaces are smooth. The minimum value of $u$ for which the block will reach the top of ...
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1answer
107 views

Physical interpretation of $Q^i = \partial _\nu T^{i \nu}$

I'm trouble with exercise 1.8 of Carroll's Space-Time and Geometry: If $\partial_\nu T^{\mu \nu} = Q^\mu$, what physically does the spatial vector $Q^i$ represent? Use the dust energy momentum ...
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0answers
64 views

Collision Between Two Particles: Writing the Mass As A Function of The Angle [duplicate]

Suppose we have two masses, $m_1$ and $m_2$, where $m_2$ is at rest, and $m_1$ is headed directly towards $m_2$. I would like to write the ratio of the masses as a function of the angle. Using ...
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2answers
750 views

How does potential energy work in the context of objects in space?

It's said that potential energy is "energy of position." If an object is sitting on a shelf five feet above the floor, its potential energy can be thought of as equal to the amount of energy that ...
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2answers
562 views

Conservation of mass energy and kinetic energy in different reference frames

With a little work it's easy to show that kinetic energy by itself is not necessarily preserved when switching between frames of reference. And it is my understanding that energy should be preserved ...
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0answers
49 views

Do these steps demonstrate that acceleration of charged particle is proportional to current?

One formulation of Maxwell's Gauss Law for electric field is: $$\bigtriangledown E = 4 \pi k \rho $$ This can be worked into the Divergence Theorem as follows: $$\int\int_{A} F_\perp \:dA= 4\pi k ...
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1answer
133 views

Hamiltonian conservation

Lagrangian formalism does not involve forces that doesn't come from a potential and Hamiltonian formalism says that even though energy is not conserved due to a force like this, the Hamiltonian is ...
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4answers
268 views

What kind of energy does superfluidity use?

Liquid helium (and other similar fluids) can "climb up" the walls of their containers. Who does the work in this case, and what kind of energy does it use? I'm sure we can't make a perpetuum mobile ...
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5answers
817 views

Conservation of Mathematical Constraints when deriving Energy and Momentum from $F=ma$

Background: Starting from $F = ma$, integrating with respect to time, and using basic calc, one can derive $\int Fdt = m (v_f - v_i)$ Starting from $F = ma$, integrating with respect to distance, ...