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Questions tagged [energy-conservation]

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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Is the universe a closed system?

Most places say it is. But according to the multiverse hypothesis, gravitons and other quantum particles could travel from one universe to another. Wouldn't matter-energy then be transferred and thus ...
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At infinite separation do charges get kinetic energy?

When charges are infinitely separated, the potential energy is reduced to zero. As we know that loss in potential energy is nothing but the gain in kinetic energy . So do charges start flowing at ...
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Where does the kinetic energy of an object annihilated in a matter antimatter reaction go?

A bullet is moving at $0.99 c$. Now we annihilate it by having it collide with an equal mass of antimatter. (assume instant and perfect reaction) This will turn both bullet and antimatter into photons....
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Can the laws of conservation of energy and momentum be thought of as projections of matter wave onto the time and spatial axes? [on hold]

I noticed that when we think of traveling or stationary particles (or objects) as lines on the ($x$, $t$) diagram, so that the velocity $v$ of a particle defines the slope, and assume they have ...
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Tritium decay energy too small for quark model

So $^3$H turns into $^3$He through $\beta^-$ decay, right? The energy of this reaction is only 18.6 keV. However, according to the constituent quarks model, the only thing different between $^3$H ...
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Question on energy conservation from the stress tensor of a classical scalar field

I am struggling to answer an old general relativity exam question, which is as follows: "Consider a scalar field $\phi(t,x^i)$ with potential $V(\phi)$ on a general spacetime. Its stress tensor is ...
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Momentum of solar panel after photon is absorbed [on hold]

A solar panel installed on a car of mass 1 kg lying on a friction free road. Let the efficiency of the solar panel be 100%.let the momentum of photons be $m_o$ and and the car be at rest before ...
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Pump raising water from a tank, through a nossle [closed]

I have no idea how to do this question or what it is asking. Can anybody explain it to me please?
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1answer
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Perpetual magnetic attraction

There are two magnets, M1 and M2. Let's say that there is an attractive force existing between M1 and M2. M1 is attached to a surface, while M2 is free to move. Now slow this motion down, and M1 is in ...
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Mechanical Energy Concept Problem

When an object is at rest on a smooth horizontal "surface" (floor) its potential energy $P_E$ equals zero and, as the object is at rest, its kinetic energy $K_E$ is also zero. When the same object is ...
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How is energy conserved in an elastic collision between two unequal masses?

In an elastic collision between two masses, if one mass is much heavier than the other, then the heavier mass will continue to move with same velocity while the lighter mass doubles its velocity. How ...
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What is the significance of maximum compression in spring?

A block of mass $m$ sliding on a smooth horizontal surface with a velocity $v$ meets horizontal spring of spring constant $k$ fixed with wall. Find maximum compression of spring. Why is the word ...
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Basic Question on Black Holes and the Potential Loss of Conservation of Energy/Mass [duplicate]

I am reading Kip Thorne's "Black Holes and Time Warps" and I have a basic question on a topic he is discussing while talking about Einstein and Eddington's disbelief in black holes. So the general ...
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Why doesn't a hydraulic lever violate conservation of energy?

Suppose I apply some force on one side of Hydraulic lift where area is less, and the fluid in the lift raises some heavier object on the other side where area is more, Now work done is $Force\times ...
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Kinetic Energy Comparison Marble Question

A uniform marble of mass M rolls down a symmetric bowl, starting from rest at the top of the left side. The top of each side is a distance h above the bottom of the bowl. The bowl is rough enough to ...
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A problem of energy conservation in GR

The following discussion with the Minkowskian space-time. It is known that the conservation is described by $P_{\xi}=\int_{\Sigma} T_{a b} n^{a} \xi^{b}$, where $\Sigma$ is a Cauchy surface and $n^a$ ...
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Destructive interference of light and destroying energy?

I've had a hard time with destructive interference of light, and the possibility of destroying energy. I've read countless articles here and elsewhere, leaving me with the answer of something to the ...
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Is a rocket engine most efficient when its velocity is equal to the exhaust velocity?

I was reading Dale's answer on this question and from the answer I concluded the following: 1- When the rocket is traveling at a velocity much less than the exhaust speed, most of the chemical energy ...
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Where does the energy for osmosis come from? [duplicate]

A thought experiment: A U-shaped tube with semi-permeable membrane at the base. The tube is completely thermally isolated from its surroundings. The liquid (solvent) is at some temperature $T$. ...
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Kinetic Energy change in particle to wall collision

The Question A particle of mass m is moving in the +x direction with a speed v. It is approaching a massive wall which is also moving in the +x direction, but with a speed u < v (see figure) The ...
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Angular Momentum vs Kinetic Energy [closed]

Suppose we have a spinning top, with angular velocity $\omega$, its speed is null and there is no force applied to it. Let $J$ be its moment of inertia. We can say $E_k = \frac{1}{2}J\omega^2$ and $L ...
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Does energy conversion happen when an electromagnet attracts an object?

I am doing science with my 6th grade son, and there is a circuit with an electromagnet (as below) When asked to describe the energy conversion when the switch is turned on, part of the answer ...
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Getting a Conserved Quantity from a Lagrangian [duplicate]

So I've been messing around with the implications of Noether's theorem, and though I conceptually get what it's saying, I'm having a hard time actually using it to retrieve a conserved quantity from a ...
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Why do I get negative height?

A baseball is thrown from the roof $50.1^\circ$ above the horizontal. The initial velocity is $11.5\ \mathrm{m/s}$. I'm trying to find how high the ball goes. I choose positive direction upwards. $$...
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Chemical energy

A physics problem I am solving has a rocket which launches and releases a certain chemical energy $u$. What is chemical energy? How would I incoporate it into the equations of conservation of energy? ...
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Perpetual Motion Machine 1: Magnetic Superconductor Loop Generating Energy with Piezoelectrics [closed]

I know there are a lot of perpetual motion machine designs out there, but I haven't found one that looks like this one. I will use this diagram. Assume the entire ring is made of a superconducting "...
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1answer
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Destructive interference in vacuum (energy conservation) [duplicate]

The following question was proposed by a student durante a lecture for a grad course in EM. What happens to the energy of an EM wave during destructive interference in vacuum, in regards to the ...
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What is the formal definition of energy in physics? [duplicate]

What is the formal definition of energy in physics? My question is what is the definition of energy in physics in general ? Moreover is energy a naturally occurring quantity or is a term defined in ...
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3answers
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Rotational KE vs Translational KE [closed]

Q. Horizontal electric field E = mg/q exists as shown in the figure and a mass m is attached at the end of a light rod. if mass m is released from the position as shown in the figure. find the angular ...
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1answer
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Rolling on a track in vertical plane

A sphere of mass m and radius r is released from rest at point A on a track in vertical plane. The track is rough enough to support rolling between A and B and from B onwards it is smooth. My ...
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4answers
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Potential Energy in conservation of energy [closed]

In energy conservation theorem, everywhere $PE=mgh$ is used. What I found is that change in PE= mgh. According to me, while calculating total energy, instead of change in PE, total PE should be used ...
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Question about Helmholtz's paper “ON THE CONSERVATION OF FORCE”

Below follows the exact extract from Helmholtz's paper "On the conservation of force". Let us now imagine, instead of the system $A$, a single material point $a$, it follows from what has been just ...
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Confusion regarding the conservation of mechanical energy

If it's always the case that---when all forces are derivable from a potential energy---$$\Delta T = W = -\Delta V$$ so that $$\Delta \left(T + V\right) = 0,$$ why is energy not conserved when $V$ is ...
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Is energy conserved in a Van der Pol oscillator?

The Van der Pol Oscillator is governed by a 2nd order ODE with nonlinear damping. The 'position' of the oscillator is the solution to $$x''(t) = \mu (1 - x^2(t)) x'(t) - x(t)$$ Here $\mu$ controls ...
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Is this statement true for all conservative forces or just attractive conservative forces?

'Change in potential energy is always equal to the negative of the work done by a conservative force.'
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How to calculate velocity of an object at the bottom of a circular hill if there is an energy loss due to friction?

Object slides down a circular hill and loses some of its initial mechanical energy due to friction. As an object slides down, friction increases and force in direction of motion decreases until the ...
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Mechanics Question - Bullet fired into Block

I am studying for the GRE and came across this question (Question 93 from Sample Exam 2 in Kahn and Anderson's Conquering the GRE): A bullet of mass 5 g is fired at a block of wood of mass 1 kg, ...
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How is energy conserved in this case?

Consider a permanent magnet introducing a magnetic field at some fixed angle to a loop of wire on a spindle. If a fixed current is allowed to run through the wire, the lorentz force introduces a net ...
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How does $dS \ge 0$ imply a system cannot receive more energy than work supplied?

I am reading a book that asserts the following: The second law arises from an equation of the form $$dS = \mu\,dE + \mu\,dW \tag{1}$$ where $dS$ is the change in entropy, $dE$ the change ...
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Energy in inertial frame of reference

If a body A is moving with a constant velocity v and an observer on that body A observes another body B to be at rest then the kinetic energy of B is zero. So is energy dependent on the frame of ...
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3answers
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How does flow work arises in enegy balance using Reynolds Transport theorem?

I will use the below mentioned form of Reynolds Transport theorem(usually derived in Fluid Mechanics context) to give a relation between a Control mass system(no mass in or out) and Control Volume(...
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What happens to the energy of an absorbed photon?

Photons are electromagnetic wave fields that travel at the speed of light. When they are absorbed by an object, their momentum and energy is transferred to the absorbing object. What is that ...
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How should energy loss in a hydraulic jump be calculated?

The energy loss in a hydraulic jump is still calculated with the old equation of Bresse from year 1860; (I.e., equation 7 in this paper from 2017) $$ \frac{\Delta E}{E_1} = \frac{(\sqrt{1+8Fr^2}-3)^3}...
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Can this sum be done without integrating?

As by using conservation of mechanical energy, we can just be concerned about the final and initial position and the velocity could be found out. Am I not right? But I am not getting the correct ...
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Why does energy conservation give me the wrong answer in this inelastic collision problem?

Suppose there are two objects. The first object is stationary and has the next parameters: v1=0 m/s, m1= 4kg. The second object is heading towards the first one with speed v2= 8m/s and mass m2= 60 kg. ...
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Energy conservation when exchanging meson

In Prob 1.2 of Griffith's "Introduction to Elementary Particles" 2nd ed., it says that: However, I don't understand why it states that "they must temporarily violate the conservation of energy by ...
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How does conservation of momentum work here? [closed]

Imagine 2 oppositely charged particles attracted to each other, say an electron and a nucleus, assume their force of attraction is very large compared with any external applied force we're about to ...
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Do black holes violate the conservation of mass?

This question has confused me for quite some time now. I have searched it up online, and the basic answer is: 'Mass is a form of energy. When black holes die they release the amount of energy that ...
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Conceptual question regarding velocity in projectile motion vs. up a ramp

I have a question regarding conservation of energy in regards to two different scenarios: In the first case I was told that a ball with an initial velocity, V, was launched at an angle, $\theta$ and ...
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Is that reasonable to use kinetic energy and potential energy of a fluid system to define the laminar, transition and turbulent region?

This is my assumption ( I don't know whether it exists or not ): I suppose a fluid system always contains two types of energy, i.e., kinetic energy and potential energy. The kinetic energy of a ...