Questions tagged [energy]
Energy is the conserved quantity associated to time-translation invariance and represents the work a system is capable of doing. Use this tag for questions about energy, and consider adding the [energy-conservation] tag if it is specifically about its conservation.
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Minimizing the action - particle in a potential well
Consider a particle starting at rest in a potential well, the problem is restricted to one dimension.
The lagrangian is $L=T-V$, set $V$ to 0 in the starting location, we get $L(t=0)=0$.
If the ...
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How to Calculate the Stopping Distance of Train on a Complex Track [closed]
I am attempting to determine when a train should begin braking to reach a certain point at a complete stop. There are several factors that need to be considered that make this more complicated and I ...
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A man standing in an elevator at the third floor of a building. What is the primary factor that determines the man's kinetic energy?
Misconception in Elevator Kinetic Energy Question
I recently encountered the following question in a physics entry test exam:
Question: A man standing in an elevator at the third floor of a building. ...
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Emotional Energy Theory [closed]
Emotional energies are those whose action modifies us on an emotional level, they can make you feel euphoric, tired, energetic, irritable...
Zeroth principle: If you are in sensory contact with an ...
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Discrepancy in answers regarding a problem on adiabatic expansion of an ideal gas [closed]
One mole of an ideal gas ($C_{v,m} = \frac52 R$ ) at $300 K$ and $5 atm$ is expanded adiabatically to a final pressure of $2 atm$ against a constant pressure of $2 atm$. Final temperature of the gas ...
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Work done by a spring on objects of different masses [closed]
If a spring is compressed between two trolleys of masses m and 4m on a smooth one dimensional surface is released, will the work done on both masses be equal regardless of the trolleys' masses?
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About the Energy? [closed]
How Scientist Discovered that energy in the system is a constant things without doing any work it doesn't change?
Also why KE is $(1/2) mv^2$?
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What wouild the fail points be prohibiting construction of a photonic capacitor for use as a power source? [closed]
Physical considerations for construction of a basic photonic capacitor.
Arbitrary shape and structural materials. (For example purposes- could be a 4 inch diameter evacuated aluminum sphere ...
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Energy Conservation Spring Problem [duplicate]
One end of an unstretched vertical spring is attached to the ceiling and an object attached to the other end is slowly lowered to its equilibrium position. If S be gain in spring energy and G be loss ...
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How hot can one heat a single atom?
Imagine a single hydrogen atom in an infinite vacuum being hit by thermal radiation from all directions. Then we crank up the heater, increase the radiation, and continue to do so without limit.
Q1. ...
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What does the equation of the law of conservation of total mechanical energy with deformation energy look like?
I considered task with elastic collision of two balls ($m$ and $M$) on horizontal surface.
It needs law of conservation of total mechanical energy and law of conservation of momentum to solve this ...
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When do we use $ΔU = mcΔT$?
So we usually use $Q = mcΔT$, and in a specific task they used $ΔQ = ΔU + ΔW$ and wrote ΔQ = mcΔT + pΔV. So basically ΔU = mcΔT. I am a bit confused on what formula to use for ΔU when? I have been ...
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Would conjectured warp drive bubbles protect space craft interiors from collision with space debris?ris?
I seem to have read (unable to cite source) that a vessel in a curved spacetime bubble would accumulate radiation forward to the direction of travel and external of the bubble interior. The question ...
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Motion of the COM of 2-body system
We have a block on a wedge like this;
The block is given an initial velocity $v$,and the wedge is also movable .
So when the block reaches its maximum height on the wedge,the block and wedge are both ...
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Total Work Done and Energy calculation "error"? - Bike cycling up bridge [closed]
I have this question that i have calculated to be 3620.4 J (which is the W_total). The question defines the variables as such:
$$ m = 80 kg $$
$$ V_0 = 4.5 m/s $$
$$...
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Prediction and Work
I've had this idea that if you can predict something, you can theoretically extract work from it. For instance if you know a certain mass is going from point A to point B, you can place something in ...
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Home position of a wind turbine
I've recently seen a still standing wind turbine which appeared to be in "home position", that is, one wing going straight to the top and the other two going down at 60°:
Intuitively, this ...
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Waves, energy and matter
What is the physical reason why a wave (mechanical or electromagnetic) can transport only energy and not matter?
Is it perhaps because if a wave could transport matter, then a larger amount of energy (...
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When a submarine blows its ballast and rises, where did the energy for the ascent come from?
Suppose a submarine fills its ballast tanks with water at the surface and in so doing becomes negatively buoyant. It sinks, presumably converting gravitational potential energy into kinetic for the ...
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Help with proving the von Roos kinetic energy operator is Hermitian
When calculating the kinetic energy of a variable mass quantum system, such as a semiconductor heterostructure where mass has spatial variance due to the effective mass approximation, von Roos (Phys ...
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About $1^{\rm st}$ order phase transitions in Landau theory
Suppose that we have a magnetic system described by the following free energy ($\eta$ being the order parameter, in this case the magnetization)
$$\mathcal{L}(\eta,t,h)=at\eta^{2}-w\eta^{3}+\frac{b}{4}...
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What is energy, and why and when is it conserved? [duplicate]
In which field of physics can I find a solution to this question:
What is energy, and why and when is it conserved?
When I asked a similar question to my professors and peers, they said, 'We ...
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What is the potential energy and kinetic energy for capacitor and inductor?
I always thought that $\frac{CU^2}{2}$ is potential energy stored in a capacitor and $\frac{LI^2}{2}$ is kinetic energy stored in inductor. But here and here (page 12) as understand it correctly for ...
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Vacuum energies in different colliding vacuum bubbles?
I got two questions regarding this paper
There, it's analyzed how colliding vacuum bubbles could result in new bubbles and how could this affect their vacuum energy.
Specifically, it considers an ...
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The kinetic energy of rigid body
I need to determine the kinetic energy of ball rolling with out slipping on a cylinder, and when I searched for it I found that the kinetic energy is:
$$ T = \tfrac12mv^2 + \tfrac12I\omega^2 $$
But ...
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Minimum work done on a mass to take it along a path [closed]
A particle of mass 1 kg is taken along the path ABCDE from A to E (see figure). The two "hills" are of heights 50m and 100m and the horizontal distance AE is 20m while the path length is ...
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Energy and force, the basic formula
Energy and force, the basic formula
Can someone explain to me this chain of equalities?
Namely the $v$ in the 4th term and the next term: $$\int d/dt \ ((m/2) v^2) \ dt$$
$$E = \int F \ ds = \int m a \...
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Acceleration in spring-mass system [closed]
So the original question is to find the maximum elongation in the spring:
https://imgur.com/a/hI4o3HH
The solution used $F_{net}=ma$ for both the masses.But,they assumed same acceleration for both of ...
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Elongation of a spring [closed]
Here are two springs,both initially at their relaxed length. But,the 2nd one is connected with masses $m_1$ and $m_2$. I pull on both the springs on both sides with a constant force $F$. What will be ...
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Could thermal or quantum fluctuations in the far future ionize matter?
Consider a linearly expanding universe that expands essentially forever
In the very far future, would it be possible for quantum or thermal fluctuations to ionize atoms separating electrons from atoms ...
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Could a crossterm in the energy–momentum relation be missing? [closed]
Could it be that $E^2 = (mc^2)^2 + (pc )^2$ is really $E^2 = (mc^2)^2 + 2mpc^3 + (pc )^2$ or $E^2 = (mc^2)^2 - 2mpc^3 + (pc )^2$ so that it comes from $E^2 = (mc^2 + pc)^2$ or $E^2 = (mc^2 - pc)^2$? ...
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Calculating instantaneous stored energy in an inductor
Given the current through a $10 \,\text{H}$ inductor at $t = 25 \,\mu\text{s}$ is $5 \,\text{A}$ and that at $t = 0 \,\text{s}$ is $3 \,\text{A}$. Calculate the instantaneous stored energy in the $10 \...
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Do I use the law of inertia or the work-energy theorem?
My textbook (Physics Part I: Textbook for Class XI, NCERT) has the following question:
A trolley of mass 300 kg carrying a sandbag of 25 kg is moving uniformly with a speed of 27 km/h on a ...
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Are faster or heavier objects in motion difficult to slow down due to kinetic energy or inertia?
I know that faster objects with more mass have a greater kinetic energy than slower objects with less mass. Does the kinetic energy or the inertia play a greater role in how difficult it is to stop ...
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Electric power transfer
I'm reading electric power from the book "Fundamentals of physics textbook by Robert Resnick" and I have a question about the interpretation of the text.
First it says "The principle of ...
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Type of collision of two billiard balls
My textbook has the following question:
If the potential energy of two billiard balls depends only on the seperation distance between their centres, is the collison elastic or inelastic? (Note: we ...
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Evaluating the integral of the total energy of the EM field in a nonlinear dielectric
In general, the energy of the electromagnetic field is given by [1]:
$$ H = \int \vec{H}\cdot d\vec{B} + \vec{E}\cdot d\vec{D}. \tag{1} $$
This integral can clearly be split into the magnetic and ...
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F. Mandl writes that change in the fundamental thermodynamic equation $dE=TdS-pdV$ need NOT be reversible. Is it wrong？
Principles of theremodynamics for irreversible processes clearly state that the entropy changes satisfy relation $dS>dQ/T$. Thus we in general have for a irrevesible thermodynamic process $dE+pdV=...
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Change in mass in chemical change
In a chemical change, whenever energy is absorbed or released, there must be a change in mass . Howsoever small it may be.
So in that case which sub-atomic particle(s) is responsible for the change is ...
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Is energy and work the same in numerical basis? [duplicate]
Consider this formula:
P=W/t
where p is power, W is work, and t stands for time.
and also this formula:
E=Pt ,
where E is energy, P is power, and t is time.
so by the substitution property, you can ...
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Conservation of energy and velocity dependent potential [closed]
Question: Why is energy not conserved when the potential is dependent on velocity? If not, then why not?
Attempt: I know that if the potential is dependent on time, then energy is not conserved. A ...
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How to calculate the surface tension limit theoretically?
When an object is submerging in a fluid with strong surface tension (e.g. a plastic cup sinking in a lake), it's very easy to see the surface tension in action. However, at some point, the surface ...
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Interface excess free energy using spherical harmonics
I am a bit struggling to calculate a quantity involving spherical harmonics $Y_{lm}(\theta,\varphi)$. We parameterize as surface in spherical coordinates as:
\begin{equation}
\xi(\theta,\varphi) = R \...
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Do gravitational waves cause matter to radiate?
Gravitational waves distort the "fabric" of spacetime. In doing so, it seems they can cause particles to accelerate.
On the other hand, textbook electromagnetism predicts accelerated charges ...
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Slow expansion: Work done by gas vs. Work done by environment
Suppose an ideal monoatomic gas expands and fills a container of volume $V,$ where one wall is a movable piston.
The text-book scenario of "reversible adiabatic expansion" 1 is that there is ...
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Why do we only care for (and count) energy eigenstates and not superpositions in quantum statistical mechanics?
Consider one quantum particle in a 3D box in the microcanonical ensemble. It's energy eigenvalues are:
\begin{equation*}
\varepsilon = \frac{h^2}{8mL^2}
(n_x^2+n_y^2+n_z^2)
\end{...
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Clarification on Work-Energy Theorem
The professor derived the Work-Energy Theorem using the following steps:
$F=ma$
$Fd=mad$
$W=m(v^2−u^2)/2$
$W=\frac{1}{2}mv^2−\frac{1}{2}mu^2$
$W=ΔKE$
So, it seems that work only changes the kinetic ...
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Number of particles in a given energy interval
While reading statistical mechanics from beiser,I encountered statements like *If $n(E)$ is the number of particles with energy $E$,then the number of particles with energy between $E$ and $E+ dE$ is $...
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Why does the electron jump to a higher energy state when it comes back again to a lower energy state? [duplicate]
When an electron is excited it jumps to a higher energy state. Why does the electron come back to the ground state? If it had to come back because nature prefers a ground state then why did it even ...
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How much energy per second would a nuclear reactor need to power a fighter jet?
Fighter jets typically require high amounts of thrust compared to other aircraft. Lets take the F-22 for example, and lets assume the F-22 powered by a nuclear reactor and the F-22 powered by a ...
