# What is Energy: Definition and 999 Discussions

In physics, energy is the quantitative property that must be transferred to a body or physical system to perform work on the body, or to heat it. Energy is a conserved quantity; the law of conservation of energy states that energy can be converted in form, but not created or destroyed. The unit of measurement in the International System of Units (SI) of energy is the joule, which is the energy transferred to an object by the work of moving it a distance of one metre against a force of one newton.
Common forms of energy include the kinetic energy of a moving object, the potential energy stored by an object's position in a force field (gravitational, electric or magnetic), the elastic energy stored by stretching solid objects, the chemical energy released when a fuel burns, the radiant energy carried by light, and the thermal energy due to an object's temperature.
Mass and energy are closely related. Due to mass–energy equivalence, any object that has mass when stationary (called rest mass) also has an equivalent amount of energy whose form is called rest energy, and any additional energy (of any form) acquired by the object above that rest energy will increase the object's total mass just as it increases its total energy. For example, after heating an object, its increase in energy could be measured as a small increase in mass, with a sensitive enough scale.
Living organisms require energy to stay alive, such as the energy humans get from food. Human civilization requires energy to function, which it gets from energy resources such as fossil fuels, nuclear fuel, or renewable energy. The processes of Earth's climate and ecosystem are driven by the radiant energy Earth receives from the Sun and the geothermal energy contained within the earth.

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1. ### B E. Potential Energy: Uniformly Charged Hollow Sphere and Point Charge

I was doing a problem with this one detail. It says that the electric potential energy of an uniformly charged hollow sphere and a point charge is (at the surface of the hollow sphere; both positive): $$U = k \frac{q_1 q_2}{r}$$ I guess this assumes that the hollow sphere is a point charge. Now...
2. ### A charge q approaching two stationary charges q1 and q2

I'm only confused about one part of this problem and that is setting up the conservation of energy equation. In the solution, they just wrote this: $$\frac{mv_o^2}{2} = - k \frac{q_1 q}{r} + k \frac{q_2 q}{l + r}$$ where ##r## represents the distance at which the force created by the negative...
3. ### B Is there any physics topic explaining how objects break into parts?

I am curious to know if there is any existing topic explaining how objects break into several pieces when force is applied.
4. ### Is there a limit on how much energy a photon might have in a FOR?

So there was this question: The first option seems to be the only correct answer. $$\lambda_e=\dfrac{h}{\sqrt{2m(KE)}}$$. The answer would be correct if ##KE \approx eV## The option mentions that ##eV>>\phi## so ##\phi## can be ignored. But I don't think that necessarily means that the...
5. ### Circular motion of a Weightless rod

I used law of conservation of energy to calculate (d theta/ dt)^2 (from:mgasin theta=1/2m(d theta/dt.a)^2+1/2mu^2(u is the velocity of the C ring at time=t)), but wasnt able to find u(velocity of C).Is there any relationship between the tangential velocity of B(d theta/dt.a) and velocity of C(u)...
6. ### Conservation of energy of a bow and arrow

I thought the initial elastic potential energy would be converted to the arrow's kinetic energy but it also appears that the bow has some velocity.
7. ### Block on top of moving slab, with a rough surface - When does v_b=v_s?

For the first question I thought of using an energy balance, there is friction ##\Rightarrow \Delta E_m = -W_f##. Both at the start and at the end, the block has no velocity. Therefore ##E_{\text{initial}}= \frac 1 2 m_s v_{s,i}^2## and ##E_{\text{final}}= \frac 1 2 m_s v_{s,f}^2##. This means...
8. ### I Matter Anti-Matter Annihilation

The pure energy coming from a collision and how is it measured or is it just "A Formula" and is any of that energy Dark Energy or even Dark Matter, the reason for the question is because the Dark Energy/Matter vs normal Energy/Matter seems to align with what is seen now after the Big Bang...
9. ### Energy analysis of the system (leaking bucket from Morin's "Mechanics"

Hi, Can you please help me understand how the formula of energy decreasing during a sand leaking is obtained? One of possible solution to this problem, suggested in the textbook, states that when the bucket moves from x to x+dx (d is negative), there are two components responsible of energy...
10. ### Energy analysis of this system (curved ramp up and a spring)

For point one it's clear that I have to use energy => ##ΔE_{AB} = W_{friction}## ; ##\frac 1 2 mv_0^2 - \frac 1 2 mv_1^2 = mgμ_d d## After that there is the path BC, but I don't know how to analyze it from an energetic standpoint. Then after BC the block will now have a different velocity, I...
11. ### Speed of charged balls after collision

I tried to apply energy conservation . $$\frac{-kQq}{l}=\frac{m}{2}(v_1^2+v_2^2)-\frac{kqQ}{2r}$$ Now conserving momentum : $$0=mv_1-mv_2$$ Solving for ##v_1=v_2=v'## we get : $$v'=\sqrt{\frac{kQq}{m}\left(\frac{l-2r}{2r}\right)}$$ Since the balls are elastic , so they should collide...
12. ### I I have a question about gravity -- If the value of the energy momentum tensor (Tμν) becomes zero, can it become gravitational-free?

R μν − 1/2g μν R= 8πG/c^4T μν In this formula, if the value of the energy momentum tensor(Tμν) becomes zero, can it become gravitational-free?
13. ### Rpm and power for this electric motor, gearbox and pulley system

i got confused because if the connections btw the parts
14. ### The Unified Theory of Energy

If someone believes they have developed a somewhat unified theory that brings together various physics concepts into a Unified Theory of Energy, but they are concerned about being dismissed or criticized by members who strongly adhere to conventional academic views, where in the Physics Forum...
15. ### B Can energy be stored in a single particle indefinitely?

Can energy be stored in a single particle without it being lost over time? I mean, photons would be an exampld in principle, but they get redshifted as the universe expands and become less energetic as time goes by We could store that energy in form of kinetic energy for individual...
16. ### Why isn't bond dissociation energy/bond enthaply measured in Newtons?

I understand every bond chemically has a length and energy to break, and energy is Newton*meters. Is the Bond enthaply/Bond disassociation energy equivalent to the force needed to break the bond * the bond length? Why don't we say, to break the bond from O to H we need to put magnets on left of...
17. ### Oscillation of system of three charges

I tried to take angles and proceed by energy conservation But this doesn't seem to lead me anywhere . Here , the length of threads is ##l## each and ##2\theta## is the central angle. ##y_1## is the displacement of the charges attached at the extreme ends of the threads respectively while ##y##...
18. ### B Does energy exist?

TL;DR Summary: does the quantity we refer to as energy actually exist energy is a property of a physical system but does it actually exist, like does an object actually lose or gain a quantity that we refer to as energy after an interaction, or is it just a mathematical concept, are those...
19. ### Internal energy of a gas and kinetic energy, "typical velocity"

Source: Shankar Yale OCW physics I have three questions here: 1. K_avg is 3/2kT, sure. But isn't this the kinetic energy of one particle only? So why isn't the answer multiplied by avogadro's number (because one mole). 2. When doing the "typical velocity" derivation, I noticed that they used...
20. ### IPHO classical mechanics: A mass falls on an exoplanet

Attempt: I assume that the position of the mass ##M## after it is realised its position is given by the position vectors from the origin, ##\vec m = -m(t)~\hat m## if ##m(t) > 0## or equivalently ##\vec m = m(t)~\hat m## if ##m(t) < 0## Either one we can use for energy conservation (I am...
21. ### I A. P. French "Matter and Radiation: The Inertia of Energy"

I am reading A. P. French's book: "Special Relativity". Currently I am focused on the section: "Matter and Radiation: The Inertia of Energy." Under the heading: "Matter and Radiation: The Inertia of Energy", French writes the following: In the above text by Young...
22. ### I It's still not clear to me what's the limit of light propagation

Hi, I've been asking questions about light here for years, and I still don't understand the limit of propagation of light, does anyone have advanced on this field? I really would like someone to explain me how it's possible for light to propagate forever, since it's probably emitted in perfect...
23. ### I Can the expansion of the Universe reduce the energy of a photon to 0?

Is it theoretically possible for the expansion of the universe to red-shift the energy of a photon all the way to zero? If so, what happens to the photon? Or does the photon's energy only approach zero as an asymptote?
24. ### Gravity Based Electricity Generator

Hello community, I am currently designing a gravity-based electricity generator and exploring the feasibility of different setups using a 500 kg mass. The core of my project is to harness the potential energy from this mass as it is lowered from a significant height. I am considering several...
25. ### B The relationship between mc^2 and mc^2 x 1/2

Kinetic energy = 1/2 m V^2 I was thinking about this and thought another formula… E = mc^2 These look very similar except for the multiplication by 1/2. Let’s say you take a kilogram ball of uranium and accelerate it to the speed of light. I know, I know. You can’t. But let’s say you did...
26. ### B Too much energy -- thought experiment

1/ Have a closed hydraulic circuit. Two columns of fluid. One hot, one cold, which are connected at the bottom and top. Let's have small heat loss and small hydrodynamic resistance to fluid flow and incompressible fluid. I heat the lower interconnection, I cool the upper interconnection. I have...
27. ### B Energy required to bring 1 kg 1000 meter above the surface of the Earth

How can i calculate the Energy required to bring 1 kg 1000 meter above the surface of the earth ?
28. ### I What is the zero point for Potential Energy and how to find it from integral limits?

from what I understand, work is the integral of the force with respect to displacement along the path of displacement $$\Delta w=f*\Delta r$$ and is defined so that the change in kinetic energy is the work, as for potential energy it is the work done to change the position of the object from...
29. ### I Charged box mass-energy

A charged, metallic box has an energy content higher than an uncharged box, due to the energy stored in the electric field (which is equal to the work that has to be done to bring the charges from "infinity" to the surface of the box). So, due to the mass-energy equivalence, a charge box has a...
30. ### Using conservation of energy and momentum in projectile motion

I am going to use this coordinate system: According to the answer of the book, I think no force is acting on this projectile: Let's say at top of it's trajectory its velocity is ##u##. Conservation of energy : $$2E_0=\frac 1 2 m_1 v_1^2+\frac 1 2 m_2 v_2^2$$ Conservation of momentum in...
31. ### B Percentage of known energy in the Universe

It is said that the universe is made up of approximately 4.9% ordinary matter, 26.8% dark matter and 69.3% dark energy. Why isn't ordinary energy included in this "pie"? I suppose it is included within ordinary matter, but could it be calculated what % are particles with mass and what % is pure...
32. ### How to calculate gamma ray energies following beta decay?

This is what I have for my solution thus far: "For gold to transition to mercury, it must exchange one of it's neutrons for a proton via ##\beta##-decay. In order to find the energy of the emitted gamma rays, we must first find the excess rest energy ##Q## present after the ##\beta##-decay. The...
33. ### Find at what rate the orbit radius will grow

In order to change the radius, additional energy is required, the total energy of mass m on a circular orbit is given by: $$E_{total} = - \frac {GMm} {2R},$$ The change in energy between orbits ##R## and ##R_{0}## is: \Delta E_{total} = \frac {GMm} {2} \cdot \left( \frac 1 R_{0} - \frac 1 R...
34. ### What is the energy equation in Schrodinger's Spherical equation?

I attempted the problem by first finding the radial, theta, and phi equation for the ground state of a hydrogen atom. I multiplied the three equations to get the wave equation. From there, I took each derivative in the Schrodinger Spherical equation and found that ## \frac {\partial^2 \psi}...
35. ### I Move Stress-Energy tensor to the left side of EFE?

Is it possible to move Stress-Energy tensor to the left side of EFE? R=T => R-T=0. Relativists move cosmological constant Λ to the right side of EFE. Can we move SE tensor to make a vacuum?
36. ### B Does a photon's energy change as it refracts?

I believe that when a photon is refracted it slows. Why is it that the wavelength decreases but that its frequency stays constant? Does this imply that the photon has not lost any energy in the process of slowing down given that E=hf? Thanks
37. ### B What happens to a photon's energy when it is refracted?

I believe that when a photon is refracted it slows. Why is it that the wavelength decreases but that its frequency stays constant? Does this imply that the photon has not lost any energy in the process of slowing down given that E=hf? Thanks
38. ### I Infinite flow with capillary tubes?

I was watching this YouTube video by the channel The Action Lab: At one point it shows this capillary tube phenomenon: It got me immediately thinking: Conservation of energy much? What's stopping that second tube from being bent into draining into the leftmost tube, thus creating an...
39. ### Lab Exercise: Measuring "g" using Conservation of Energy

Hi, so this is a lab in which we used an air track at an angle and a glider to gather some data through various trials, ultimately to calculate "g". L_glider = 10.15 cm x (photogate activation point) = 547.5 mm or 54.75 cm x_0 (release point) = 1800.0 mm or 180.00 cm (Δx)_midpoint = | x - x_0 |...
40. ### Rolling Sphere On Incline

I did some algebra and got the final equation mgh=1/2MR^2omega^2+1/2(2/5MR^2)omega^2, then plugged in the numbers and got an omega value of 111.09. Using 1.7 as h instead of cos(17)*1.7=1.63 I got 114.09. Both answers were wrong. Am I missing something, or did I just screw up the math?
41. ### Do you graph the energy levels of a ##1s^1## atom the same as a ##1s^2## atom?

From the first equation, there are 5 constants, e, ##\pi##, ##\varepsilon_0##, ##n^2##, and 32. The only difference is m, where helium has around four times the mass of hydrogen. What I don't get is if there is a difference between the energy levels of the hydrogen and the ionized hellium? Also...
42. ### B Energy paradox? Conservation of energy violated?

If you have 3 observers, one at rest 0c, one at 0.8c, and one that moves between these 2 frames of reference. The observer that moves between the frames starts at 0c and accelerates all the way up to 0.8c. As it accelerates, the observer in the stationary frame sees its momentum, relativistic...
43. ### I The answer for the total mass of the Universe?

it requires less energy to impart acceleration to an object than to the entirety of the universe, This would break the principle of relativity since the amount of energy to move an object should be the same as the amount of energy to move the rest of the universe. This observation implies a...
44. ### I Vacuum up-tunneling with high-energy events?

I was reading these papers by Sean Carroll (https://arxiv.org/abs/1405.0298; https://arxiv.org/abs/1505.02780) in which, among other things, he argues against vacuum up-tunneling occurring in the universe. He only acknowledged that it would be possible in the first moments of the universe while...
45. ### B I have a question about energy & ignoring friction losses

If the energy itself ignores loss resistance friction and the energy moves forward, can infinity go far at a constant speed?