Energy Definition and 999 Threads

  1. xpell

    Calculating Energy Released by Sodium Reacting with Water

    Hi! After watching a couple videos about the explosive reaction of sodium in freshwater and seawater, I felt curious and wanted to calculate how much energy is released. However, my Chemistry is super rusty and I think that my result is way too high (equivalent to almost 2 g of TNT per gram of...
  2. S

    MHB Work energy principle and power

    Particle X of mass 2 kg , and particle Y of mass m kg are attached to the ends of a light inextensible string of length 4.8m. The string passes over a fixed smooth pulley and hangs vertically either side of the pulley. Particle X is held at ground level, 3m below the pulley. Particle X is...
  3. E

    How to find the heat energy produced in a resistor?

    i found current to be 2/5 A. then 2/5A multipled by 6s = 2.4C. and since one joule per coulomb is equal to the unit of p.d, i get 2.4J. I don't get 4.8J(supposed answer)
  4. Ranku

    I Energy-momentum tensor as energy density

    Can the energy-momentum tensor of matter and energy be cast in terms of energy density of matter and energy, similar to how the energy-momentum tensor of vacuum energy can be cast in terms of the energy density of vacuum energy?
  5. V

    B Potential energy in standing wave compared to traveling wave

    From hyperphysics, "The unique point in the case of the traveling wave in the string is the element of the string that is at the maximum displacement as the wave passes. That element has a zero instantaneous velocity perpendicular to the straight string configuration, and as the wave goes "over...
  6. S

    MHB Work energy principle and power

    A car of mass 1200 kg accelerates up a hill inclined at 10 degree to the horizontal. The car has initial speed of 10 m/ s and final speed of 12 m/s after 60 s. Air resistance and friction may be ignored. Find the average power generated by the engine. The and in the textbook is 36000 W, iam...
  7. L

    B Work and energy: conceptual doubt

    Suppose I am sliding a block very slowly on a rough surface. If the block has traveled ##d## distance then work done by me is ##W_1=\mu mg d## and that by friction is ##W_2=-\mu mg d##. Now the energy transferred from me to block is ##\mu mgd## and that taken by friction from block is ##\mu mgd...
  8. M

    B Why does it require an infinite amount of energy to reach the speed of light?

    I don't need equations, I would just like to pose a question which contradicts the above statement (I know I am wrong btw, I want to see where I am going wrong).My understanding of space (not near any gravity and therefore no spacetime curvature) is that a body in motion will continue to move at...
  9. S

    MHB Work energy principle and power

    A golf ball of mass 45.9g is hit from a tee with speed 180km/ h. The ball rises to a height of 20m having traveled along a curved path of length 61.875m. At the highest point of its path the ball is traveling at 144 km/h a) Find the magnitude of the avg resistance force acting on the golf ball...
  10. meher4real

    Solving Photon Energy with Equations: Am I Right?

    Hi Guys ! Previous question was to determine the energy of photon with wavelength = 820nm I found that E = 2.42x10^-19 J = 1.51 ev using the equation E=hc/wavelength - I tried to solve the above problem with various equations but all failed. The closest equation i tried is...
  11. P

    A Carnot engine between the Earth's poles and equator

    Earth absorbs and releases a massive amount of energy as a part of the day night cycles.(~100 Peta watts of solar radiation received) And the temperature difference between the poles and equator drive weather phenomenon, which happens at an energy scale beyond technology. I am wondering if one...
  12. S

    MHB Work energy principle and power

    A golf ball of mass 45.9g is hit from a tee with speed 50 m/ s. The ball lands in a pond 5m lower than the tee. When the ball lands in the pond it has traveled a curved path of length 160m. The resistance acting on the ball has magnitude 0.3N a) Find the speed of the ball just before it hits the...
  13. S

    MHB Work energy principle and power

    A sky diver of mass 80 kg falls 1000m from rest and then opens his parachute for the remaining 2000m of his fall. Air resistance is negligible until the parachute opens. The sky diver is traveling at 5 m/ s just before he hits the ground. Find the average resistance force when the sky diver is...
  14. D

    A Relationship between Larmor precession and energy eigenstates

    Quantum mechanically, a spin 1/2 particle in a uniform magnetic field has two energy eigenstates ##\ket{up}## and ##\ket{down}## and rotational degrees of freedom (distinct from the energy eigenstates) about the axis of the magnetic field. this can be derived from the Pauli matrix commonly...
  15. S

    MHB Work energy principle and power

    A football is kicked from the ground level with speed 15m/s and rises to a height of 1.45m. Assume that air resistance is negligible a) Find the speed of the ball when it is 1m above the ground Increase in GPE= loss of KE mgh=1/2mu^2-1/2mv2 10×(1.45-1)= 1/2×15^2-v^2/2 V= 14.7 m/ s But textbook...
  16. S

    MHB Work energy principle and power

    A box slides down a smooth ramp. The height of the ramp is 20cm and the length of the ramp is 2.5m. The box starts from rest. What is the speed of the box when it reaches the bottom of the ramp? I don't understand how to solve this. Pls help
  17. Paulpaulpa

    I A ray crossing 2 media of different indices and energy conservation

    The ##I_i## are the intensity of the rays, in other words energy per surface units per radians by seconds. The d##\Omega## are the solid angles The equation p75 isis what I don't understand. I suppose that each side represent the energy going and out of the surface dS but I don't understand...
  18. F

    Kinetic and potential energy of a particle attracted by charged sphere

    Hello, I have a particle at point A with charge ##q_A##, and an unmovable sphere of radius ##R_B## at point B with a volumic charge density ##\rho##. The distance from particle A to the centre of the sphere in B is ##r##. Both objects have opposed charges, so, the particle in A, initially at...
  19. S

    MHB Work energy principle and power

    A child of mass 40kg slides down a playground slide. The child starts from rest at the top of the slide, 2m above the ground. At the bottom of the slide it's slope levels off. a) Find the child's loss of GPE I got the ans 800J there is a constant resistance of 112N throughout b) find the...
  20. S

    MHB Work energy principle and power

    A girl of mass 50 kg travels down a water slide. She starts at the top with a speed of 2 m/s and descends through a vertical height of 5m. a) Assuming that there is no resistance, find her speed when she reaches the bottom of the slide I got the ans 10.2m/ s b) the girl's actual final speed is...
  21. S

    MHB The work energy principle and power

    A box of mass 25kg is pulled 5m across a smooth floor by rope with tension 22N. The rope is inclined at 40 degree to above the horizontal. There is a frictional force with average value 12N. The box starts from rest. Find the final speed. Iam getting the ans 1.97m/s. The textbook ans is 1.39 m/s...
  22. K

    I How to measure a 10 Hz energy splitting of two energy levels

    Hello! If I have 2 energy levels split by something of the order of 10 Hz (they can be connected by an electric dipole moment i.e. ##\Delta J = 0## and they have different parities), what would be the best way to measure this difference (even 10% error would be good, but the lower the error the...
  23. stevendaryl

    I A short derivation of the relativistic forms of energy and momentum

    I've been noodling around with derivations of the relativistic energy and momentum, and I almost got it down to just a few lines. But not quite. I'm going to work in one spatial dimension, for simplicity (even though some derivations require a second spatial dimension) Let's assume that there...
  24. Vandenburg

    B How does conservation of energy apply at the nuclear level?

    Electrons rotate around a nucleus for long periods of time. Where does the energy for this motion come from? Ok, I realize that electrons don't actually rotate around the nucleus, like a tiny solar system. But if the electron is wave function, it's still constantly vibrating, constant...
  25. Paige_Turner

    B Energy & Pendulums: Is Negative Energy Real?

    Does "absorbing" negative energy from gravity cancel the upward momentum of the pendulum? Or what? Where can I see energy being negative? Does it repel stuff? It sounds impossible.
  26. Phys pilot

    I Energy distribution plot of neutrinos in beta decay

    Hello, When you have a beta decay you get the typical continuos spectrum representing counts against the kinetic energy of the electron. But what's the shape and how I get the spectrum of the kinetic energy of the neutrinos? Thanks
  27. S

    MHB Calculate the Length & Potential Energy of a Sliding Box on a Ramp

    A ramp rises 10cm for every 80cm along the sloping surface. A box of mass 50 kg slides down the ramp starting from rest at the top of the ramp. The coefficient of friction between the ramp and the box is 0.03 and no other resistance acts. The box is traveling at 2 m/s when it reaches the bottom...
  28. S

    MHB Decrease in Potential Energy of a Sliding Tile

    A tile of mass 1.2 kg slides 3m down a roof that makes an angle of 35 degree to the horizontal. Find the decrease in potential energy. Iam getting the ans 24.8J PE = mgh= 1.2× 12 sin 35 ×3 The ans in the textbook is 20.6J
  29. S

    MHB Calculating Kinetic Energy After Collision of Balls A & B

    Ball A of mass 2kg, is moving in a straight line at 5 m/s. Ball B of mass 4kg is moving in the same straight line at 2 m/s. Ball B is traveling directly towards Ball A. The balls hit each other and after the impact each ball has reversed its direction of travel. The kinetic energy lost in the...
  30. N

    A Relationship between energy density and cosmological constant

    According to the wiki entry on Planck units, https://en.wikipedia.org/wiki/Planck_units, the energy density of the universe, 1.8 × 10−123, is 1/16th the cosmological constant, 2.9 × 10−122. Is there a theoretical reason for this precise relationship?
  31. H

    Degeneracy of the energy level

    Energy of the One-dimensional box: ground state: En = (n^2*h^2) / (8mL^2), where n=1 twice the ground state: 2* En = 2 [(1^2*h^2) / (8mL^2)] Energy of the Three-dimensional box: En = (nx^2 + ny^2 + nz^2) *h^2 / (8mL^2) = 2 (1^2*h^2) / (8mL^2) As stated, twice the ground state energy of one...
  32. George Keeling

    I Uncovering a Decent Proof of E=mc^2

    Sean Carroll says that in SR the time component of the 4-momentum of a particle is its energy. It is of course also ##mc^2dt/d\tau##. He uses that to prove that ##E=mc^2##. Which begs the question why does ##E=p^0##? Misner, Thorne, Wheeler do roughly the same thing. I find these 'proofs'...
  33. A

    I Atomic Structure: Why Does Energy Decrease with Orbit Number?

    I have read that an electron requires certain minimum energy of threshold frequency to move an orbit However the energy needed decreases with increase in shell number The transition energy is reduced with each orbit For example The energy to shift an electron from 1st to 2nd orbit is much...
  34. H

    Can I use the principle of conservation of energy for this problem?

    ddd
  35. Haorong Wu

    I Gradient Energy: Definition & Classical Mechanics

    In page 40 of Spacetime and geometry by Sean M. Carroll, when consider the classical mechanics of a single real scalar field, it reads that the field will have an energy density including various contributions: kinetic energy:##\frac 1 2 \dot \phi^2## gradient energy:##\frac 1 2 (\nabla...
  36. P

    I Do nucleons have a lower energy state when bound in a nucleus?

    But when I look at the definition of binding energy that doesn't make seem to make sense. It looks as though they had more energy when they were together and when they were separated that energy turned to mass (the mass defect)? Am I looking at this right? I also don't understand this...
  37. T

    Understanding the Energy Required to Lift Walls

    Please help, I'm really struggling to understand how to work this... work = mass * gravity * change in height w = m*g*(h2 - h1) Total energy = Sum of potential energy + Kinetic Energy work = 1/2Kinetic Energy * Potential energy ^2 w = (1/2 * K) * U^2 m*g*(h2-h1)=(1/2*k)*U^2 rearanged =...
  38. H

    B Distribution of energy in the electric field surrounding an electron

    I am thinking about how an electric field has energy associated with it. If a single electron exists alone in a remote vaccuum, I believe it has it's own electric field surrounding it, and that this field has an energy content associated with it. My question is; does this electric field store...
  39. J

    Programming Languages used in Graduate School? (quantum, high energy, and nuclear physics)

    Hello, I have 1 undergrad year left and was wondering what the most used programming languages in grad school are. I still don't know which grad school will accept me so I haven't bothered to ask them directly as my application will be in November when I get my GRE scores. I am Interested in...
  40. Ashshahril

    Change in heat and internal energy

    ΔQ=mC(T3-T1) But, will this C be Cp or Cv. Both pressure and volume changes. So, neither of them can be. Feeling so confused
  41. K

    I Smallest energy we can measure

    Hello! Assuming we can bring 2 energy levels very close to each other (e.g. by applying a magnetic field), what is the practical limit (in terms of lab equipment) on the smallest energy difference that we can measure? And what is the relative error on it, that can be obtained? For example if the...
  42. bob012345

    A Rippling Graphene Harvests Thermal Energy?

    It is claimed 2D Graphene sheets can be used to harvest thermal energy while being at the same temperature as the surroundings in seeming contradiction to Feynman's *argument (in the popular accounts). However, the academic papers referenced below make the claim everything is well within known...
  43. B

    I How can there be an Uncertain Momentum at a Specific Energy?

    It's been a few years since I failed my physics degree but I still really want to reach an understanding of QM, and I'm currently going through a QM textbook. One thing I cannot understand no matter how much I think about it, is momentum uncertainty. In classical mechanics a specific kinetic...
  44. itchybrain

    Question about potential energy: gravity and a vertical spring

    Let's say a mass is gently laid on top of a massless spring. The spring compresses. There is a change in the height of the mass. Therefore, there is a change in the gravitational potential energy: a decrease. The compressed spring now has potential energy (it has gained energy). The change...
  45. S

    How does gravity convert potential energy to kinetic?

    If I hold a ball above the ground, it has potential energy. Once gravity pulls on it, it becomes kinetic. What is gravity and how does it convert one kind of energy to another?
  46. F

    Electrostatic energy in finite space

    Electrostatic energy involves a volume integral and a surface integral The question is how to apply this formula to a finite space in which case the 1st term (surface integral) won't vanish. Let's apply to a capacitor and enclose the capacitor by a closed surface. Calculate the energy integral...
  47. yucheng

    Is the potential energy of an ion in a lattice ##U_0## or ##U_0/2##?

    Potential energy in a two-dimensional crystal Consider the potential energy of a given ion due to the full infinite plane. Call it##U_{0}##. If we sum over all ions (or a very large number##N##) to find the total##U##of these ions, we obtain##N U_{0}##. However, we have counted each pair twice...
  48. R

    How Can I Analyze Particle Movement Given Potential Energy and Force Function?

    Knowing that ##F(x)=-\mathrm{d}V(x)/\mathrm{d}x##, I found that ##F(x)=-2.4x^3+1.35x^2+8x-3##. But it was the only thing I could find. How can I analyze what will be the type of movement with the information presented by the question statement?
  49. D

    Energy, work, power and efficiency when throwing a ball upward

    I don't have an idea of where to start. I tried to do Ein = 1/2 mv^2 ------ (1/2)(1)(10)^2 ----------- 50 but i don't know where to go from here
  50. R

    What is the potential energy of a system with N cubes at equidistant heights?

    I have tried to apply the conservation methods, but I am not understanding what the statement is asking for.
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