# What is Energy and its consevation: Definition and 46 Discussions

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1. ### I Energy and momentum of a photon in a medium

According to this paper: https://academicjournals.org/article/article1380534636_Salih.pdf, the researcher states that energy of a photon is dependent on a medium while momentum of a photon is independent. However according to https://arxiv.org/ftp/physics/papers/0607/0607094.pdf, the research...
2. ### Calculating the spring force constant K

So, my question is pertaining more to a specific part of this problem than actually calculating ##P## which I get to be ##P = \frac{kh}{2} - mg##. But I need ##P## in terms of a multiple of ##mg## so I need to find ##k##. The solution attached uses the fact that when the object comes to a rest...
3. ### Loop problem with skier on ramp going into a loop-the-loop

I have an assignment in physics and I can't understand the difference between the 2 questions. That's the question A skier with mass m=70 kg stands on a ramp of height h. At the foot of the ramp, there is a loop with a radius of r=15 m. When entering the ramp, the speed is v1=0 m/s. a) For...
4. ### Conservation of momentum and mechanical energy on an inclined plane

So i am tried to conserve momentum and use conservation of mechanical energy but won't there be psuedo force acting on the block if i am solving from non inertial frame ?. If i ignore the pseudo force and simply use C.O.M.E and include the K.E of the wedge and solve normally i do get the...
5. ### I Cosmic strings increasing internal energy?

I was reading an article by Edward Harrison, which tackles the problems of conservation of energy at cosmological scales. At some point (point 2.4) he cites several article, including one by Rees and Gott, which he says indicates that the internal energy of a comoving volume (e.g. a cosmic...
6. ### B What is the need for the concepts of Work and Energy (KE)?

What is the need to introduce the concept of work and energy when the motion can be completely understood by the concept of force and acceleration and momentum and velocity and displacement, etc? Why do we need to understand the same thing once again in terms of Work and energy? Also the kinetic...
7. ### B Work done in lifting and lowering an object

##\Delta K=K_f-K_i=W_a+W_g##.(##W_a##, work done by applied force and ##W_g##, work done by gravity) In case of uniform motion with velocity u, kinetic energy is equal. Change is zero. ##W_a=-W_g## If one force transfers energy into the system then the other takes out of the system. Energy of...
8. ### Calculating the velocity of a head-on impact between two suns

Firstly I would like to start with solving the problem with energy conservation principle which most solutions to the question show. -Gmm/r= 1/2 mv^2 +1/2mv^2 -Gmm/2R Where m= mass of planet r= initial seperation v= final velocity. R=...
9. ### I How Does Heat Transfer Impact Our Sensation of Temperature?

We know heat is the motion of molecules of a gas for example. As temperature increases this motion increases and the gas expands. We know this gas would be hot if we touch it. So I want to ask how does the fast motion of molecules somehow translates to the heat that we feel. Why does it feel...
10. ### Rail Car with a Sail in the Wind

I solve the following problem, there is a particle of mass ## m_p ## and velocity ## \vec{v}_p ## which collide with sail installed on rail car with mass ## m_c ## resting in the frame of reference associated with it before the collision. The cart is fixed on straight rails for which the vector...
11. ### What type of ball can bounce and travel the most?

From what I have investigated, I know that dimples do a large part of the process of going far, and elasticity for sure is something of the main bounce thing, but things that confuse me like if the ball is empty affects both characteristics, or if the same dimples affect the bounce of the ball...
12. ### I Can the energy in two waves cancel out? If so, why?

I know the answer would be yes, but why? In class, I learned that energy is scalar and cannot be negative (at least in undergraduate class). Thus adding two sources of energy should result in a higher level of energy in general. But here for wave, if we have 2 waves that do destructive...
13. ### Is Kinetic Friction a Conservative Force?

If a block slides down an inclined surface under the presence of the kinetic friction, does that mean the total energy lost by the block is equal to the work done by the kinetic friction? Thanks in advance.
14. ### I Energy in Everett's Many-worlds Interpretation

Hi, Is anybody able to explain how energy is "distributed" in the many-worlds interpretation. I'm using scare quotes as I think this may be the wrong line of thought. It's tempting to imagine energy being distributed amongst subsequent branches as the wave function evolves but I'm not certain...
15. O

### Chemical Engineer - MBA in Chemical Industry

Experience in the fertilizer industry, with focus on environmental issues and energy recovery, clean technologies, process design and simulation, GHG balance. Published several studies on low carbon technologies as well as on Kyoto Protocol Communications.
16. ### Is it possible to apply energy conservation here?

Homework Statement Homework Equations Kinetic Energy =1/2*m*v^2 Spring Potential Energy = 1/2*k*x^2 Gravitational Potential Energy = m*g*h The Attempt at a Solution I am thinking to solve this problem using energy conservation but I feel that it is not possible to apply energy conservation...
17. ### Beyond Viscous Drag, Cavitation, Laminar Flow, & Reynolds

When using Computational Fluid Dynamic (CFD) modeling you are able to see incredible complexity in water energy transfer. The Thermodynamic energy transfer occurs at much higher velocity so it does not apply in most of the cases I am looking at. What are the other parasitic energy transfers that...
18. ### Bullet shooting vertically into a block

Homework Statement A 28-g bullet is shot vertically into an 6-kg block. The block lifts upward 5mm (see the figure). The bullet penetrates the block and comes to rest in it in a time interval of 0.0010s. Assume the force on the bullet is constant during penetration and that air resistance is...
19. ### Potential Energy for "stone + field" physical system

Consider the classical scenario a stone falling in the Earth gravitational field. Classically we attach a Potential Energy to the stone and using the law of conservation of (mechanical) energy we are able to evaluate the dynamic of the falling stone. This model assume a stone in a "external"...
20. ### Hi, help me to check my answers, dynamics, energy and work

Homework Statement Whats is the final speed of the second box(mass = 1kg) when the first one (mass = 5 kg) has descended a distance equal to 0.6 meters in the rough ramp inclined 60° respect the x axis. Consider the spring is compressed a length x = 0.2 meters. The second box is tied to the...
21. ### Find the angle for which both disks are no longer in contact

Homework Statement A mobile disk of radius R and mass M is moving above another immobile vertical disk of the same radius. Initially the mobile disk is at rest at the highest point above the immobile disk and then it starts rolling without slipping. Assuming the mobile disk never slips, find...
22. ### Analyzing Elastic Collisions w/ Conservation of Energy and Momentum

Homework Statement There is a 4 kg mass that has a speed of 6 m/sec on a horizontal frictionless surface. The mass collides head-on and elastically with an identical 4 kg mass initially at rest. The final speed of the first 4 kg mass is: (a) 0 m/s (b) 2 m/s (c) 3 m/s (d) 6 m/s Homework...
23. ### Maximum amount of energy the neutron can lose

A 1 keV fast neutron (relative mass 1) in a moderator collides elastically with a helium atom He (relative mass 4) at rest. What is the maximum amount of energy the neutron can lose? My answer is 16/25 of 1ke but while deriving this answer I simply solved based on the question as if the...
24. ### Using Energy and momentum conservation to derive the equation

Homework Statement I need to find the intial velocity of a ball, given the angle the pendulum bob swings through. I need to derive this equation. [/B] V0=4.43mtotalL1/2{1-cosΔθ}1/2/mball Homework EquationsThe Attempt at a Solution I have barely any attempts since I can't even think where to...
25. C

### Conservation of Energy: Spring pushing a block up an incline

Homework Statement "Consider a 250 gram block on a 10 degree frictionless incline and in contact with a spring of constant 1.2 N/cm. If the block is launched from rest by the spring with an initial compression of 6cm, how fast is the block moving at the point of release from the spring? How...
26. T

### Block sliding down hill splits into two blocks

I am currently working on this problem and I am stuck as to how to approach or solve it. The problem is that a block of mass 11000kg is sliding down a slope with a height of 1000m, angle of 40 degrees and the coefficient of friction between the block and the slope is 0.1. The block splits...
27. ### Newtonian energy integral and suitable boundary conditions

I have a (somewhat) strange energy equation which has the following form: KE = A + B W + C \exp(-D W), where A,B,D are known constant, C is an unknown constant to be determined and kinetic and potential energy are given by KE and W respectively with W\equiv W(r) i.e. is a function of...
28. P

### Piezoelectric energy harvesting systems

Hello, I am an student of Engineering and I come a cross of a doubt that I would like to investigate a bit more. I would like some information or some directions to the right path for the following. I would like to know how could I calculate the harvesting energy of an area for a piezoelectric...
29. ### Kinetic energy and potential energy look very dissimilar

The more I think about it the less clear the respective natures of kinetic energy and potential energy (and of their sum, the so-called total energy) become. The thing is I have the impression that once you try to go a bit further than the usual description of "scalar values assigned to systems...
30. ### Calculating the Wavelength of Monochromatic Radiation in Compton Scattering

Homework Statement Compton scattering can be used both to measure the direction and energy of photons in nuclear physics experiments. For a particular preparation a spectrum of Compton scattered electrons was measured which clearly corresponded to a generally monochromatic gamma radiation. The...
31. ### Mass sliding on frictionless inclined plane compresses a spring

Homework Statement A body with mass m start from repose and slide along a plane without friction. The plane forms an angle with the horizontal. After sliding a distance d unknown, the mass touches the extremity of a spring non compressed and non stretched with no mass. The mass slides a...
32. ### Finding the total mechanical energy

Homework Statement A bead is sliding on a surface. At point A it is 80 cm above the ground, at point B it has just hit the ground and at point C it is 50 cm above the ground. At point A it has a speed of 200 m/s, so what will its speed be at point B and C? Homework Equations W=F.d Kinetic...
33. ### How can energy be lost to friction?

hi everyone! I'm new here. I was wondering how energy can be lost to friction? How would one even calculate that? Thanks!
34. ### Height in conservation of energy problem

Homework Statement A very Slippery ice cube slides in a vertical plane around the inside of a smooth, 20 cm diameter horizontal pipe. The ice cube's speed at the bottom of the circle is 3.0 m/s Vi = 3.0 m/s Height at top= 2(.20) = .40 Vf = ? Homework Equations KE(initial) + PE(initial) =...
35. D

### What are the latest technologies for reducing carbon fuel requirements?

Hey all, I've been in the energy and emissions conservation and reduction space for over a decade, just searching for new technologies resulting in the reduction of carbon fuel requirements.
36. ### Excess Energy Saving from a constant energy source.

Hello everyone. So i live in Lebanon, and since a long time now we have constant power cuts, almost half the day each day. For that the Lebanese made up a system in which each neighborhood has power generators providing electricity to the residents when the power is out. Usually the owners of...
37. ### Doubts on Work-Energy theorem for a system

While studying energy conservation on Morin I found this explanation about the work-energy theorem for a system. Using Koenig theorem $$\Delta K_\textrm{system}=\Delta K +\Delta K_\textrm{internal}$$ so we have I've got two main question on that: Why are only external forces considered for...
38. ### Why do conservative forces try to reduce potential energy?

I do not understand the reason why a conservative force always "tries" to reduce the potential energy of a system at its minimum (forgive me if I said it in a wrong way). The explanation I gave me is: since for a conservative force, from the definition of potential energy, W=-\Delta U that...
39. ### Gaussian gun: Where does the extra kinetic energy come from?

Probably a stupid question, but I do not get it... Here you can see an experiment demonstrating the Gaussian gun: Here I understand how momentum is conserved, i.e. the smaller mass of the ejected ball times its high speed is equal the bigger total mass of the recoiling masses times its...
40. ### Ball bounce angle on inclined surface

Homework Statement Ball strikes inclined plane of infinite mass with velocity v vertically. Elastic collisions. Velocity and direction after collision? One way of solving is take components along and perpendicular to inclined plane and then solve easily. Is there any way to solve is using...
41. ### Problem about equality of energy formulas

Above they use the equality that the massless energy hv is equal to einsteins energy. Both have the same velocity. How can not this mean that einsteins formula has more energy then the other. I have added a proof of einsteins formula just in case it can be used: In the proof above I don't...
42. ### Mass on Incline: Kinetic & Potential Energy

Homework Statement Attached Homework Equations kinetic energy = (1/2) m v^2 potential energy = mgh The Attempt at a Solution Did I do this correctly, At the top, kinetic energy is 0 since it starts at rest. At the bottom we choose the potential to be zero So using conservation, mgz=...
43. ### Speeds regarding 2D inelastic collision

Homework Statement You are called to the site of a collision between a small car and a half ton truck on a lonely country road. You are given the following information: a) On clean dry pavement at a temperature of 20 C, the coefficient of kinetic friction is 0.600 on locked wheels. b)...
44. ### Is Conservation of Energy the Key to Solving This Tricky Problem?

I find this problem kind of tricky. I think it must be a, since a change in momentum of the car causes the same change in momentum of the earth.
45. ### Two Toilet Papers dropping-Rotational Inertia

Homework Statement Okay, so I'm supposed to take two fresh rolls of toilet paper and drop them. One of which, I am supposed to let unravel while falling. The other, I just drop on its own. I'm supposed to find the height at which both will drop and hit the ground at the same time. Homework...
46. ### Basketball (Energy and its Conservation?)

Homework Statement A basketball player shoots a 3-point shot. The 0.625 kg ball leaves the player's hands with a speed of 9m/s. It rises 2.4m before descending towards the basket. at the top of the ball's arc, what is its speed Homework Equations Im at a chapter named Energy and its...