1. ### Plane pendulum: Lagrangian, Hamiltonian and energy conservation

Hello! I need some help with this problem. I've solved most of it, but I need some help with the Hamiltonian. I will run through the problem as I've solved it, but it's the Hamiltonian at the end that gives me trouble. To find the Lagrangian, start by finding the x- and y-positions of the...
2. ### Energy conservation for objects hanging from a pulley

I have some conceptual questions about this task. In order to get the correct result (I checked the textbook answer) in part (a) I had to assume that the speed for each block is the same at all instants. And that if one block moves down x meters, the other one will move up that same amount of...
3. ### Energy Conservation of a Vertical Spring

Using conservation of energy, 0.5kx^2=mgh=mgx 0.5kx=mg 0.5kx=mg, x=0.15, m=9, g= 9.8 So isn't it k= 1176N/m? For this problem, I understand that you can't use conservation of energy, but why? There is gravitational potential energy at the top and spring elastic energy at the bottom, and no...
4. ### Closest approach from initial velocity and impact parameter

Here were my assumptions: Energy and angular momentum are both conserved because the only force acting here is a central force. The initial angular momentum of this particle is ##L = mv_0b## and we can treat E as a constant in the homework equation given above. I solved for the KE (1/2 mv^2) in...
5. ### I Does action at distance in electromagnetism violate energyconservation

Consider two charges A and B separated at distance D. charge B is attached on spring and can move towards and away from charge A. Now charge A is brought closer to charge B and then it is taken back to its original position. Work done in this process is zero because of conservative forces. If...
6. ### I Conflict of domain and endpoints in Noether's theorem

In the derivation of energy conservation, there is the transformation ##q(t)\rightarrow q'(t)=q(t+\epsilon)##, whose end points are kind of fuzzy. The original path ##q(t)## is only defined from ##t_1## to ##t_2##. If this transformation rule is imposed, ##q'(t_2-\epsilon)=q(t_2)## to...
7. ### I If a magnet does not feel the Lenz force, will there be a violation of energy?

Consider the following, a magnet and a charge is attached to a platform, which is constrained to move only up and down direction. Now, if magnet is moved towards charge, due to changing magnetic field, an electric field will be created. This electric field will impart force on charge. since the...
8. ### Object A is stationary while objects B and C are in motion.

Homework Statement Object A is stationary while objects B and C are in motion. Forces from object A do 10 J of work on object B and –5 J of work on object C. Forces from the environment do 4 J of work on object B and 8 J of work on object C. Objects B and C do not interact. What are ΔKtot and...
9. ### I Energy conservation and friction

Hi, I just started learning physics at university and so I'm looking for help on a simple energy conservation problem. On the bottom right-hand of the image I attached below, you should see that it asks whether the initial speed would increase or decrease if the object was of a greater mass...
10. ### I Energy stored in space-time or space?

I was talking to a graduate physics student about the issue of energy conservation in an expanding universe. I paraphrased the argument against energy conservation as follows - Suppose we have a photon in outer space that is very far from earth. The universe is expanding (by this I meant that in...
11. ### B Do magnets lose strength by pulling them apart?

When two magnets already sticked to each other being pulled apart, energy is applied, so I assume that the energy is stored like some sort of potential energy which will be turned back into kinetic energy when they accelerate towards each other to stick back together. So the magnet itself...
12. ### I Understanding an electron's emission of electromagnetic waves

Hi to everybody ! I was thinking about something which confuses me about wave emission. The question is simply the following: Does an electron emit light when it accelerate? or just during its deceleration? or maybe when acceleration and deceleration alternates in some order? I'm not really...
13. ### A Energy conservation on Cosmological scales

I'm curious to know whether anyone with good maths has anything to say about Dr Philip Gibbs' covariant formula for conserved currents of energy, momentum and angular- momentum derived from a general form of Noether’s theorem? I'm not a pro mathematician, but it looks relatively robust to me...
14. ### B Calculating the frequency of a free electron

I feel like I must be missing something obvious, but I can't figure it out. I have the speed of an electron, and to calculate its frequency i used p = h/λ, then subbed in p =mv and λ= v/f. Giving me the equation f = mv2/h. However, I also could use E = 1/2 mv2 and E = hf to give me the equation...
15. ### I Derivation of the energy principle from Gregory Classical Mechanics textbook

I'm working through Gregory's Classical Mechanics and came across his derivation of energy conservation for a system of N particles that is unconstrained. We get to assume all the external forces are conservative, so we can write them as the gradient of a potential energy. There's a step he...
16. ### Finding angular velocity using conservation of energy

Homework Statement In the document below I need to try and find the angular velocity I need help on part b Homework Equations F=ma KE_Translational= 0.5mv^2 KE-Rotational= 0.5Iw^2 Assuming g=10m/s^2 The Attempt at a Solution I have the answer key attached with the question but I am not sure...
17. ### Conservation of momentum - need help with understanding

I just want to state that i DID solve the problem. I just seek understanding of it. I'd be really grateful if someone could answer two of my questions at the end of this post. The problem I've solved here is just to show what I'm dealing with. 1. Homework Statement Object 1 is moving towards...
18. ### Mechanics - Hooke's law and energy conservation

Homework Statement One end of a light elastic string of stiffness mg/l and natural length l is attached to a point O. A small bead of mass m is fixed to the free end of the string. The bead is held at O and then released so that it will fall vertically. In terms of find the greatest depth to...
19. ### I Velocity addition and conservation of the energy

Hello everyone, For some time I'm a little bit confused about (at the first view) a very simple question, which is about the conversation of the energy of moving objects (in terms of special relativity). As an example lets talk about firearms. If the mass of the gun M1 is infinitely higher...
20. ### Starting height of marble rolling around a loop the loop

Homework Statement The marble rolls down a track and around a loop-the-loop of radius R. The marble has mass m and radius r. What minimum height h must the track have for the marble to make it around the loop-the-loop without falling off? (Use any variable or symbol stated above along with the...
21. C

### Energy conservation in a magnetron

I am trying to understand how energy is conserved in a magnetron. If the electrons are following a circular path in the magnetic field and never reach the anode will there be zero current between the anode and the cathode? If so how does the device consume power from the external circuit...
22. ### Relativity question -- an electron e− and a positron e+ collide, annihilate, and produce Bosons...

Homework Statement In a particle physics lab, an electron e− and a positron e+ collide, annihilate, and produce a W+ boson and a W− boson. Just before the collision, the electron and positron have a total energy of E = 100 GeV each, with velocities pointing along the +x-axis and -x-axis...
23. ### Potential gravitational energy coordinate axis

Homework Statement I know that potential gravitational energy is relative to the reference point that I decide to choose (like in the picture below). But then if, for instance, I set my reference point in the ceiling and my vertically down y-axis to be positive. What would the potential...
24. ### Permanent magnets

First off, I am new here, so please don't tear me apart too hard if I miss or misinterpret something, I'll cotton on eventually :D As I understand it, when a permanent magnet repels an oppositely charged object, such as another oppositely charged permanent magnet, no energy is technically being...
25. ### Minimum Kinetic energy of the hydrogen atom

Homework Statement A hydrogen atom collides with another hydrogen atom at rest. If the electrons in both atoms are in the ground state, what is the minimum kinetic energy of the hydrogen atom such that the hydrogen atom at rest will have its electron in the first excited state after collision...
26. ### Velocity of particle leaving wedge

Homework Statement In the given figure, a wedge of mass 2m is lying at rest on a horizontal surface. The wedge has a cavity which is the portion of a sphere of radius R. A small sphere of mass m is released from the top edge of the cavity to slide down. All surfaces are smooth.Prove the maximum...
27. ### Conservation of momentum - Vertical spring

Homework Statement A block of mass 200g is suspended through a vertical spring. The spring is stretched by 1.0 cm when the block is in equilibrium. A particle of mass 120g is dropped on the block from a height of 45 cm. The particle sticks to the block after the impact. Find the maximum...
28. ### B Is the energy from the mass energy equivalence unbiased?

Let's assume that a system has zero total momentum. The following relationship between mass and energy should apply: E=mc^2. If a system is overall at rest, does that mean that any internal changes to that system, assuming they leave the system with non-negative mass, will not be able to...
29. ### Decay of a particle of mass M into two particles

Homework Statement A particle of mass M and 4-moment P decays into two particles of masses m1 and m2 1) Find the total energy of each particle (lab frame). 2) Show that the kinetic energy T1 of the first particle in the same reference frame is given by T_1= \Delta M (1 - \frac{m_1}{M} -...
30. ### I Conservation of energy in refraction

Hello, This has been bugging me for some time now, so I would be interested to see what I have been missing so far. Imagine a single ray of light (made up of many photons) hitting a perfectly non-absorbing (for this wavelength of light) spherical dielectric object, which has finite mass. The...