Read about conservation laws | 29 Discussions | Page 1

  1. F

    Annihilation: calculation of photon energies

    I set up this problem this way: ##p_a^{\mu}=(E, \sqrt{E^2-m^2}, 0, 0)## ##p_b^{\mu}=(m, 0, 0, 0)## ##p_c^{\mu}=(2E_\gamma, 2E_\gamma, 0, 0)## I have chosen to consider the two photons as a single particle of energy equal to ##2E_\gamma##. At this point I applied conservation of the...
  2. S

    A What would it mean if symmetries in physics would not be fundamental?

    Physicist Joseph Polchinski wrote an article (https://arxiv.org/pdf/1412.5704.pdf) where he considered the possibility that all symmetries in nature may not be fundamental. He says at page 36: "From more theoretical points of view, string theory appears to allow no exact global symmetries, and...
  3. Hamiltonian299792458

    A Block in a groove problem

    since there is no external force in the x-direction linear momentum can be conserved. Hence I get the equation $$0 = mv^2 - 9mV^2$$ where ##v## is the velocity of B towards the right and ##V## is the velocity of A toward the left. also the conservation of energy gives $$1/2(9m)V^2 + (1/2)mv^2 =...
  4. Like Tony Stark

    Two rods, each with a free and a fixed ball and a spring

    Since there are no external forces, the angular momentum (##L##) and linear momentum (##P##) are conserved. Let's call the left rod ##A## and the right one ##B##. If all the balls were fixed, I'd write ##L_0=L_f## ##L_A+L_B=(I_A+I_B)\omega_f## From this equation I can find the final angular...
  5. Like Tony Stark

    Conservative forces vs friction

    Hello I've written that homework statement as an example to illustrate my doubt: How can I tell if a force is conservative or not? I've read that, if the curl of the force is 0, it's conservative. But what about the friction force (##f=\mu N##)? Its curl is also zero, but it's not conservative...
  6. T

    Isospin conservation

    I got the I3 values for the tau(minus) to be -1, as charge is -1 and Y=0. For muon(minus) i got I3 to be -1 too using the same equation and the anti electron neutrino to have an isospin of zero (since Q=0, Y=0). This shows I3 to be conserved (which is needed for strong interaction i believe)...
  7. isabelle3

    A bullet hits a rod attached to a pivot at one of its ends...

    My initial thought was to use the conservation of energy law since there're no external forces acting on the system bullet + rod. The rod is in rest, the bullet is moving. Then after the collision, the bullet and the rod are rotating around the pivot together, so the kinetic energy of the bullet...
  8. KingOfDirewolves

    Classical Mechanics Problem Based On Conservation Of Energy

    I have tried using Conservation of Energy but I'm getting incorrect answer.
  9. M

    A Conservation laws in Newtonian and Hamiltonian (symplectic) mechanics

    In Newtonian mechanics, conservation laws of momemtum and angular momentum for an isolated system follow from Newton's laws plus the assumption that all forces are central. This picture tells nothing about symmetries. In contrast, in Hamiltonian mechanics, conservation laws are tightly...
  10. Abhishek11235

    Solving this problem using the energy method

    I attempted the solution using force method. I got correct. However, I was stuck at the alternative way to solve problem using energy method. As shown in screenshot 2, he tells that the energy of system changes due to 2 ways: - The tension T - Leaking of mass As shown in screenshot 2 ,the...
  11. Harperchisari

    I The energy conservation issue with parallel charged plates with a hole.

    A while back I thought of an issue with parallel charged plates. Imagine this: a set of opposite charged resistive plates with holes in the center. In theory, there is a finite amount of energy required to push a positive charged particle through the hole in the positive plate (in theory it...
  12. F

    Motion in a vertical loop

    $$mg(0.45) = mg(R + R \cdot cos(\frac{π}{3})) + \frac{1}{2}mv^2$$ $$v^2 = g(0.9 - 3R)$$ The centripetal acceleration during the "flying through air" will be given by gravity $$mg \cdot cos(\frac{\pi}{3}) = \frac{mv^2}{r}$$ $$R = \frac{1.8}{5}$$ But my book says $$ R = \frac{1}{5}$$
  13. ?

    I Rotating sphere which separates into hemispheres

    Hi all, The scenario I'm considering is a solid sphere (of uniform density) rotating with constant angular velocity when it abruptly splits into two hemispheres along a cut which contains the rotation axis. The hemispheres will begin to separate; if, for example, we consider the rotation to be...
  14. stevendaryl

    A Intuitive Meaning of the Coleman-Mandula Theorem

    This theorem is summarized here: https://en.wikipedia.org/wiki/Coleman%E2%80%93Mandula_theorem I sort of understand the mathematical content of the theorem, that But what I don't understand is, intuitively, what sort of possibilities are ruled out. I've heard it said that flavor conservation...
  15. L

    Relativistic Dynamics Problem - Reference Frames

    Homework Statement Two images are attached. The first image details the problem. The second image has an x',y' coordinate system depiction of the problem. Homework Equations The total energy of a particle is defined as E = mc^2, with m = γ*m_0. The Attempt at a Solution If the x', y'...
  16. Phylosopher

    I Conservation laws from Lagrange's equation

    My question is related to the book: Classical Mechanics by Taylor. Section 7.8 So, In the book Taylor is trying to derive the conservation of momentum and energy from Lagrange's equation. I understood everything, but I am struggling with the concept and the following equation...
  17. Manolisjam

    3 balls in a moving mechanics problem

    Homework Statement Consider 2 balls A,B on the same line . and they are connected to a third one G with a rope L. AG, AB. now the system monves in the effect of the mass of G and its projection to the line AB is in the middle. No friction. mass of A=mass of B=m and mass of G=2m .FInd the time...
  18. Manolisjam

    Classical Mechanics Problem with balls

    Homework Statement Consider 2 balls A,B on the same line . and they are connected to a third one G with a rope L. AG, AB. now the system monves in the effect of the mass of G and its projection to the line AB is in the middle. No friction. mass of A=mass of B=m and mass of G=2m .FInd the time...
  19. R

    Elastic and inelstic collisions conceptual questions.

    Homework Statement The questions showed in the pictures ask me whether the collisions in the drawings could be elastic or inelastic, I am not given any mass, the angles are a little vague but I think B and C are supposed to be π/2 and π respectively. For velocity, I am just given the direction...
  20. J

    A Conserved quantity for a particle in a homogeneous and static magnetic field

    The equation of motion for a charged particle with mass ##m## and charge ##q## in a static magnetic field is: ##\frac{d}{dt}[m{\dot{\vec{r}}}]=q\ \dot{\vec{r}}\times \vec{B}## From this, we can see that ##\frac{d}{dt}[m\dot{\vec{r}}-q \vec{r}\times \vec{B}]=0## and so the following quantity is...
  21. parshyaa

    I What does Noether's theorem actually say?

    I don't know much about classical physics(such as lagrangian function), but as i was reading conservation of energy, i came to this theorem and it tells that if a system is symmetrical in certain transformations(such as translation, rotation etc) then it will have a corresponding law of...
  22. entropy1

    B Photon absorbtion and conservation

    If a (polarized) photon is absorbed by a polarization filter, does its energy go into the filter? I am wondering if that is the case to obey conservation laws. And if it passes, is its original polarisation direction somehow conserved?
  23. S

    I Conservation of information

    Hi everyone, I was watching Susskinsd's lectures posted on youtube about Quantum mechanics and he started explaining his -1st basic law of physics saying: the conservation of information means basically that if you start with 2 quantum states (vectors) which are observably different, or...
  24. referframe

    I Conservation laws during particle decay?

    I believe that conservation laws, like for energy and momentum, are obeyed during the particle decay process, e.g. the total energy of the new output particles is equal to the energy of the one input particle. But is that relationship subject to quantum fuzziness? Suppose we, somehow, prepare...
  25. R

    I Compton effect: how can it take place?

    English is not my native language. So, I hope to be understood. :-) The Compton effect is the dynamics in which high-energy incident photons (X or gamma) are scattered by electrons of certain materials, like graphite. The electrons are supposed to be free, as they are only weakly bounded by...
  26. GamrCorps

    Conservation of momentum/energy of stacked balls

    Homework Statement A tennis ball and basketball are dropped from a height of 1m (the tennis ball on top of the basketball). The tennis ball has a mass of 75g and the basketball has a mass of 1kg. When dropped separately, the tennis ball bounces to a height of 0.5m and the basketball to a...
  27. E

    Conserved quantities in the Korteweg-de Vries equation

    Homework Statement Consider the Kortweg-de Vires Equation in the form $$\frac{\partial \psi}{\partial t}+\frac{\partial^3 \psi}{\partial x^3}+6\psi\frac{\partial \psi}{\partial x}=0$$ Find the relation between the coefficients ##c## and ##d## , such that the following quantity is conserved...
  28. S

    A Deducing decay processes and Feynman diagrams using Lagrangian and conservation laws

    The decay processes of the ##W## bosons are completely governed by the charged current interaction terms of the Standard model: $$\mathcal{L}_{cc} = ie_{W}\big[W_{\mu}^{+}(\bar{\nu}_{m}\gamma^{\mu}(1-\gamma_{5})e_{m} + V_{mn}\bar{u}_{m}\gamma^{\mu}(1-\gamma_{5})d_{n})\\...
  29. S

    Air-Track Carts & Spring (Energy)

    Homework Statement The air-track carts in the figure(Figure 1) are sliding to the right at 2.0 m/s. The spring between them has a spring constant of 140 N/m and is compressed 4.1 cm. The carts slide past a flame that burns through the string holding them together. What is the final speed of...
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