What is Conservation laws: Definition and 113 Discussions

In physics, a conservation law states that a particular measurable property of an isolated physical system does not change as the system evolves over time. Exact conservation laws include conservation of energy, conservation of linear momentum, conservation of angular momentum, and conservation of electric charge. There are also many approximate conservation laws, which apply to such quantities as mass, parity, lepton number, baryon number, strangeness, hypercharge, etc. These quantities are conserved in certain classes of physics processes, but not in all.
A local conservation law is usually expressed mathematically as a continuity equation, a partial differential equation which gives a relation between the amount of the quantity and the "transport" of that quantity. It states that the amount of the conserved quantity at a point or within a volume can only change by the amount of the quantity which flows in or out of the volume.
From Noether's theorem, each conservation law is associated with a symmetry in the underlying physics.

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  1. mokeejoe5

    Relativity and conservation laws

    Does the relativity of simultaneity imply the impossibility of non local conservation laws?
  2. Buzz Bloom

    Neutrinos and Conservation Laws

    I am confused about the current physics regarding neutrinos and implications about the conservation laws of mass-energy and linear momentum. I have read the threads listed for similar discussions, including, "How does conservation of energy/mass apply to neutrinos?", and none of them seem to...
  3. S

    Electron-Positron Annihilation Conservation Laws

    What ensures conservation of energy and conservation of momentum in Electron-Positron annihilation? If e=mc2 and momentum = mass*velocity, couldn't the energy equivalence of the electron and the positron be converted to make one photon, or more that 2 photons? I've read that two photons are...
  4. 1

    The Conservation Laws - Particle Physics

    Homework Statement Question: Figure 1: QUESTION ABOVE. Homework Equations The Attempt at a Solution :[/B] P + P → K+ + K- + P + P Firstly, I just wanted to check that this is correct. I found the answer pretty quickly by googling it but have since spent a long while trying to understand...
  5. N

    Electrons and Consequences of Conservation Laws

    Hi Guys, I was wondering; if electronic charge is conserved by Kirchoff's Current Law, then does this mean that the total number of electrons traversing any given circuit at any time is constant?
  6. G

    Two particle system involving conservation laws

    Homework Statement A billiard ball of mass M is initially at rest on a horizontal frictionless table. Another ball of mass m < M and velocity ##\vec{v}## in the positive x-direction hits the first ball in a pefectly elastic collision. After the collision, the balls move with (unknown)...
  7. D

    How does conservation laws work in physics

    Hello, this thought I'm having might be difficult to explain, but i will do my best. Ever since i learned about conservation of energy, conservation of mass (or mass plus energy due to E=mc^2) and conservation of linear/angular momentum, i always found it weird that things in nature can...
  8. D

    Conservation Laws in Rotational Motion

    Homework Statement 15. A solid sphere of mass 6.0 kg is mounted on a vertical axis and can rotate freely without friction. A massless cord is wrapped around the middle of the sphere and passes over a 1.0 kg pulley and is attached to block of mass 4.0 kg, as shown. What is the speed of the...
  9. M

    Conservation laws - Ball hitting a rod about its end

    Homework Statement Homework Equations v = wr K_total = 1/2Mv^2 + 1/2Iw^2 L = Iw p = mv The Attempt at a Solution a. No friction or other outside forces are acting on the system, so linear momentum is conserved. The collision is elastic, so kinetic energy is conserved. There...
  10. G

    Measurements of a superposition: Conservation laws?

    Hi, Assume we have a source that emits several copies of the same quantum state which is a superposition of several eigenstates of the Hamilton operator with different energies. We can calculate the expectation value of the energy of this state and therefore also the energy the source...
  11. Radarithm

    Do black holes violate conservation laws?

    I was arguing today with a friend and the argument seemed pretty pointless because we had nothing to back up our facts with, so I thought about hearing your opinion(s). Do black holes violate laws such as the conservation of matter + conservation of energy? I'm currently leaning towards 'yes'...
  12. H

    Conservation Laws and Symmetries Particle Physics

    2) A π0 of kinetic energy 350 MeV decays in flight into 2 γ rays of equal energies. Determine the angles of the γ rays from the incident π0 direction. Not sure where I am going wrong but my answer is not correct. Energy of π0 meson E = 350 MeV Rest mass Energy of π0 meson E0 = 135...
  13. S

    Time travel + conservation laws

    Doesn't traditional time travel violate conservation laws? If I could in effect materialize in the past, then the universe, being an isolated system would have suddenly gained matter/energy out of nowhere! Also, the atoms that make up my body would then be in two places at once since they...
  14. R

    The Primacy of Conservation Laws: Rethinking the Concept of Force in Mechanics

    Hello, I am hoping for some input concerning shall we say the philosophy of mechanics, particularly with relation to the concept of force. Most textbooks treat force as being something of a primary concept. A fundamental aspect of nature much like energy and mass and therefore something...
  15. M

    Conservation Laws (Linear & Angulat Momentum)

    As we are being introduced to this new lesson, it gets difficult sometimes to indicate which component of either (linear or angular momentum) is conserved. Is there a strict rule to help me indicate which is which? Hmm, if not, can you give me the logical way through it? Thanks in advance...
  16. M

    Combining conservation laws (momentum and energy)

    Homework Statement A 15kg block is attached to a very light horizontal spring of force constant 500 N/m and is resting on a frictionless horizontal table. It is struck by a 3kg stone traveling horizontally at 8m/s to the right, whereupon the stone rebounds at 2 m/s horizontally to the left...
  17. Y

    Combination of conservation laws, and older things. Need Help.

    Homework Statement A small wooden block, of mass M, lies in the middle of a horizontal table of length L and height h above the floor. The coefficient of kinetic friction between this block and the surface of the table is μ. A bullet, of mass m, is shot with a horizontal velocity into...
  18. N

    Four-Momentum Invariant and Conservation Laws Yielding Contradictory Results

    Hello PF community! I'm having trouble with what strikes me as an inconsistency within conservation of energy, conservation of momentum, and the four-momentum invariant equation (E2-p2c2 = m2c4). For the sake of this question, I'll be using non-relativistic mass--i.e. mass is the same in all...
  19. nukeman

    Conservation Laws - Finding angle ?

    Conservation Laws - Finding angle ? Homework Statement Having trouble with question 2, a) - See below Homework Equations The Attempt at a Solution Which conservations laws am I suppoe to use to figure this out? THanks!
  20. naima

    How are intertwinners related to conservation laws?

    In "The Algebra of Grand Unified Theories" http://arxiv.org/abs/0904.1556" john Baez (see page nb 9) shows how the conservation of eigenvalues (T3 isospin) appears if there is an intertwiner between the representation spaces of the symmetry group. I discovered intertwiners for the first time...
  21. Q

    Understanding the Invariance and Limitations of Conservation Laws

    It seems to me that there is a difference between, say, the law of conservation of linear momentum or energy and the law of conservation of angular momentum. The first two are valid in any frame of reference and their invariance is a direct consequence of the relativity principle, whereas...
  22. Q

    Conservation Laws vs a Spinning 'Disk Brake'

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  23. B

    How does the concept of mass-energy equivalence relate to conservation laws?

    Hi How many conservation laws are there ? 1. Conservation of energy 2. " " momentum 3. " mass 4. " charge ? TIA
  24. bcrowell

    Sum over histories, including histories that violate conservation laws?

    It's been 20 years since I took field theory in grad school, and I didn't really understand it all that well even then, so I'm basically looking for a very low-level explanation of the following issue... In the sum-over-histories approach, there is the question of which histories to include...
  25. Z

    Question regarding a contradiction in conservation laws

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  26. P

    Absorption of Photon by a Free Electron - Impossible - Conservation Laws?

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  27. R

    Conservation laws, Noether's theorem and initial conditions

    Hello, everybody! During the whole of my undergraduate study of physics, this one thing always bothered me. It concerns the interplay of conserved quantities, symmetries, Noether's theorem and initial conditions. For a system of N degrees of freedom, governed by the usual Newton's laws...
  28. bcrowell

    Conservation laws in GR: a messy picture?

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  29. B

    What is the correct charge conservation at a vertex in a decay process?

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  30. M

    Particle physics conservation laws

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  31. B

    Two conservation laws but how are they compatible

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  32. C

    Is conservation laws are more fundamental than Newton's second law?

    Is conservation laws are more fundamental than Newton's second law in Newtonian mechanics? I know from the point of view of Noether's theorem conservation laws are more fundamental. But all the conservation laws can be derived from the F= ma equation. And from these conservation laws I can't...
  33. D

    This invention should work ELSE the conservation laws are wrong.

    Last time people concluded that 'I proved the law of conservation of energy and momentum wrong by my invention'...but the reality is strictly opposite. If this invention DOES NOT work THEN then 2 laws are wrong. This invention follows all rules and regulation governed by the physics book...
  34. C

    Random walks and conservation laws in average

    Perpetual motion idea: Sit on a bench with a collimated laser, single slit and detector screen. This prepares particles with identical and symmetric spatial wave-functions, so each measurement transfers an independent, random quantity of transverse momentum (from a symmetric distribution) to...
  35. C

    Conservation laws in mechanics

    Homework Statement A 0.2 kg disc slides down from smooth track of height 1.8m. It arrives at a rough 1 kg block resting on a smooth surface. The friction between them is 1.2N. Find the distance traveled by the disc on the block before it comes to rest relative to the block. Homework...
  36. K

    Finite speed of signal transmission and conservation laws

    Hi all, This is my first post, so please forgive me if this has already been discussed. There is a question that has perplexed me for years, and I am hoping someone at this forum can shed light or point me in the right direction. If two like charges separated by a distance, d, quickly and...
  37. T

    Time Travel and Conservation Laws

    I'm doing some research into time travel for a presentation I have to give in a month or so, and I'm currently looking at the compatibility of Time Travel and the Laws of Conservation. Sending an object back in time would increase the mass - and hence the energy - in the Universe at this...
  38. S

    Do resistors obey conservation laws?

    Do resistors in series and parallel obey conservation laws? why? I think yes, because I add individual resistance for the effective resistance ... but why? :| what's a true and deeper explanation for this??
  39. F

    Conservation laws (tricky, conceptual,not homework)

    Homework Statement Consider a small body of mass m placed over a larger body of mass M whose surface is horizontal near the smaller mass and gradually curves to become vertical at height h. The smaller mass is pushed on the longer one at speed v and the system is left to itself...
  40. C

    Momentum and the Conservation Laws

    A 4.00 kg steel ball strikes a wall with a speed of 9 m/s at an angle of 60.0° with the surface. It bounces off with the same speed and angle (Fig. P8.9) If the ball is in contact with the wall for 0.200 s, what is the average force exerted on the ball by the wall? F\DeltaT = \Deltap p =...
  41. T

    Question about conservation laws & Feynman Lectures

    I just got a copy of Feynman's lectures on Physics the other week. They are very interesting. Near the end of the 4th chapter, he begins discussing conservation laws. One of the laws he gives is the conservation of baryons. I noticed early in the first volume, the lectures were published in...
  42. F

    Exploring Conservation Laws in Closed Systems

    In classical mechanics (also in QM I guess), the fundamental laws of conservation (energy, momentum and angular momentum. tell me if I forget something) are valid only if the system is closed and the net external force is zero. WE define what the system is. We can include or exclude things...
  43. Phrak

    Conservation Laws: Matter and Antimatter Annihilation in a Sealed Box

    Place equal amounts of matter and anitmatter in a box on a scale. It's a very good box; it's very reflective, and light doesn't get in or out. Allow all the stuff to annihilate to photons. The box floats away. Has a conservation law been violated?
  44. L

    Conservation Laws in Rotational Motion Physics

    Homework Statement A uniform solid sphere, with diameter 28 cm and mass 2.5 kg, rolls without slipping on a horizontal surface, at constant speed of 2.0 m/s. 1) What is the rotational kinetic energy? 2) What is its total kinetic energy? 3) What is its angular momentum? Homework...
  45. P

    Conservation Laws and Projectile Motion

    Homework Statement A cannon in a fort overlooking the ocean fires a shell of mass M at an elevation angle, theta and muzzle velocity, v0. At the highest point, the shell explodes into two fragments (masses m1 + m2 = M), with an additional energy E, traveling in the original horizontal...
  46. H

    Weak interaction - conservation laws

    Hi, i just need to get together all the conservations/violations that the weak interaction conserves/violates. I know about parity, charge conjugation and basic properties such as charge and lepton number but i was wandering if anybody could think of any more. No mathematics please! Thanks.
  47. J

    Collapses against conservation laws

    In the time evolution defined by the Shrodinger's equation, the expectation value of the energy is conserved. However, in a collapse of the state, where state vector gets projected onto some subspace of physical states, the expectation value is not conserved. Does this mean that through...
  48. M

    Reynolds Transport Theorem and Conservation Laws.

    The Reynolds Transport theorem (RTT) is usually applied to derive the conservation of mass, momentum and energy in a fluid. But when I try to apply the RTT to other physical quantities, I get weird results. Can anyone see where I'm going wrong? As a simple example, take the physical...
  49. P

    Energy and momentum conservation laws problem

    Homework Statement Two balls go vis-a-vis (each of them speed is v) and strike. Hit is absolutely elastic. After the hit, one of the balls changes his motion direction 30 degrees. I need to find the direction of the other ball and both ball speeds after the hit. The Attempt at a Solution...
  50. M

    Conservation Laws for Particles in Circular Cylinder

    Homework Statement A particle moves in a conservative field of force produced by a mass distribution. In each instance the force generated by a volume element of the distribution is derived from a potential that is proportional to the mass of the volume element and is a function only of the...
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