What is Third law: Definition and 377 Discussions

The third law of thermodynamics states as follows, regarding the properties of closed systems in thermodynamic equilibrium: The entropy of a system approaches a constant value as its temperature approaches absolute zero. This constant value cannot depend on any other parameters characterizing the closed system, such as pressure or applied magnetic field. At absolute zero (zero kelvins) the system must be in a state with the minimum possible energy. Entropy is related to the number of accessible microstates, and there is typically one unique state (called the ground state) with minimum energy. In such a case, the entropy at absolute zero will be exactly zero. If the system does not have a well-defined order (if its order is glassy, for example), then there may remain some finite entropy as the system is brought to very low temperatures, either because the system becomes locked into a configuration with non-minimal energy or because the minimum energy state is non-unique. The constant value is called the residual entropy of the system. The entropy is essentially a state-function meaning the inherent value of different atoms, molecules, and other configurations of particles including subatomic or atomic material is defined by entropy, which can be discovered near 0 K.
The Nernst–Simon statement of the third law of thermodynamics concerns thermodynamic processes at a fixed, low temperature: The entropy change associated with any condensed system undergoing a reversible isothermal process approaches zero as the temperature at which it is performed approaches 0 K. Here a condensed system refers to liquids and solids.
A classical formulation by Nernst (actually a consequence of the Third Law) is: It is impossible for any process, no matter how idealized, to reduce the entropy of a system to its absolute-zero value in a finite number of operations.
There also exists a formulation of the third law which approaches the subject by postulating a specific energy behavior: If the composite of two thermodynamic systems constitutes an isolated system, then any energy exchange in any form between those two systems is bounded.

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  1. String theory guy

    Newton's Third Law for book on a table

    This problem is from MIT's module again. I have a doubt about Newton's Third Law. How is the normal and weight of the book not Newton's 3rd Law pairs? When the book is placed on the table, I imagine that the book's weight exerts a force on the table and the table exerts and equal and...
  2. M

    I Why does Kepler's Third Law exist?

    Hi; I understand Kepler 3 in terms of the relationship that it demonstrates. I have researched the internet but can't find a reason why this relationship exists. Is it somehow a consequence of some type of gravitational balance, if not is there some other mechanical reason? Thanks Martyn
  3. C

    B Newton's third law (equal and opposite force)

    Something about Newton's third law confuses me, when I hold my phone and I move it around it's velocity is changing, therefore because Newton's first law it's acted on by a force, and because Newton's second law, the force is directly proportional to the mass and acceleration of that object...
  4. P

    I Exploring Newton's Third Law in an Imaginary Magnetic Field

    Hi, here's a theoretical problem that I am trying to find a satisfactory answer for. Imagine a coil that is temporarily switched on an off and generates a magnetic field that permeates through space. Now imagine a charged particle passing through this field, at time that the coil is already...
  5. J

    Newton's third law -- I have trouble with some real-life examples

    I have gathered that forces always occur in pairs and are equal in magnitude but opposite in direction and that according to the second law a force is the product of the mas and acceleration of the object exerting the force. My problem is with getting this make sense with real life examples. If...
  6. rudransh verma

    B Same case of Newton’s third law

    Previously I have posted two threads on Thread 'Monkey climbing up the rope' https://www.physicsforums.com/threads/monkey-climbing-up-the-rope.1012065/ and Thread 'Car's maximum acceleration on a road is proportional to what?'...
  7. S

    Kepler's Third Law vs Conservation of angular momentum

    The classic way to go about this problem would be to use Kepler's laws and thus find the new time period of earth. However I encountered this question in a test on rotational motion which deals with conservation of angular momentum. The equation used here would be I1ω1= I2ω2 Replacing I with MR2...
  8. P

    How do you identify the systems in which Newton's Third Law is obeyed?

    eg. For a ball in free fall (ignoring air resistance) there is only one force acting if the system is just considered to be the ball. It seems necessary to go to the system which includes the Earth for identification of the reaction force to be made. Is there any rule for how big a system must...
  9. I

    Grade 11 Question: Newton's Third Law -- Calculating acceleration of an airboat

    for c) i need help d) Given: Let [forward] be positive. F 1 = — 250 N F 2 = 360 N Unknown: a = ? Equation: F net= F1+ F2 F net= m a Solve: F net= (— 250 N) + 360 N = 110 N F net= m a a = F net/ m = 110 N / 220 kg= 0.5 m/s [forward] Statement: The acceleration of the boat is 0.5 m/s [forward].
  10. A

    Does Newton's Third law apply to torque/rotation?

    For example, let's say i have force acting on an object about some pivot. That object will apply an equal force about the same pivot, at equal lever arm, back. So there it seems like if the forces are in line, acting on the same pivot point, there is an equal and opposite torque. However, if i...
  11. jaumzaum

    Is Newton's third law valid for rotation / Torque?

    Is third Newton law valid for rotation / Torque? I mean, can we say that for every torque there must be another torque with equal magnitude and opposite direction? This can only be true for contact forces or radial forces, as these forces will create a reaction that will cancel the torque...
  12. R

    The violation of Newton's third Law

    Let us consider two point charges: one of which moves (in the simplest case) rectilinearly with a constant speed and the other is at rest. The electric field of the first charge is renormalized due to the effects of retardation and generation of a solenoidal electric field (see Am. J. Phys...
  13. chucho11028

    Understanding the Third Law of Newton — How can the forces be equal?

    Hello guys, The third law says: "For every action, there is an equal and opposite reaction. The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second...
  14. LCSphysicist

    Where does Newton's third law fail?

    I have read in some books and articles of classic mechanics that the Newton's third law does not always hold, could you say an example to me?
  15. Baran

    Newton's third law -- Calculate the recoil of a paintball gun

    I've been stuck on this for sometime now, and am unsure how to approach this question.
  16. R

    Working with Newton's Third Law and friction

    I know that the normal force and Fg are unbalanced in this case, but I don't get how to set up the equilibrium equations with that. I struggle with determining what you set the equations equal to when its either balanced or unbalanced forces you're dealing with.
  17. Petronius

    Newton's third law problem/kinematics (Airboat problem/no friction)

    Thank you very much your time! I first found the force of the air on the boat using the principle of Newton's third law and the fact that no friction is involved. Faction= -Freaction 150 N backwards = -150 N backwards 150 N backwards = 150 N forward I then sought to determine the...
  18. K

    I Separating Masses in Kepler's Third Law

    Hey everyone! I have been looking everywhere to try to find the answer to this question so I thought I'd pose it here. When we discuss finding the mass of orbiting bodies, it's easy to find the combined mass of the system using Kepler's Third Law in the form M1+M2=(4pi^2)(a^3)/((G)(T^2). My...
  19. MrDickinson

    Question about Newton's Third Law and and the Force between 2 objects

    m1 is the large cruise ship m2 is the tug boat The question doesn't state it explicitly, but I assume that both masses are undergoing acceleration because the tug boat is experiencing a change in velocity. I assume the system accelerates uniformly, and I assume that m1 and m2 accelerate...
  20. J

    Trivial question regarding Newton's Third Law and Elasticity

    1. Drawing Free Body Diagrams for all components we get : 2. Following this we can find total elongation using ##\Delta L = \frac {1}{AY}(F_1*L_1 + F_2 *L_2+ F_3 *L_3) ## My questions : a) I am assuming that the internal forces (3t) are neglected in the FBD because of Newton's third law whereby...
  21. Callista

    Conceptual check: Newton's third law

    Homework Statement A spherical rubber balloon inflated with air is held stationary, with its opening, on the west side, pinched shut. (a) Describe the forces exerted by the air inside and outside the balloon on sections of the rubber. Homework Equations ? The Attempt at a Solution The air...
  22. Callista

    Why doesn't Newtons third law also apply to the frame?

    Homework Statement A locomotive has broken through the wall of a train station. During the collision, what can be said about the force exerted by the locomotive on the wall? a)The force exerted by the locomotive on the wall was less than the force exerted by the wall on the locomotive. b) The...
  23. sams

    A How to calculate Saturn's mass from Kepler's third law?

    This is not a homework. In Chapter 8: Central-Force Motion, in the Classical Dynamics of Particles and Systems book by Thornton and Marion, Fifth Edition, page 325, Problem 8-19, we are asked to calculate the mass of the planet Saturn. In the instructor's solution manual, the solution for this...
  24. Zeke Bevan

    Newton's Third Law Problem: Two masses, a rope and a pulley

    Homework Statement Two blocks are attached to opposite ends of a massless rope that goes over a massless, frictionless, stationary pulley. One of the blocks w/ a mass of 1.5 kg accelerates downward at (3/4)g. A. What is the mass of the other block? Homework Equations There were no given...
  25. B

    Apparent (clearly false) contradiction - Kepler's Third Law

    Homework Statement When considering a satellite in geosynchronous orbit, its speed is zero across (relative to) Earth's surface. From Kepler's third Law: T2=(4π2r3)/(GM), we can derive that v2=GM/r This would tell us that as the radius of a satellite to Earth's centre increases, its velocity...
  26. A

    Trouble understanding Newton's Third Law in Pulleys

    Hi everyone. I found this image online that made me question a couple of things that might be a bit humiliating to ask: I always thought that tension is simply a pulling force, meaning that the mass is suspended because the tension force pulling the mass is equal to the weight force of the...
  27. M

    Newton's Third Law: Acceleration of box and worker

    Homework Statement On a horizontal frictionless floor, a worker of weight 0.900 kN pushes horizontally with a force of 0.200 kN on a box weighing 1.80 kN. As a result of this push, which statement could be true? A ) The box will not move because the push is less than its weight. B ) The worker...
  28. R

    Elastic collisions and Newton's third law

    Hi everyone, I am new to this forum, and I'm having a hard time understanding Newton's third law and collisions, first of all I want to say that it is not homework and that I do know the basics of physics, vectors, energy, work, and momentum I also know and understand Newton's first and second...
  29. S

    How Does Newton's Third Law Apply When a Bat Hits a Ball?

    The following reasoning leads to an apparent paradox; explain what’s wrong with the logic. A baseball player hits a ball. The ball and the bat spend a fraction of a second in contact. During that time they’re moving together, so their accelerations must be equal. Newton’s third law says that...
  30. F

    Question about the Hamiltonian and the third law of thermodynamics

    The third law of quantum mechanics states that a system at absolute zero temperature has zero entropy. Entropy can be conceived as an expression of the number of possible microstates that can produce an identical macrostate. At zero entropy, there should be exactly *one* microstate configuration...
  31. A

    What Are the Correct Units for Kepler's Third Law?

    I am confused about the units used in Kepler's 3rd law. Is T supposed to be in years or days? Is R supposed to be in kilometers or meters? Is there ever an instance where one combination of units is preferable over another (for example, if you want to use the answers from Kepler's third law to...
  32. L

    Newton's third law - normal force

    Homework Statement A book is placed on a chair which is standing on the floor. An iPad is placed on the book. The floor exerts a significant force: A) only on the book B) Upwards on chair and downwards on book C) ONLY upwards on chair D) upwards on chair book and iPad E) downwards on...
  33. H

    Tension in a String: Why is the Scale Reading 10N?

    [Source] Both these masses are of 1kg each and the scale reads in Newtons. The intuitive answer for the reading of the scale is about 20N because it is being pulled by 10N from each side. However, this is clearly not true as the scale reads about 10N. Why is it so? I believe that I am...
  34. H

    How does the reaction force of friction affect the ground?

    Suppose I am trying to push a heavy block on ground and I haven't applied a force stronger than the maximum static friction. The force of friction acts on the block and its equal and opposite force acts on the ground. Does the reaction force of friction(acting horizontally on the ground)...
  35. E

    Understanding Newton's Third Law in Action: A Truck Pushing an SUV

    Homework Statement A 2,300 kg truck pushes a 2,500 kg SUV, and they both move at 3.75 m/s2. I am to find the force that the SUV exerts on the truck. Homework Equations F=ma The Attempt at a Solution I solved this problem already, it was just taking the acceleration I had found (3.75) and...
  36. PhysicsKid0123

    Center of mass, Newton's third law, and rocket thrust

    Can someone clarify this for me-- how is the motion of the center of mass unaffected by internal forces, as is stated towards the end of the paragraph in the picture I've provided. It seems to me that motion of the center of mass directly affected by these internal forces. Specifically, the...
  37. A

    Some calculations in Newton's Third Law

    Hello, 1- IF we have an object has 200 kg and we also have a machine has 5 kg and pushes that object with force of 600 n , in Newton third law it must be another force =-600n then : the acceleration of the object must be 3 m\s^2 and the acceleration of the machine is 120 m\s^2 , Am I wrong...
  38. A

    A Re-interpretation of the third law of thermodynamics

    Consider the first paragraph of this paper - https://arxiv.org/abs/gr-qc/0611004: A fundamental problem in thermodynamic and statistical physics is to study the response of a system in thermal equilibrium to an outside perturbation. In particular, one is typically interested in calculating the...
  39. N

    Problem applying Newtons third law

    Homework Statement Two people each with a mass of 70kg are wearing in line skates and holding opposite ends of a 15m rope. One person pulls forward on the rope by moving hand over hand and gradually reeling in more of the rope. In doing so, he exerts a force of 35N (backwards) on the rope. This...
  40. M

    New Explanation to How Newton's Third Law is Satisfied in Magnetism

    <Moderators note. Moved from the quoted thread to avoid detracting> Newton third law does apply in magnetism, and this is proved by applying the conservation of momentum principle. This will be clearer when the causes/origin of the magnetic force is understood as described in the work "Two new...
  41. parshyaa

    Newton's third law is frame independent?

    how can you say(prove) that Newton's third law is frame independent. you will say that as real forces are frame independent , therefore Newton's third law is also frame independent, so tell me how real forces are frame independent?
  42. Z

    Kepler's Third Law and Motion of Two Point Masses

    I posted this before but I think it was in the wrong, place, so sorry for the duplicate :O I'm trying to work through some equations in the paper 'Gravitational Radiation and the Motion of Two Point Masses' (Peters, 1964) but I can't get out the right values 1. Homework Statement For a binary...
  43. Z

    Kepler's Third Law and Motion of Two Point Masses

    I'm trying to work through some equations in the paper 'Gravitational Radiation and the Motion of Two Point Masses' (Peters, 1964) but I can't get out the right values 1. Homework Statement For a binary star system with each mass = 1 solar mass, the equations give the results: Separation ~ 10...
  44. Awais gul

    My Question is about Newton's third law of motion

    Hello, I am Awais gul, a student of science subjects and learning the basic of Physics , chemistry and computer science.I have a Question about Newton's third law of Motion. Question : If we Throw a ball on a wall it goes back but if we throw a ball on sand why it does not go back? Newton's...
  45. Mr Davis 97

    Newton's Third Law textbook -- question about the sign of a force

    Imagine that I am pushing on a wall. Then my textbook says that by Newton's third law, ##F_{AB} = F_{BA}##, where I am B and the wall is A. Isn't this wrong? Shouldn't it be that ##F_{AB} = -F_{BA}##?
  46. Akshat

    Exceptions to the Laws of Physics: Newton's Third Law

    I've been noticing that there are exceptions to every scientific law. For Example, with Newton's First and Second Laws are totally invalid at the quantum level, but I'm failing to find such an example for Newton's Third Law. Is anyone able to help me?
  47. Lothlorien

    How far can we go with space drives and the third law?

    Hi all. I'm new here and I appreciate any help on this question: Under the premise of E=mc^2, m=E/c^2 and the more energy you gift your propulsion device and the faster the exit speed I'm trying to figure out exactly why making some sort of high speed mass driver system as a propulsion drive...
  48. D

    Kepler's third law implies force proportional to mass

    Homework Statement Show that Kepler's third law, \tau = a^{3/2}, implies that the force on a planet is proportional to its mass. Homework Equations 3. The Attempt at a Solution [/B] I haven't really attempted anything. I'm not sure what the question is going for. What can we assume and use?