# What is Second law: Definition and 491 Discussions

The second law of thermodynamics establishes the concept of entropy as a physical property of a thermodynamic system. Entropy predicts the direction of spontaneous processes, and determines whether they are irreversible or impossible, despite obeying the requirement of conservation of energy, which is established in the first law of thermodynamics. The second law may be formulated by the observation that the entropy of isolated systems left to spontaneous evolution cannot decrease, as they always arrive at a state of thermodynamic equilibrium, where the entropy is highest. If all processes in the system are reversible, the entropy is constant. An increase in entropy accounts for the irreversibility of natural processes, often referred to in the concept of the arrow of time.Historically, the second law was an empirical finding that was accepted as an axiom of thermodynamic theory. Statistical mechanics provides a microscopic explanation of the law in terms of probability distributions of the states of large assemblies of atoms or molecules. The second law has been expressed in many ways. Its first formulation, which preceded the proper definition of entropy and was based on caloric theory, is Carnot's theorem, credited to the French scientist Sadi Carnot, who in 1824 showed that the efficiency of conversion of heat to work in a heat engine has an upper limit. The first rigorous definition of the second law based on the concept of entropy came from German scientist Rudolph Clausius in the 1850s including his statement that heat can never pass from a colder to a warmer body without some other change, connected therewith, occurring at the same time.
The second law of thermodynamics can also be used to define the concept of thermodynamic temperature, but this is usually delegated to the zeroth law of thermodynamics.

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1. ### I Kepler´s 2nd law -- Do any two planets sweep out equal area in equal time?

I mean. For example, Earth in one month sweep the same area* than Jupiter in one month? *The line joining the Earth with the Sun than the line joining Jupiter with the Sun. I think yes, but is not what 2nd law says. I think in the fact that the aceleration just depends on the distance to the...
2. ### Circular Motion - Newton's Second Law: Bead on a Rotating Hoop

For whatever reason, I'm having a hard time conceptualizing this problem. I understand that the tangential components of all forces involved need to cancel out in order for the bead to be stationary. I also understand that there is a mgsinθ in the negative θ-hat direction. What I don't...
3. ### B Newton's second law -- rockets

I am having difficulty understanding the information below. In the second line it states that F=0 as there is no external force on the system. However it later calculates acceleration of the rocket. How can Force=0 if there is acceleration? (This is the first time I have encountered the...
4. ### Friction, Mass and Acceleration: Analyzing Block Motion

The block starts to slide if friction can no longer hold the block. F=u*n and F=(m1+m2)a so: (m1+m2)a=uN=>am1+am2=uN=>am2=(uN)/(am1) So:am2=(uN)/(am1) is the force. The answer is F=(u*m1g(m1+m2))/m2 I do not see how the acceleration terms are canceled. Is my answer equivalent to this?
5. ### Newton's 2nd Law: Why is Resistance Considered a Positive Force?

regarding the last question. I know that resistance is a negative force because it goes in the opposite direction to the movement of the boat. So whenever, I want to apply Newton's 2nd law of motion: the sum of forces = m a I should write - F resit = m.a. However, they have considered the...
6. ### I Kepler's second law derivation from angular momentum conservation

Many texts state that in an elliptic orbit you can find angular momentum magnitude as $$L = r m v = m r^2 \frac {d \theta} {dt}$$ I wonder if $$v = r \frac {d \theta} {dt}$$ is valid at every point. I understand this approximation in a circumference or radius r but what about an arc...
7. ### I Newton's second law and pressure wave propagation

Imagine a long deformable rod which has just been hammered on the top end (the bottom end is clamped to Earth). Consider a time interval $dt = t_{2} - t_{1}$ in which the pressure wave is traveling somewhere within the length of the rod (meaning some portion of the object has already "felt" the...
8. ### A Is there a generalized second law of thermodynamics?

Hi Pfs, There are different kinds of entropies. I discoved the free entropy. https://arxiv.org/pdf/math/0304341.pdf the second law says that the total entropy cannot decrease when time goes by. Is it always the same "time" for the different entropies? the author, Voiculescu, wrote articles...
9. ### B Newton's second law about cars

The car that accelerates must enter according to the equation below the external force, while such an external force does not seem to exist. The force of friction between the car and the road is in the opposite direction of motion ؟
10. ### I Magnetic fields are exceptions to the second law of thermodynamics?

Einstein famously said “{Thermodynamics} is the only physical theory of universal content, which I am convinced, that within the framework of applicability of its basic concepts will never be overthrown.” I don't think any of us want to argue with Einstein, but it's worth noting the "within the...
11. ### B Newton’s second law as one complete law

https://www.feynmanlectures.caltech.edu/I_08.html Page 9-3. It says “In these terms, we see that Newton’s second law, in saying that the force is in same direction as the acceleration,is really three laws, in the sense that the components of force in the x, y,z directions is equal to the mass...
12. ### I Elementary Ideas About Entropy -- Is this textbook example correct?

Summary:: An elementary example calculation involving entropy in a textbook seems wrong I was reading an elementary introduction to entropy and the second law of thermodynamics. The book gave the example of a gas in a chamber suddenly allowed to expand into an additional portion of the...

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14. ### B Newton's second law of motion -- Why is it F=m*a and not F=m+a?

Why F=m*a i.e product or multiplication and not F=m+a? i.e addition or summation?
15. ### Newton's second law (My turtle named Newton being accelerated)

https://www.khanacademy.org/science/physics/forces-Newtons-laws/Newtons-laws-of-motion/a/what-is-Newtons-second-law How do I find the horizontal right components force? It states it is 22 N but there is no reason that the left horizontal component is the same as the right. I thought the...
16. ### Newton's second law -- Crate sliding in the back of an accelerating truck

Hi everyone is able to help solve this question for my assignment in university? I've draw a free body diagram for each component of the question but now i am stuck. [Mentor Note -- Poster has been reminded to show their work when starting a new schoolwork thread]
17. ### Confused about which forces are external when Newton's Second Law is used

So I have a trolley of mass m that moves on a straight line. A sphere of mass m, is attached on the trolley with a light string of length a and it is left to oscillate. Just to give some idea of their positions: r_trolley = xi r_sphere = (x-asinθ)i - acosθj (θ is the angle between the string...
18. ### Newton's Second Law of Motion -- Three masses, an inclined plane and a pulley

I did part a and part b but stuck in part c. Could you help me? (Part a and b attached.)
19. ### Calculating Masses from Fnet: What Am I Missing?

The answer is supposed to be 2.32. I've been trying to relate the two masses but am having troubles with it. Fnet for mass one would be m1a. Fnet for mass 2 would be -m2a. The Fnets would be different though. m2=-Fnet/a for mass two and m1=Fnet/a for mass one. I can't just divide m2 by m1...
20. ### Engineering Newton’s Second Law of Motion — Collar sliding on a rotating rod

The soultion used polar corrdinates. Acceleration in polar corrdinates have radial and transeverse components.When calculating the acceleration of collar respect to the rod, the solution only calculates the radial component of acceleration. Is it because the collar is on the rod, so the...
21. E

### "Where’s your second law of thermodynamics now?"

Prof. Tong used the Liouville theorem to prove the Poincaré recurrence theorem in his notes, that given an initial point ##P## in phase space, for any neighbourhood ##D_0## of ##P## there exists a point ##P' \in D_0## that will return to ##D_0## in a finite time. To illustrate the theorem, he...
22. ### Is the Second Law of Thermodynamics Falsifiable?

My question relates to whether the 2nd law of thermodynamics is a an empirical or mathematical law. If we can reason purely from the laws of statistics that entropy can only increase, then the 2nd law of thermodynamics cannot be falsifiable and therefore shouldn't be considered a scientific...
23. ### Is the principal of superposition of forces a part of the second law?

The three laws are basic principles in Newtonian Mechanics. The principle of superposition of forces is part of the second law.But before we make the assumption of superposition of forces,we have to make another assumption which is the principal of superposition of motion.why are...
24. ### Electric circuit -- Ohm's second law

I want to know ohm's second law i search many times about it but every time the result is about ohm's first only, so can anyone help me??
25. ### A Predicting how far an object will fly

Hi, I have an experimental setup where we are taking certain different types of metals of varying shapes and sizes, weighing them, taking approximate measurements, and then blowing it off of a table of a fixed height with an Air Nozzle. The data taken down in experiment is the PSI at which the...
26. ### The Second Law of Thermodynamics and the Stefan-Boltzmann Law

I have a question about the Second Law of Thermodynamics and the Stefan-Boltzmann law. These quotes are from http://www.ces.fau.edu/nasa/module-2/correlation-between-temperature-and-radiation.php “The Stefan-Boltzmann law, a fundamental law of physics, explains the relationship between an...
27. ### Describing the second law of motion using linear momentum

Hi! This is a very simple question regarding terms of expressions. One law of motion is: F=ma Another, using L as the linear momentum, is: F = dL/dt If the first equation can be characterized (ignoring reference frames) as a "coordinate-based equation" (since is concerned with the second...
28. ### I with understanding Kepler's second law

I have a lot of questions as usually, but must begin with the difficult to understand moment which have started my explorations of the Kepler's second law. In the book of Steven Wainberg "To Explain The World", in the technical paragraph twenty one I have found the next formula which represents...
29. ### Engineering Thermodynamics - Second Law: 2 Heat Engines Connected Between 3 Metal Blocks

Hi, I posted a similar question recently and gained some insight on these types of problems. However, I am slightly stumped on how to approach this variation of the problem. So I know that: - there is no net change in enthalpy of the blocks and the engine as the processes are reversible -...
30. ### Deriving Momentum From Newton's Second Law of Motion

Hello everyone I was hoping someone could shed some light on the following:- I am trying to derive the equation of Momentum from Newton's 2nd Law. What I know is the following:- I don't know how to get from Force = Mass * Acceleration TO Momentum = Mass * Velocity. I have attempted to...

32. ### Newton's Second Law for Translation and Rotation

Answer choices: N2L for Translation, N2L for Rotation, Both, Either1. You are asked to find the angular acceleration of a low-friction pulley with a given force exerted on it. My solution = N2L for rotation 2. You are asked to find the angular acceleration of a low-friction pulley due to a...
33. ### Newton's second law along a line

As I mentioned I am self tutoring myself on a subject I studied 20 years ago. I'm sure I can answer all the questions apart from question (iii). I think I'm supposed to use a simultaneous equation. But I need to answer question (iii) to answer the rest. Any help please?
34. ### AIK, this is the best I can do for you.

Carnot's postulate: one cannot build an engine whose sole effect is to transfer heat from a cold body to a hot body. How granted this postulate can I prove that no engine beats Carnot's engine? From this postulate, I can conclude that work must be done from the surroundings and that Carnot's...

36. ### Newton’s Second Law applied: F=ma

My attempt at solving this written out with random inputs Block A- F(100)=M(10)A(10) Block B- F(200)= M(15) A A=200/15 = 13.33 This was not in line with options.. which were 4x block A (1/4) block A Equal to block A (1/2) block A 2x block A Much appreciated some help as I am a beginner
37. ### The second law of the thermodynamics and philosophy

The second law of thermodynamics tells us that the amount of useful energy in an isolated system tends to decrease. Does this imply that mankind will reach a point where it cannot longer use any kind of energy? If so, do we have an estimate of how long it will be necessary for to haven't useful...
38. ### Solving Newton's Second Law: Tension, Work and Kinetic Energy

$$\sum F_x = T - w_x - f_k = ma_x$$ $$T = mg\sin(\theta) + mg\cos(\theta)\mu_k + ma_x$$ $$T = (9.8 \frac{m}{s^2}) \cdot (\sin(41^{\circ}) + \cos(41^{\circ})) + (75kg)\cdot(0.25\frac{m}{s^2})$$ $$T = 672.91 N$$ Having found the tension force, I can find the work done by the person who's...
39. ### Relating the universal law of gravitation and Newton's second law

First, I started with F_a = m_aa_a=G \frac{m_am_b}{r^2} and F_b = m_ba_b=G \frac{m_am_b}{r^2} . Solving for their respective accelerations, I got a_a=G \frac{m_b}{r^2} = 100G and a_b=G \frac{m_a}{r^2} = 100G, meaning that the initial acceleration of the two point particles are each 100G ...
40. ### The second law of thermodynamics -- What does "from cold to hot" mean?

in Clausius formulation, what does the phrase "from cold to hot" means? I can understand it intuitively but in the language of the zero and first laws, we have not defined a temperature scale, only equivalence classes of systems that will be in equilibrium with each other (systems with the same...
41. ### Confusion in explaining Kepler's second law in terms of energy

Hello. As I understand it, Kepler's 2nd law of planetary motion can be explained through conservation of energy or conservation of angular momentum. I am having trouble with the conservation of energy explanation. We know that the sum of potential and kinetic energy of a planet in orbit around...
42. ### Kepler's second law and Wikipedia article

Hi, I'm sorry but I'm not sure if I should post it here or in homework section. It's not homework for sure. This Wikipedia article, https://en.wikipedia.org/wiki/Kepler%27s_laws_of_planetary_motion, on Kepler's laws says the following under History section in the last para: Newton was...
43. ### Is Newton's Second Law Flawed? An Exploration of Zero Force in Space

Newton's law's of motion have been used for hundred's of years, but the second law is wrong and here is why. Newton's second says that net force equals the mass of the object multiplied by the acceleration or F∑=MA. This is fairly simple and straight forward, but is easy to prove wrong. If we...
44. ### Find v(t) from Newton's Second Law and Differential Equation

<Moderator's note: Moved from a technical forum and thus no template.> Is what I have done correct ? I want to find v(t) from Sigma F = m*a. I have gravity force mg pointing downward with positive direction and resistive force R = -b*v^2 pointing upwards with negative direction are acting on a...
45. ### Applying a force in a wheel bicycle

Homework Statement We have a bicycle and lift the front wheel. And with the hand we apply a force of 10 N down and we turn the wheel. The wheel is 26 inches in diameter and weighs 1.5 kg. Find the angular acceleration, the torque of the net force and the angular velocity after a lap. Homework...
46. ### I Reversible compression of a gas - faulty reasoning?

Hello, I am trying to figure out where my reasoning falls apart in this thought experiment: To determine if a process "A" is reversible (or at the very least internally reversible), I try to picture a reversible process "B" that involves only heat transfer and links the same two endpoints that...
47. ### Magnetic fields, Newton's Second Law, and GR

I have a question regarding the interactions of electromagnetic fields. Say you have two superconducting electromagnets A and B. The properties of the magnets are known such that you can tell precisely how quickly it takes each one to produce it's full strength magnetic field and how long it...
48. ### I Alcubierre Warp Drive: 2nd Law of Thermodynamics Impossibility?

It seems to me that if one had a functional Alcubierre drive and used it there would be some subluminal frame of reference in which time was going backwards for the spaceship which probably for it and for macroscopic objects in it which constitute a reasonably closed system would be a violation...
49. ### Kepler's Second Law with Angular Momentum

Homework Statement I am working on the derivation of Kepler's Second Law based on torque and angular momentum. I understand that the vector "L" is equal to the mass (m) times the cross product of the vector "r" and the vector "v." The source I am following then states that L = mrvtheta. I do...
50. ### I Data Model of Kepler's Second Law of Planetary Motion

Hello, I am completing a research project for differential equations class. I am to derive Kepler's three laws and then compare the results of the derivation with real-world data. For Kepler's second law (a planet sweeps out an equal area in an equal time), I was hoping to find orbital data for...