Second law Definition and 491 Threads

Can you provide me any source from where I can learn about the overall second law efficiency calculation of a simple Rankine cycle?

1. Homework Statement i did part a,b,c but i am confused about part d.. i have a test tomorrow and will reattempt this when i wake up but i really need a hint here..so when the worker pulls upward he is decreasing the magnitude of -mg right? so if Fn and mg initially cancel each other out...

Hi, The article below states that they made a material that blocks the motion of molecules in only one way but passing the other way is possible. See the article. https://physicsworld.com/a/blocking-the-symmetry-of-motion/ This seems to me in contrast with the second law. You can for example...

Homework Statement A person whose mass is 20 kg needs to accelerate vertically from the surface of the Earth at 5.0m/s^2 and is trying to pick which rocket he should strap to his back. How much thrust does he need if each rocket has a mass of 30 kg? Homework Equations F=ma The Attempt at a...

I’m a bit stuck with differentiating between the conservation of energy and D’Alambert. For a question I need to find the average resistance of the ground after it has been struck by an object. I chose to look at the equation as Ma + Mgh - Fr = 0. Can you advise if this is correct?

Wiki states: Photon upconversion (UC) is a process in which the sequential absorption of two or more photons leads to the emission of light at shorter wavelength than the excitation wavelength. https://en.wikipedia.org/wiki/Photon_upconversion Is it possible to have the emitted light with the...

Hi, From the movement of air molecules electricity is generated. The kinetic energy of the air molecules is converted into usefull energy and the air is cooled down. Why do windmills not violate the second law of thermodynamics?

At high school, we saw without demonstration the fundamental principle of dynamics (2th Newton's law), i.e : $$\sum \vec{F}=m \vec{a}\,\,\,\,\,\,\,\,\,eq(1)$$ after, at university, we saw another expression of this 2th Newton's law : $$\sum \vec{F}= \dfrac{d\vec{p}}{dt}\,\,\,\,\...

Hi people! A may be philosophical question, in fact my doubt comes from epistemology class: Newton´s second law is a definition of force based on 2 previously defined things: momentum and time or is a relation found by Newton between 3 previously defined things: force; momentum and time? Thanks.

Imagine we have a box suspended to the ceiling by a rope of negligible mass, the net force looks like: ∑F=T-Fg=0 T=Fg where T and Fg are equal in magnitude but opposite in direction. If we cut the rope, the box is in free fall and ∑F=Fg, and this equation is similar to T=Fg but Fg and ∑F have...

Homework Statement How does the gradient of the graph compare to the weight force? The graph is a Mass vs 1/Acceleration graph (y axis = mass, x-axis = Acceleration, It was mentioned to do this.) Homework Equations Explain by referring to the formula for Newton's Second Law. The Attempt at a...

Does a system with zero entropy represent the thermal equilibrium at some temperature = 0K? Does the second law of thermodynamics entail that the system will eventually evolve to higher entropy? e.g. a system of 7 magnetic dipoles of paramagnetic spin-1/2 particles in an external magnetic...

From what I know, the law says that disorder increases over time. But, when a building is constructed the disordered bricks,cement etc. take form of the ordered building. Am I wrong or is this an exception?

I began reading Mehran Kardar's Statistical Physics of Particles and about halfway through the first chapter, there was a discussion on the second law of thermodynamics. He makes no mention of the old tenet that 'the total entropy in the universe must always increase' (I'll refer to this as the...

Homework Statement The two forces F⃗ 1 and F⃗ 2 shown in (Figure 1) act on a 29.0-kg object on a frictionless tabletop. Suppose that F1 = 10.8 N , and F2 = 19 N . A. Determine the magnitude of the net force on the object for the diagram (a) in the figure. B.Determine the angle between the...

Hi Guys, I solved two questions below. Just need some feedback to make sure I solved them correctly. Thanks a bunch. 1. Homework Statement A car of mass 1.4 tonnes (1400 kg) traveling down the highway at 120 km/h has a F air of 1800 N acting against it. What force must be applied by the...

Homework Statement A 20-kg block with a pulley attached slides along a frictionless ledge. It is connected by a massless string to a 5.0-kg block via the arrangement shown in Figure. (a) draw complete free body diagrams for the masses and Find (b) the acceleration of each block and (c) the...

Hey guys is my solution correct for the question below? 1. Homework Statement Consider a 5.0 kg watermelon that is being accelerated at 2.0 m/s2 [up] by a cable. Find the tension in the cable. Homework Equations Fcable = m.a The Attempt at a Solution Fcable = m.a = 5.0 kg(2.0) = 10N

Can someone please help me solve this question below. 1. Homework Statement How much force is needed to push a 20 kg box across a floor at a constant speed of 0.5 m/s [forward] against a frictional resistance of 4 N? Homework Equations Fnet = m.a The Attempt at a Solution Since the velocity...

I wonder why Newton second law, define force as mass x acceleration, acceleration is the second time derivative of displacement, why he didn't define the force as mass x higher order time derivatives of displacement

Dear all, I've never really understood how exactly Liouville's theorem about time-conservation of phase space volume can be reconciled with the second law of thermodynamics. Recently I came across this popular article, http://www.necsi.edu/projects/baranger/cce.pdf "Chaos, Complexity, and...

Hello, I'm going through Kirchoff's second law, and stumbled across this example on this page: In the image above, you can see that the blue arrows (both arrow 1 and arrow 2) indicate the direction that the current goes in. However, I am wondering why the current cannot go as per the red...

Homework Statement Homework Equations F = ma The Attempt at a Solution My value for P does not make any sense. Why is this so?

Hello Pf's members Suppose we isolated a methane's molecule at a room with 213 kelvin. According to second law, methane's molecule tends to break it's bond for reaching whole system (methane's molecule and surroundings) to maximum entropy. On the other hand for breaking these bonds we need...

Homework Statement A human holds a ball of mass 8kg at his hand, so that the forearm is perpendicular to the upperarm. The distance between the elbow and the center of mass of the forearm is 0.15 meters, the distance between the elbow and the muscle is 0.05 meters and the mass of the forearm...

Why if a hot object did not emit radiation then second law of thermodynamics would be violated?

Homework Statement An inventor claims to have invented four engines, each of which operates between constant-temperature reservoirs at 400 and 300 K. Data on each engine, per cycle of operation, can be found in the table below. Of the first and second laws of thermodynamics, which (if either)...

Homework Statement Hi, I am doing this lab this week, but before I do it I have a few questions. ( it has to be completed in class so I want to be prepared). First, these are the three pages of the lab questions...

Homework Statement Initially, the system of objects shown in figure is held motionless, the pulley and all surfaces and wheels are frictionless. Let the force F be zero and assume that m1 can move only vertically. https://drive.google.com/file/d/0B6xlSNQyGbU5M3FTSXpNSVdmT2c/view?usp=sharing...

Homework Statement A 7kg mass is hung from a 1.5 m long massless string. It is released from rest with the string displaced by an angle of 18 degrees from vertical. a) Make a free body diagram of the forces on the mass just after it is released. b) Find the tension in the string. c) Find the...

Hi, I have a thought experiment which seems inconsistent with the 2nd Law of Thermodynamics, so there is probably a flaw in the idea somewhere. Imagine a single molecule of a heavy gas (such as SF6) in a pipe 2.5 metres high and 100mm diametre. The pipe is upright and under the normal...

Hi PF! I was reading my book and I understand the following $$\sum \vec{F} = \frac{\partial}{\partial t} \iiint_{CV} \rho \vec{u} dV +\iint_{CS} \rho \vec{u} ( \vec{u_{rel}} \cdot \hat{n}) dS$$ ##CV## is a control volume, ##CS## is control surface, ##u## is velocity, ##u_{rel}## is velocity...

Homework Statement A chain of mass M and length L is suspended vertically with its lowest end touching a scale. The chain is released and falls onto the scale. What is the reading of the scale when a length of chain, x , has fallen? (Neglect the size of individual links.) Homework Equations...

Homework Statement A block of mass ## m ##, attached to a rope, is dropped at the point A. When the block reaches the point B, the tension is ## T = 2 \cdot m \cdot g ## and the rope broke at that same point. If the length of the rope is ## L = 6cm ##, evaluate the height ## h ## where the...

I was reading an answer on Quora written by Richard Muller ,a retired professor from Berkeley https://www.quora.com/Does-Richard-Mullers-new-time-theory-account-for-the-second-law-of-thermodynamics-or-any-of-times-arrows/answer/Richard-Muller-3?srid=E7fv What are your views ? I am from...

Apologies for what I assume is a rookie question. Given F=ma if you have a 1 kg object traveling in e.g. space say x @ 100m/s then F = 0 on that object. If you have another 1kg object say y sitting at 0 m/s, F=0. If they impact there must be force involved? Does negative acceleration give you a...

GUT's violate baryon number b/c baryon conservation is an emergent phenomenon second law of thermodynamics clearly applies to macroscopic systems, and are the result of the many ways in which atoms of macroscopic systems can be arranged. what if second law of thermodynamics though is just...

Dear all, I'm trying to think about applying the second law of thermodynamics to a system which is not isolated, but has an energy flowing inwards and an equal (!) energy flowing outwards, such that the total energy does not change (total energy flux is zero). Can we still apply the second law...

Homework Statement There are two forces on the 2.08 kg box. F1 is 20.5N. acceleration is happening in the third quadrant with a magnitude of 12.1m/s^2 and with an angle of 25.9 (in the third quadrant). Find the second force in vector notation. Homework Equations Fnet=ma The Attempt at a...

qrev/T = ΔS here what does ΔS signify?does it mean change in entropy of system or surroundings? how is entropy of system,surrounding and universe related to each other and which entropy is used in gibbs free energy equation?

Hello! This is an old problem from the Estonian-Finnish open physics olympiad (2013) and I hope you will be able to lend me some help. 1. Homework Statement Sun-rays are focused with a lens of diameter d = 10 cm and focal length f = 7 cm to a black thin plate. Behind the plate is a mirror...

I read this what-if XKCD article recently: (Fire from Moonlight) http://what-if.xkcd.com/145/ ..so here's a nice little brain-teaser about radiating bodies in thermal equilibrium! Let's say you have a large, perfectly reflecting elliptical cavity (a prolate ellipsoid). At one focus, you...

Homework Statement Using Newtons second law, if an object with mass M is free falling (the only F acting upon it is its own weight), calculate the object's acceleration. What would happen to its acceleration should its mass duplicate? Homework Equations F=ma The Attempt at a Solution So...

Homework Statement An airplane with a mass of 5.32 x 10^4 Kg accelerates down the runway at 1.76 m/s^2 [forward]. Calculate the net force acting on the airplane. Homework Equations Fnet=ma The Attempt at a Solution I did this by multiplying the two given values, but my answer was 93632N...

Homework Statement A student tries to raise a chain consisting of three identical links. Each link has a mass of 200 g . The three-piece chain is connected to a string and then suspended vertically, with the student holding the upper end of the string and pulling upward. Because of the...

Homework Statement An object of mass m is allowed to slide down a frictionless ramp of angle Θ, and its speed at the bottom is recorded as v. If this same process was followed on a planet with twice the gravitational acceleration as Earth, what would be its final speed? Multiple Choice...

I, earlier were studying Newton's second law F=ma, or rather the free fall of objects, F=mg. A thought occurred to me, how do I apply this formula to the moon with the Gravity of 1.622m/^2 and came up with F=m(g/6), however, g/6 = 1.63m/s^2 Normally I'd just use F=ma, but assuming people one...

http://phys.org/news/2015-02-big-quantum-equation-universe.html Some people claim that universe has existed for eternity to get around the Big Bang theory. However wouldn't this violate either the 1st of 2nd laws of Thermodynamics which forbid perpetual motion of the 1st and 2nd kind...

Homework Statement A car travels along the perimeter of a vertical circle (radius = 0.25 km) at a constant speed of 30 m/s. What is the magnitude of the resultant force on the 60-kg driver of the car at the lowest point on this circular path? Question options: 1) 0.37 kN 2) 0.80 kN 3) 0.22 kN...

Homework Statement Given the free-body diagram below, and that the body is in equilibrium, find θ. Homework Equations F = ma The Attempt at a Solution [/B] Basically, I used Newton's second law in the y direction. Here is my work: This does not seem like the correct answer, because the...