Second law Definition and 479 Threads

Can someone help me on this: Hawking Bernstein radiation can cause a BH to evaporate. The second law of BH dynamics says that the area of the event horizon cannot decrease. Isn't this a contradiction? Also when considering the size of the event horizon, is it different depending on wheather...

http://postimg.org/image/ubkhk78fn/ Can someone explain why "the sum of the voltages equals zero for any closed path in an electrical circuit"? (an explanation in terms of electric potential, polarity, resistors etc. and not a water analogy or anything like that)

This is not a homework problem - I am trying to understand the physics of something that happened to me. Scenario: two cars are stopped at a red light. Car #1 in front, and behind it is car #2. Car #3 comes up from behind, does not stop, and hits car 2, and car 2 in turn hits car 1. I am...

Homework Statement A 0.70-kg disk with a rotational inertia given by MR^2/2 is free to rotate on a fixed horizontal axis suspended from the ceiling. A string is wrapped around the disk and a 2.0-kg mass hangs from the free end. If the string does not slip, then as the mass falls and the...

Homework Statement A block of wood floats in fresh water with 0.722 of its volume V submerged and in oil with 0.895 V submerged. Find the density of (a) the wood and (b) the oil. Homework Equations mass = density x volume f = ma conservation of mass F_{weight} - F_{buoyancy} =...

Homework Statement "The constant force resisting the motion of a car of mass 1500kg is 980N. If, when traveling at 48kmhr-1, the car is brought to rest in a distance 50m by applying the brakes, find the additional retarding force due to the brakes (assumed constant) Homework Equations...

Homework Statement "A block of mass 10kg is pulled 20m up a smooth plane inclined at 45 degrees to the horizontal. The block is initially at rest and reaches a velocity of 2.0m/s at the tope of the plane. Calculate the magnitude of the force required, assuming it acts parallel to the plane."...

Homework Statement If m1=m2=1.0 kg and θ=30°, what will be the acceleration of the system? Calculate the tension in the cord, if a block m1 lying on a frictionless inclined plane and is connected to a mass less cord. Assume m1 is moving down the plane for A), and for B) m1 is in the opposite...

Hello, I read on wikipedia that the second law represents how concentration changes with time, but I was wondering how taking the partial derivative with respect to x of the first law will give change in concentration with time?

Homework Statement There is an attachment of a FBD, and you have to solve for the following: Fg=? Fnet? Mass=? The Attempt at a Solution I know you will have to equate equations, but I don't know how to go about doing this. I do know that Fg= m x 9.8, I isolated various equations and...

Homework Statement The minimum safe distance between vehicles on a highway is the distance a vehicle can travel in 2.0s at a constant speed. assume that a 1.2x10^3 kg car is traveling 72km/h [S] when the truck ahead crashes into a northbound truck and stops suddenly. a) if the car is at the...

A clarinetist, setting out for a performance, grabs his 3.41-kg clarinet case (including the clarinet) from the top of the piano and holds it in the air with an upward force of 31.8 N. Find the case\'s vertical acceleration. Indicate an upward acceleration as positive and a downward one as...

Homework Statement As part of a safety test, an eighteen wheeler is being shot down a track by a rocket into a solid concrete wall. The truck is fully loaded, for a total weight of 95,000lbs. If the truck hits the wall at 80mph and causes a deformation of .1m in the wall before coming to a...

Hi, question here. Newton's second law is so intuitively obvious to anyone not even hearing or reading his law in words that it makes me wonder: how was it such a great revelation that many years ago? Pardon my ignorance, but I'm very curious.

I want to preface this question with the fact that I am a scum noob. How did Newton come up with the second law of motion. As I understand it, it is only derived through experiential observations. I have tried many different Google searches but I can't find anything relevant.

Hi everyone! Recently, I've been trying to understand how the error function pertains to solving for concentration in a non-steady state case (with a constant diffusivity D), but I've been having some trouble with the initial assumptions. The source I am currently using (Crank's The...

Hi people, I'm currently working on a project regarding entropy generation by (emissive only) radiative heat transfer from heat sinks. But I'm unable to find a proper treatment regarding Second Law analysis for Radiative Heat Transfer. Can someone give me the link to any book or any article...

Hello! I have another idea for how to break the second law of thermodynamics, and I don't see where this one cracks either. I would be glad if someone could explain this to me. Kelvin-Planck statement of the second law: "No process is possible whose sole result is the absorption of heat...

"The rate of change of linear momentum of a body with time is directly proportional to the net force acting on it." =>F\proptodp/dt Then how do we suddenly come to: F=dp/dt? We took the proportionality constant as 1 but why? How to determine that the constant of proportionality is 1?

Hi! I have an idea for how to break the second law of thermodynamics, and I don't see where it cracks. I would really appreciate if someone could explain this to me. Kelvin-Planck statement of the second law: "No process is possible whose sole result is the absorption of heat from a reservoir...

Homework Statement A particle of mass m moves along a straight line with constant velocity v in the x direction, a distance b from the x axis. (a) Does the particle possesses any angular momentum about the origin? (b) Explain why the amount of its angular momentum should change or should stay...

The second law of thermodynamics essentially states (paraphrased) that the universe always moves from order to disorder (increase in entropy). The problem I have with this, though, is that there is no thermodynamic definition to disorder that I am aware of. Is there such a thermodynamic...

Second law of thermodynamics states that heat does not flow spontaneously from cold to hot bodies. But a cool fluorescent bulb is perfectly capable of heating something that had already started out being warmer than the bulb itself. Is this not a contradiction to the law ? :confused:

Homework Statement see here for a situation of the problem http://i40.tinypic.com/6fbblk.jpg Homework Equations F=ma The Attempt at a Solution Ok well I've solved this problem but one thing about the solution process troubles me. Namely when trying to find the force in the bar...

Okay, we have one-thousand pounds; at sea level, and a feather. We elevate them to a thousand meters and support them on a platform. All of which is in a vacuum. We remove the platform, they both hit the earth; the limiting surface; at the same time. Newton's second law suggests that this is due...

Homework Statement see this link http://i45.tinypic.com/10gwhw2.jpgHomework Equations F=kx, F=ma The Attempt at a Solution ok first I find the total length of the pulley system, 2Sa+Sb=L, differentiating with respect to time i get, 2Va + VB=0 and thus 2Aa+Ab =0 So Aa =-Ab/2 next I first...

Homework Statement See the this link for problem statement, http://i48.tinypic.com/vphsm1.jpg Case 1 I've solved, however I'm unsure about how good my solution is for case 2. Homework Equations F=ma Fu = N * ukThe Attempt at a Solution Ok first I sum the forces in the y direction...

Homework Statement The Attempt at a Solution So pretty much for this question you want to find the actual work output of the turbine and the reversible work output. To find the reversible (theoretical) work output, you know that s2 = s1, so based on that information you can find what...

Homework Statement http://postimage.org/image/6exf0w475/ B) I have a question on this part. Would I just use the "v = v0 at" equation here because the question states "after it starts moving?" Do i need to include static/kinetic friction into this equation somehow? Homework Equations part...

To the layman 2nd law of thermodynamics states that over time systems become more "disordered." You hear a lot of people trying to argue that Evolution contradicts the second law of thermodynamics, which we all know isn't the case. But, I find it interesting that while a species has managed to...

The second law of thermodynamics states, (in simple terms) that heat always dissipates towards cold (correct me if I am wrong). Thus, the heat in an object should radiate or convect or conduct towards the cold. So if this is true, can someone explain how icicles have a tendency to move...

On an object with a mass of 3 kg a single force F acts perpendicular to the speed of the object.The object moves in a circle with a radius of 2 meters and the period is 3 seconds.a) Find the acceleration. I have the formula T=2*pi*r/v ,I find v here and then in the formula a=v^2/2 I replace v...

Hi, I noticed that universal electric motors have peculiar electrical <=> mechanical transfer function, which seems to break the second law of thermodynamics. Can anyone show where did I do the mistake ? ** Universal motor (series wounded) is a motor where rotor and stator...

Hi, I am trying to prove the second law of reflection using fermat's principle and I am not entirely sure how to start it. By the way the second law of reflection is: The incident ray, reflect ray and normal ray all lie in a single plane.

I'm just uncertain (haha), what IS Newton's Second law exactly? F=ma, or F=dp/dt, aren't these defining equations for F itself? what is a force? what would you mean a "force" acts on a object, would it be equally probable to define say, d^3Q/dt^3 = m dx^3/dt^3? does Newton's second law have...

Homework Statement In rotational dynamics, a typical problem would be along the lines of a ring with mass m and radius r rolling down a hill with angle θ to the horizontal. Find the acceleration of the ring. Homework Equations Ʃτ=I\alpha = r x (friction) ƩF = ma = mgsinθ - (friction)...

Homework Statement When jumping from a height on to a hard surface, it is advisable to bend one's knees on landing. How does bending the knees affect the time one takes to come to rest? (1 mark) With reference to Newton's second law, explain why it is a good idea to bend one's knees. (2...

when occurs an impulsive in the Newton's second law?

Homework Statement Suppose 10000 J is transferred between a large thermal reservoir at a temperature of 300 K to another large thermal reservoir at 500 K. What is the net change in entropy of the system? Write an expression for the factor by which the multiplicity of the system changes...

Homework Statement In the following figure, the boy has a mass of 40kg, and the platform he is sitting on has a mass of 15kg. If the force of the board on the boy is 180N; a) Find the acceleration of the boy b) Find the reading of the scale...

Homework Statement A heavy metal chain of mass 0.6 kg hangs over the edge of a table. When half the chain is hanging over the edge, what is the magnitude of the external net force that causes the chain to accelerate? What's the acceleration of the chain when half of it is over the edge?Homework...

There is no friction and the strings are massless. Find Tension and acceleration. 3. The Attempt at a Solution 1. +T-m1g = m1a 2. m2g-2t= m2a 3. +T - m3g =m3a 4. 2a2 = a1+a3 g(-m1+m2-m3) + 2a2 = (m1 + m2 + m3)a + a1 + a2 -29.4 + 2a2 =11a +a1+ a2

This was a return of my test and I have filled in the forces already. I got a few problems wrong and I would like to see what the correct answers are. Homework Statement 1. Write an expression and determine the value of N1, the normal force acting on M2 2. Write an expression for and...

Homework Statement So the question revolves around two blocks, one of mass A and one of mass B. The Mass A block is on a smooth horizontal surface, connected by a thin chord that passes over a pulley to the second block, of Mass B. The question asks to draw a free body diagram of both objects...

[b]1. A loaded penguin sled weighing 70 N rests on a plane inclined at 20° to the horizontal. Between the sled and the plane the coefficient of static friction is 0.25, and the coefficient of kinetic friction is 0.16. (a) What is the minimum magnitude of the force F, parallel to the plane, that...

Hi everyone! I saw in an old exam of physics : "Write and comment the second law of dynamics." And i was thinking, I know a lot about it and if i had to answer it i would have made some examples, but when he asks me to comment it ? What should i have to say ? In sense, what do you think are...

Say an object is dropped out of a plane at height h above the ground and flying at a constant speed v0... Ignoring air resistance, the only force on the object is the gravitational force, but it will still move in the direction that the plane was moving, right? So if Fg is the only force...

Homework Statement A plane, which is flying horizontally at a constant speed v0 and at a height h above the sea, must drop a bundle of supplies to a castaway on a small raft a) Write down Newton's second law for the bundle as it falls from the plane, assuming you can neglect air resistance...

Hello everyone, These days i am studying thermal physics in my classes so had a few queries to get clear about. The Kelvin statement says: “ No process is possible in which the sole result is the absorption of heat from a reservoir and its complete conversion into work. ” Yet the...

I'm curious how to solve Fick's second law of diffusion \frac{∂c}{∂t}=D \frac{∂^2c}{∂x^2}For conditions:c(x,0)=0c(0,t)=Ac(\infty,t)=0Physically this means: -c(x,t) is the concentration at point x at time t. -Initially there is no concentration of diffusing species. -At x=0 for all t the is a...