What is First law: Definition and 229 Discussions

"First Law" is a science fiction short story by American writer Isaac Asimov, first published in the October 1956 issue of Fantastic Universe magazine and later collected in The Rest of the Robots (1964) and The Complete Robot (1982). The title of the story is a reference to the first of the Three Laws of Robotics.

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  1. S

    First Law of Thermodynamics and a bullet

    1. A 3.0-g bullet traveling at a speed of 400 m/s enters a tree and exits the other side with a speed of 200 m/s. Where did the bullet's lost KE go, and what was the energy transferred? 2. ΔKE+ΔPE+ΔU = Q-W 3. The only questions I have about this problem are why does the Work=0, Heat=0, and why...
  2. G

    First Law of Thermodynamics and temperature

    Homework Statement A toy balloon is initially at a temperature of T = 20°C. Its initial volume is 0.0042 m3 (It is a 10 cm radius sphere). Assume that the pressure in the balloon always equals atmospheric pressure, 101.3 kPa. The new temp was found to be 52.23°C By how many Joules did the...
  3. C

    Question about Newton's first law

    Homework Statement Q13 All external forces on a body cancel out. Which statement must be correct? A The body does not move. B The momentum of the body remains unchanged. C The speed of the body remains unchanged. D The total energy of the body remains unchanged. Homework EquationsThe Attempt...
  4. F

    Why Does Susan Fall Forward When the Metro Brakes?

    Homework Statement Susan is standing in an empty Metro (subway) car which is traveling at constant velocity. Suddenly, because of an emergency, the driver applies the brakes. Select True or False for each of the following statements. a) If she immediately grabs onto the seat behind her (she...
  5. C

    Thermodynamics First Law Problem

    Homework Statement Consider a rock having a mass of 5 kg and a bucket containing 50 kg of liquid water. Initially, the stone is 20 m above the water, and the stone and the water are at the same temperature, T1 (state 1). The stone then falls into the water. For the system stone + water...
  6. C

    First Law Thermodynamics Problem

    When asked to calculate the change in kinetic energy of a system, do you calculate the kinetic of the entire system as if it was moving? Or do you calculate the kinetic energies of the various subsystems within the systems and then add it up so that you have the total kinetic energy?
  7. B

    What is the mathematical proof for Kepler's first law?

    I wanted to ask how come kepler say that the orbits on which planet rotates is elliptical ... just by obvious saying, what we have learned is that when the velocity is Perpendicular to force the path traced is circular...even if we say in reality the actual angle B/W velocity and force is not 90...
  8. S

    Gibbs free energy and the first law of thermodynamics

    Hi thanks for reading. I have 2 questions regarding the topics mentioned that I'm having trouble with. Hope you guys can help me out here :) 1) The first law is ΔU=Q+W so we can rearrange this to get ΔH=ΔU+PΔV so would this indicate that ΔH only measures the heat given out at constant...
  9. F

    Thermodynamics: First law and work definition

    I've been having some doubts regarding the definition of work in thermodynamics and how it is related to the change of energy in the system. I have tried asking this in physics stack exchange but I was unable to obtain a satisfactory answer; now that I am trying again in this forum I will aim to...
  10. adjacent

    Newton's 1st Law: Object at Constant Speed, No Reaction Force

    An object is traveling at constant speed.There is no force on it(It means it won't also exert any force on any object)What will happen if I put something in its path?If it does not exert a force there will be no reaction force(Newtons 3rd law).Then what will happen to it?
  11. K

    Where can we apply the first law of thermodynamics?

    Hi people, I have a question. Where can we apply the first law of thermodynamics? I mean is it applicable between 2 equilibrium points? Or between a reversible process? When we consider free expansion of a gas into vacuum, we apply the first law. But since the process isn't reversible (or even...
  12. M

    Definition of first law of thermodynamic?

    Homework Statement What is the definition for first law of thermodynamic? I was confused and which one is correct? (a) The CHANGE in internal energy of a closed system is equal to the heat that enters a system and the workdone on the system OR (b)...
  13. V

    What Happens to Work Done by Gravity in First Law of Thermodynamics?

    I began applying first law of thermodynamics (delQ=dE+delW) to a freely falling body. We see that for this case delQ=0, so dE=-delW. As dE consists of both potential and kinetic energies we see that neglecting any viscous losses, dE=0. (At max. height, KE=0,PE=mgh and at bottom most point...
  14. R

    The First Law of Thermodynamics

    Homework Statement The average thermal conductivity of the walls (including the windows) and roof of a house is 0.480 W/m · °C, and their average thickness is 24.7 cm. The house is heated with natural gas having a heat of combustion (that is, the energy provided per cubic meter of gas burned)...
  15. O

    Definition of first law of thermodynamics

    The total work is the same in all adiabatic processes between any two equilibrium states having the same kinetic and potential energy. That is another way to describe first law of thermodynamics , and define internal energy. My question is what does "the same kinetic and potential energy "...
  16. E

    Question relating Newton's First Law of motion

    Hi there! I kept on reading my textbook and i found one simple confusion in Newton's First law of motion. I want to consider a situation where a space shuttle is moving in the outerspace (assume there is no gravity) under the influence of the engines. What will happen when the engine stops...
  17. B

    The First Law of Thermodynamics VS. The Universe Created from Nothing

    If the first law of thermodynamics states you can't get something from nothing then how does Stephen Hawking and Leonard Mlodinow break that law to put forward the argument that the Universe can and will create itself out of nothing due to a proposed law of gravity? I began reading 'The Grand...
  18. H

    First law of thermodynamics applied to Compression and Expansion

    How does the first law of thermodynamics apply to the Compression And expansion of a gas at constant temperature ? If the gas is at a constant temperature and is being compressed, then ΔQ = 0 ΔU = + W(on the system) = + Are those(^^) correct ? The equation being : ΔU = ΔQ +...
  19. C

    First Law of Thermodynamics - Isobaric Cooling

    Homework Statement An ideal monatomic gas has an initial pressure of 3 atm, an initial volume of 1 L, and is at an initial temperature of 90 degrees Celsius. It first expands isothermically to 2 L and is then cooled isobarically to a point where it is adiabatically compressed to its initial...
  20. R

    Slight confusion concerning Newton's first law

    Hi, I've very recently begun physics self-teaching as I used to find it boring and thus avoided it as a subject but now find it increasingly interesting. Nevertheless, I'm having some problems regarding questions on Newton's First Law. One of the questions in the book I'm using says the...
  21. Z

    First Law of Thermodynamics (Eint = Q-W) question. I am only stuck on part (d)

    When a system is taken from state i to state f along path iaf in the figure, Q = 60 cal and W = 10 cal. Along path ibf, Q = 65 cal. (a) What is W along path ibf? (b) If W = -19 cal for the return path fi, what is Q for this path? (c) If Eint,i = 11 cal, what is Eint,f?(d) If Eint,b = 27 cal...
  22. sankalpmittal

    Clarification regarding Newon's first law.

    Hello , I am in class XI now and I am 15. Ok , so I have a doubt : Majority (no all !) of our textbooks state Newton's first law as follows : "Every object remains in its original state of rest or uniform motion unless it is acted upon by a net external force." But our teacher told...
  23. P

    Perpetual motion and Newton's first law

    This might be a very stupid question. But let me still ask. :) Perpetual motion is not possible. But by Newton's first law, a body remains in its state of motion unless an external force acts on it. Now, I wonder if the statement "perpetual motion is not possible" is true only in a practical...
  24. A

    Has the first law of thermo dynamics been broken?

    If : The total energy of a falling rock = G.P.E. (mass x gravity x height) + K.E. (0.5 x mass velocity^2) But if a rock reaches terminal velocity (i.e velocity remains the same) but height keep decreasing, then surely Kinetic energy is remaining constant and Gravitational potential energy is...
  25. W

    Apparent Discrepancy Between Two Definitions of Newton's First Law

    So I was reading through my textbook (specifically, Physics for Scientists and Engineers, Eighth Edition, Volume 1 by Raymond A. Serway and John W. Jewett Jr.) and I noticed that, in one of the "Pitfall Prevention" sections (which are usually quite helpful - not this time, evidently), it says...
  26. F

    A small, easy algebraic question about the First Law of Thermo for SISO

    O_o. I know this is a really stupid question, but how the heck did the Q - W get to the other side of the equation? Shouldn't it be W - Q? I'm going to feel really stupid when I hear the answer, I'm sure, and I'm already done with the class so it's ultimately irrelevant, but I'd love to know.
  27. J

    Definition of The first law of motion

    Text book definition is "In the absence of forces, ("body") at rest will stay at rest, and a body moving at a constant velocity in a straight line continues doing so indefinitely". My thinking. Moon is orbiting Earth in a circular path, and not in a straight line. Still that motion follows...
  28. B

    Thermodynamics help on the first law please

    Hi all I am struggling with this textbook example :( I have applied the formula but the numbers I am getting are really wrong! A pump compressing hydrogen from a low pressure to a high pressure tank where the tanks pressure is constant and H2 is an ideal gas. Hydrogen: (H2: MW = 2, Cv = 10...
  29. P

    What is the tension in the rope and how does it affect the man's equilibrium?

    A man sits in a seat that is suspended from a rope. The rope passes over a pulley suspended from the ceiling, and the man holds the other end of the rope in his hands. What is the tension in the rope and what force does the seat exert on the man? Draw a free body for the man. The tension...
  30. J

    Kepler's First law Polar to Cartesian

    Forgive me if this is in the wrong thread I'm new here. I am trying to plot an orbit in MatLab using Kepler's First law of motion. In polar form it works fine r(θ) = h^2/μ*(1/(1+e*cos(θ))) h = angular momentum μ = standard gravitational constant and e = eccentricity. The problem is I'd...
  31. G

    The need of Newtons first law of dynamics

    Newton`s first law states that an object will continue to remain in its state of rest or motion with uniform velocity unless and until acted upon by a net external force. Newton`s second law F=ma In this equation if F=0 then a=0 that is the same as the first law... So, if the first law is a...
  32. S

    Is Every Example Listed an Illustration of Newton's First Law?

    Homework Statement Which situations below are examples of Newton's First Law? a) Your herd jerking back when car accelerates rapidly. b) Hitting shoe on wall to knock off snow. c) An apple hanging motionless on a tree. d) All of the above I know Newton's first law states that...
  33. B

    Kepler's First Law of Planetary Motion

    I'm reviewing my old calculus textbook and I stumbled upon a proof of Kepler's First Law of Planetary Motion which uses vector valued functions along with all of the operations to demonstrate the material. I understand the math and how to to DO it but what I am struggling with is why. It goes...
  34. johann1301

    Newtons first law should not be termed an expression of inertia

    Newtons first law should not be termed an expression of inertia LAW I. EVERY BODY PERSEVERES IN ITS STATE OF REST, OR OF UNIFORM MOTION IN A RIGHT LINE, UNLESS IT IS COMPELLED TO CHANGE THAT STATE BY FORCES IMPRESSED THERON. - «Isaac Newton`s Principa» In case inertia could be removed from a...
  35. O

    First Law of Thermodynamics for Irreversible Processes

    Hello everyone. I have found quite a lot of conflicting information about the first law applied to irreversible processes and in particular whether the dS term in the equation dU = TdS - pdV only accounts for the entropy transferred or for the total entropy, i.e. dS_transferred + dS_created...
  36. P

    Explain Kepler's 1st Law without Math, Momentum/Energy

    I can understand the mathematics behind the deduction of "expanded" (using Newton's laws) Kepler's First law that shows the orbits can be only parabolic, hyperbolic, linear or ellipses, but a friend (who doesn't know derivatives and differential equations) asked me why, taking in account only...
  37. J

    First law of thermodynamics and quasi-static processes

    why is du = δq - δw valid for all processes while du = δq - p dv valid only for quasi-static processes ?
  38. L

    How Does Spontaneous Decay Relate to Newton's First Law?

    Homework Statement Newton's first law of motion deals with inertia of bodies. According to this law, " Every body continues in its state of rest or of uniform motion in straight line, unless compelled by an external force." Homework Equations The Attempt at a Solution My question...
  39. M

    The first Law of Thermodynamics seems confusing to me.

    Hi, The first law of thermodynamic is sometimes written as \DeltaU = \DeltaQ-P \DeltaV and sometimes as \DeltaU = Q+W, and sometimes \DeltaU = Q - W. I am confused about all these. I also know that W is positive when the system in question does work to its surrounding and negative...
  40. O

    Differential form of the first law of thermodynamics

    Hi everyone, I'm new here. I'm an italian student who has an exam in applied thermodynamics soon. Through the whole course as well as the Physics one I have faced a lot of equations expressed with differentials, basically all of them. I have never been taught how to use them though. For...
  41. C

    Where Does the Energy Come From for Motion Without External Forces?

    Forgive me if I'm missing a basic concept, A body does have a natural tendency to move, but a body does need energy to move! Where does this energy come from when no external force is applied on it? Please help, I've been pondering over this doubt since a week..
  42. B

    How to integrate Joule's First Law (Joule Heating)?

    Homework Statement I know the current distribution through a rectangular cross section of metal. It varies only with respect to y. I need to find a heating function for the metal. (This is only the relevant part of a larger problem). Homework Equations Q=I2R The Attempt at a Solution I...
  43. H

    Is Newton's first law a particular case of the second?

    There is not much to explain my question, really. First law says that the velocity of a body remains constant if there is no external force acting, which is just equivalent to second law, as a null force is equivalent to null acceleration, as mass cannot be null.
  44. L

    First law of thermodynamics in layman's term

    The first law of thermodynamics dU=dQ-dW could be expanded to dU=TdS-PdV where U is internal energy, T is temperature, S is entropy, P is pressure, and V is volume. I am trying to understand the terms one by one in plain English. So: V is how big the container is...
  45. A

    Does the first law of thermodynamics apply to the cosmological universe?

    How can the conservation of energy be applied to the entire universe? How can we define the energy of the universe? Is this definition unique for different observers? What is the difference between the thermodynamics universe (system+environment) and the cosmological universe?
  46. C

    First law of thermodynamics applied to a submarine

    First law of thermodynamics Hallo, i hope someone can help me with the following question: A submarine contaiins 1000m^3 of air and has a temperature and pressure of 15°C and 0.1MPa respectively. Due to the cold seawater a heatflow of 60 MJ/h occurs. The machines on the otherhand add...
  47. C

    First law of thermodynamics applied to a submarine

    First law of thermodynamics Hallo, I hope someone can help me with the following question: A submarine conatins 1000m^3 of air and has a temperature and pressure of 15°C and 0.1MPa respectively. Due to the cold seawater a heatflow of 60 MJ/h occurs. The machines on the otherhand adds...
  48. D

    First Law of Thermodynamics - easy q's

    Homework Statement The internal energy of a gas is 600 J. The gas is compressed adiabatically, and its volume decreases by 400 cm3. If the pressure applied on the gas during compression is 4 atm, what is the internal energy of the gas after the adiabatic compression? Homework Equations...
  49. M

    Thermodynamics closed system - first law

    Homework Statement 1kg of water that is initially at 90 degrees celsius with a quality of 10% occupies a spring loaded piston cylinder device. The device is now heated until the pressure rises to 800kPa and the temperature is 250 degrees c Determine the total work done during this process...
  50. B

    Thermodynamics, first law, pressure increase, volume increase

    Homework Statement How much heat energy is needed to heat oxygen, mass=10g which is on temperature t=27°C to make the volume go up by 3 times while pressure is constant, and after that increase the pressure 2 times while volume is constant. Specific heat capacity of oxygen is cp= 908 J/KgK and...
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