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. R

    Applying first law of thermo to a closed spring piston

    Homework Statement A cylinder having a piston restrained by a linear spring (of spring constant 15 kN/m) contains 0.5 kg of saturated vapor water at 120ºC. Heat is transferred to the water causing piston to rise. If the piston’s cross sectional area is 0.05 m2 and the pressure varies linearly...
  2. K

    Thermodynamic properties - Isochronic process - First law

    Homework Statement An amount of water substance having a mass of 1 kg is contained in a vessel at a pressure of 1 MPa. It is heated for some time after which the pressure is 3 MPa. The volume of the vessel is 0.1 m3. Assuming no work is done by or to the substance, calculate the final...
  3. B

    What force causes a body to move into the windshield when a car suddenly stops?

    Homework Statement This is not a specific problem, I was just wondering, what unbalanced force acts on your body when you are in a car traveling at a constant velocity and the car comes to a sudden stop? This has to do with Newton's First Law, but I'm unsure of what force applied to your body...
  4. O

    First Law of Thermodynamics and Melting-Ice Scenario?

    In order to explain the change in energy of an ice cube removed from a refrigerator and placed on the kitchen counter, I consider it as a closed system (can exchange only energy but not matter). For the first law of thermodynamics, ∆U = Q+W. I'm pretty sure that the sign of Q is positive due to...
  5. a lone fishy

    Relative motion or Newton first law?

    Homework Statement An observer on the ground sees a person on a train just leaving the station in the east direction. explain how the observer on the ground could see the person moving to the west while the train is moving east Homework EquationsThe Attempt at a Solution At first I thought...
  6. D

    Why is delta-H equal to q at constant pressure?

    Homework Statement In my lecture notes(beginner thermodnyamics), we just got introduced to the first law(DeltaU=q+w) and two scenarios. One at constant volume which yields the equation Delta U = qv. I understand that The second scenario is at Constant pressure and it says that At constant...
  7. rpthomps

    First Law of Thermodynamics - Piston

    A cylinder of cross-sectional Area A is closed by a massless piston. The cylinder contains n mol of ideal gas with specific heat ratio and is initially in equilibrium with the surrounding air at temperature T and pressure P. The piston is initially at height h1 above the bottom of the cylinder...
  8. Y

    How is Fick's First Law derived?

    How do you derive it? I'm looking for the form J = -D(dC/dX) From the image, I am assuming left side is of higher concentration. N represents the number of molecules. My work: Mass balance Net molecules in from concentration gradient = Net = N(x) - N(x+Delta X) Net concentration in =...
  9. L

    Question on Newton's first law

    An object with no resultant force can still be moving at constant velocity. Why? Is it because of its kinetic energy or has it got nothing to do with energy?
  10. J

    How can balanced forces result in constant velocity?

    Homework Statement Hi everyone. I am a grade 11 physics teacher. This week, I will be teaching about Newton's first law in class. I'm having trouble explaining one thing. Students generally understand that a hockey puck moving on ice will continue to move with constant velocity. In this case...
  11. L

    First law of thermodynamics applied to a closed system

    Homework Statement - A well insulated rigid steel tank contains water (only) - A resistance heating element (240V, 3.67Amp) designed to heat the water as well as inlet and outlet valves for - the water. - At state-1, the water is in a saturated liquid-vapor mixture state at P(abs)=100kPa...
  12. L

    First law of thermodynamics & steady flow devices

    Homework Statement The motherboard in a laptop is run by a PSU of 20V/4.5A. This motherboard is inside a plastic case where there is not heat transfer through the casing. To stop the motherboard from over heating a single (motherboard powered) 10W electric fan is mounted inside the casing. The...
  13. C

    First law of thermodynamics at constant volume (ideal gas)

    If c_v is the specific heat at constant volume, authors substitute this into the first law as follows: c_v d\theta + pdv = dq How can one deduce that equation for any case? Since the specific heat at constant volume is used, the equation would be valid only where there is no expansion i.e...
  14. N

    Regarding efficiency of heat engines

    I tried to calculate the efficiency of a engine using the formula : Efficiency = (Net Work Done) / (Heat absorbed) And then I tried to calculate the efficiency of respective Carnot engine by two ways : 1) Efficiency = (Net Work Done) / (Heat absorbed) 2) Efficiency = 1 - (Temperature Lowest /...
  15. G

    Fick's First Law (Diffusion Problem)

    1. A sheet (with a thickness of 0.05 cm) of MgO lies in between layers of Ni and Ta to avoid reaction between these two metals . At 1400 ºC, ions of Ni are created and diffuse through the ceramic MgO to the Ta. Find the number of ions that go through the MgO per second, knowing that the...
  16. S

    First law of thermodyamics: undestanding required

    Homework Statement Understanding the first law of thermodynamics Homework Equations The Attempt at a Solution [/B] I have been doing a self-study course on heat transfer. I have been reading from the book by Incropera and all. I understand that the first law of thermodynamics forms the very...
  17. S

    (simple) Applying constant force - Newton's first Law

    Another silly question... Assuming a surface with no friction in space (you get the idea), I push/kick/give a 1kg block on this horizontal plane a force of 1N. It would start to accelerate in the direction i pushed it with at a rate of 1m/s² forever until another force acted upon it. So far so...
  18. copypacer

    Newton's First law - Dropping a feather and a hammer....

    <<Mentor note: Edited for language>> 1. Homework Statement If I have a feather and a hammer and drop it at the same time from the same height (with out air resistance, so basically in a vacuum) Why would it land on the ground at the exact same time? Homework Equations F = ma The Attempt at...
  19. B

    Thermodynamics First Law Question

    Homework Statement Can someone help me confirm if I answered correctly please? a) A sample of gas is enclosed in a cylinder by a piston. The cylinder is given 225J of energy which expands and pushes the piston 16cm outwards against an atmospheric pressure of 1.01x105Pa. i) give the equation...
  20. Rwindsor1

    Trouble with definition of Newton's First Law

    The lecturer in my dynamics class defined Newton's First Law to be 'There exists at least one inertial frame with respect to which mass m moves in a straight line with a constant velocity. In this frame no net force acts on m.' This has confused me; I thought inertial frames could not...
  21. A

    Thermodynamics, open system first law problem

    Homework Statement >A gas flows through a small orifice in a pipe as shown above. On the higher pressure side, the gas is at 1Mpa and temperature of 300K. The pressure reduces to 1kPa after it flows through the orifice. The equation of state for the gas is v=\frac{RT}{P}+10^{-6}T^2 If the...
  22. L

    Keplar's First Law: Explaining Orbit and Eccentricity

    I know that Keplar's First law is true, but it doesn't occur to me why do stars orbit the center of gravity. Is there a proof for that, or a way for me to visualise why? Also, to measure the eccentricity of the orbit, e^2=1+(2E(L^2))/(GM(m^2)). How did they derive this equation?
  23. DrVirz

    Another Thermo First Law Question

    Hi all, I have another thermo q I am stuck on. You can see the question and my attempted answer below. Part 1 is fine, Part 2 i don't know where to start, and part 3 I thought was correct however the answer is wrong! I think I am doing something wrong with the U2 part of internal energy, just...
  24. DrVirz

    How to Solve the 4th Thermodynamics Question on Heat Capacity?

    Homework Statement Hi all, please see uploaded pics. I can do the first 3 questions but can't get my head around the 4th. Any help is appreciated.
  25. S

    Thermodynamic question: first law

    Estimate the kilowatt pump you would need to pump 450 L/min of liquid water at 20c through a rise in elevation of 100m. Assume the water is pumped adiabatically and with negligible changes changes in temperature and kinetic energy. answer given to us is -7.35kw.. Question.. what formula should...
  26. M

    How does an object maintain constant velocity according to Newton's First Law?

    Newton's first law says that when an object is at rest or moving in constant velocity, it will stay that way unless external force is applied. So, net force is zero. What I don't get is on an object moving in constant velocity. How is the object begin moving? Is netforce applied initially...
  27. I

    First law of thermodynamics & state functions

    Homework Statement 1 kg air at the pressure ##10^6##Pa and the temperature ##125^\circ C = 398K## expand until the volume is 5 times larger. The expansion is done with change in heat at every moment being ##1/4## of the work done by the gas. Calculate the end pressure.Homework Equations ##dU...
  28. A

    Thermodynamics:based on first law

    Homework Statement In an adiabatic steam nozzle,steam is expanded from 10 bar and 473k to an exit pressure of 5 bar.nozzle has an isentropic efficiency of 90%.neglect kinetic energy at the inlet.assuming equilibrium condition at exit,find the velocity of nozzle at exit? given data : i)at nozzle...
  29. A

    How to use the first law of thermodynamics for simple mechanical problems

    I'm confused about what exactly is Q and U and their signs. Consider a block initially having some kinetic energy which we stop and we want to find by how much amount its temperature increases. Then Q=dU+W where dU is change in internal energy. (I'm using the sign convention that heat absorbed...
  30. avito009

    Newton's First Law: Movement in Space Without Friction

    Everything should move in a straight line if there is no friction. But when an aircraft goes into space it needs to move burning its fuel. Why does it not move constantly in straight line? So is the reason behind the fact that a rocket needs to be propelled, that the gravity of the sun and...
  31. V

    Newton's First law and KT of Gases

    One of the assumptions of Kinetic Theory states that: A gas is a substance composed of many particles flying about randomly, colliding with one another and with the walls of the container. My instructor mention that we might be required to Know, state, and apply this theory to one of newtoms...
  32. Avatrin

    Reversibility/quasistatic and the first law

    Hi The thing I am struggling with is why for reversible or quasistatic (depending on what book you use): Q = TdS and W = -PdV Thus, dU = TdS - PdV What about reversible/quasistatic processes makes -PdV the only work the system can do? While I know \frac{1}{T} = (\frac{dS}{dU})_{N,V}, I don't...
  33. K

    The first law of thermodynamics and the dry adiabatic lapse rate

    Say there is a parcel of air rising and cooling, what is going? Is it that the net kinetic energy (Temperature) of the parcel is distributed over the (new and larger) area/volume? Or Is it that the parcel is undertaking work as it presses outwards and so looses a corresponding amount of kinetic...
  34. L

    Newton's First Law: At Rest or In Motion?

    For Newton's first law, which one is correct? 1. All objects that are at rest, stay at rest; all objects that are in motion stay in motion, unless acted upon an unbalanced force. Or 2. All objects that are at rest, stay at rest; all objects that are in motion stay in motion, unless acted...
  35. M

    Understanding Newton's First Law of Motion in Physics

    Won't the sentence be complete if its mentioned, with respect whom the particle remains at rest or moves with constant velocity?
  36. Adjoint

    Why do we need Newton's First law? And how the First law works?

    This must have been asked before. But never mind here I ask it again to convince myself. Newton's Second law says \vec{F} = m\vec{a} Now if we put \vec{F} = 0 here we get \vec{a} = 0 which is Newton's First law. So why do we need to state First law as a separate law? Before I asked this I did...
  37. R

    The First Law of Thermodynamics

    I am working on this problem: Q: During an isothermal expansion, a confined ideal gas does -150 J of work against its surroundings. which of the following describes the heat transfer during this process? (a) 150 J of heat was added to the gas; (b) 150 J of heat was removed from the gas; (c) 300...
  38. L

    First Law of Thermodynamics help

    Homework Statement Steam flow through a pipe at a pressure if 8bar and a temperature of 340C. A valve is opened and steam is allow to enter an insulated enclosure whose volume is 0.5(m^3), until the pressure equals the quantity of steam that enters the enclosure. Homework Equations Q...
  39. M

    Why is using Newton's First Law failing me?

    A factory worker pushes a 29.7kg crate a distance of 5.0m along a level floor at constant velocity by pushing downward at an angle of 32∘ below the horizontal. The coefficient of kinetic friction between the crate and floor is 0.24. What magnitude of force must the worker apply to move the...
  40. S

    I've been staring at this Newtons first law assignment for hours

    And I'm starting to feel kind of dumb here... here is the picture to go along with the question Question: Draw a free-body diagram for Point A, showing all the forces acting at A. Resolve the vectors into components, where necessary. Using the first condition of equilibrium, solve...
  41. I

    Is the First Law of Thermodynamics Applicable to Work Flows?

    I used the formula Qin-Qout=Wnet and therefore had: 100-Qout =20 , so Qout=80 J.. They got -80J and I got 80J but is my answer still correct?
  42. P

    The First Law of Thermodynamics and Mechanical Work

    So I have a doubt that's been bugging me for a very long time, and my teacher's response to it was vague. And I would be very grateful if someone would help me clear the misconceptions I have. Now, first of all, according to the first law of thermodynamics, \DeltaU = Q + W, where U is the...
  43. E

    Newton's first law and projectiles questions

    I just don't get these questions...please help. () are what I think the answers are but I am not positive. Question set 1: Beginning from rest, Mia Edwards runs the 200 meter dash with an acceleration of 0.9 m/s2. 1. define inertia. (inertia- the tendency of an object to resist change)...
  44. G

    First Law of Thermodynamics issue

    Homework Statement In an experiment to simulate conditions within an automobile engine, 0.155 mol of air at a temperature of 780K and a pressure of 3.20x106 Pa is contained in a cylinder of volume 310 cm3. Then 670 J of heat is transferred to the cylinder. A) If the volume of the cylinder...
  45. N

    Newton's First Law of Motion is Wrong (Proven Wrong in This Thread)

    Please note this is a very serious thread. Please note Newton's First Law: First law: When viewed in an inertial reference frame, an object either remains at rest or moves at a constant velocity, unless acted upon by an external force. Here is an experiment to prove this First Law...
  46. R

    First Law Thermodynamics Problem

    Homework Statement For an hour , an athlete with 50kg of weight produces a work of 75kcal pedaling on the gym bike. During this period of time he consumes 60L of oxygen. Calculate: a) The power produced by the athlete b) The variation of the internal energy of the athlete knowing that for...
  47. P

    First law of thermodynamics of gas

    The first law can be stated as ΔU = Q + W and we adopt a sign convention as follows, if the system under study does work then W is negative and if work is done on the system W is positive. Now consider the example of a gas expanding against a constant external pressure p. W would be negative as...
  48. S

    How Is Work Calculated in an Isothermal Expansion of an Ideal Gas?

    Homework Statement 5 litres of an ideal gas which is initially at 300K and 10atm, and is expanded to a final pressure of 1atm. Find work done, change in internal energy, heat absorbed and change in enthalpy of the system if the process is isothermal and reversible. Homework Equations...
  49. E

    Can Gravitational energy be derived from First Law of Thermodynamics?

    I'm not sure if I'm right .. Here's my thought: 1. The First Law of Thermodynamics is the only single law which could derive energy formula. Furthermore, we can't, solely, use Newton's law to derive Kinetic Energy. It will make us fail to explain the existence of thermal energy.(*) 2. In...
  50. J

    Thermodynamics first law closed system

    1) A rigid walled tank 5m3 contains helium at 10 bar. The cylinder is heated from 10 °C to 50 ° C. What is the work done during the heating cycle.
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