Work Definition and 999 Threads

In physics, work is the energy transferred to or from an object via the application of force along a displacement. In its simplest form, it is often represented as the product of force and displacement. A force is said to do positive work if (when applied) it has a component in the direction of the displacement of the point of application. A force does negative work if it has a component opposite to the direction of the displacement at the point of application of the force.
For example, when a ball is held above the ground and then dropped, the work done by the gravitational force on the ball as it falls is equal to the weight of the ball (a force) multiplied by the distance to the ground (a displacement). When the force F is constant and the angle between the force and the displacement s is θ, then the work done is given by:




W
=
F
s
cos
⁡

θ



{\displaystyle W=Fs\cos {\theta }}
Work is a scalar quantity, so it has only magnitude and no direction. Work transfers energy from one place to another, or one form to another. The SI unit of work is the joule (J), the same unit as for energy.

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  1. Jay529

    Solving for Work Done When Accelerating a Mass

    I wanted to make sure these were the correct equations to use for they were not given. I had to research to get this far and only had the below statement to go off. The problem is an object with initial velocity = "0." It travels 0.495m in 0.1625sec and weights 8.4 kg or Initial velocity (u) =...
  2. S

    Work done by a balloon that is rising

    the answer just takes wd = 1N x 20m = 20J. My question is why does it not account for the weight? In physics, when we say work done by the upward force, do we just take that force or must we calculate what the net force is? the next part of the qn then asks, "gpe + ke = work done by this...
  3. L

    I First law of thermodynamics -- two different expressions for dQ

    Okay, so the first law ##dU=dQ+dW##. We all know that ##dU=C_VdT##. So that means that we have: $$dQ=C_VdT+PdV$$ Now I have a problem. We also have that $$dU=\left(\frac{\partial U}{\partial V}\right)_TdV+\left(\frac{\partial U}{\partial T}\right)_VdT$$ and substituting that in to ##dQ=dU+dW##...
  4. martonhorvath

    B The power output of a vehicle when descending hills

    By applying some simplifications, the power ##P## of a body moving at a speed of ##v## at an incline of ##\alpha > 0^\circ## can be expressed as: ##P = P_{acceleration} + P_{friction} + P_{gravity} + P_{air} = (ma + mgsin(\alpha) + C_{rr}mgcos(\alpha) + 0.5C_dA\rho v^2)\cdot v## If we would...
  5. joejoe121

    The specific work done on an ideal gas during an adiabatic process in a piston-cylinder system

    a) 0 b) (P2-P1)V c) Cp(T2-T1) d) Cv(T2-T1) < Ans I don't believe its B because if volume is constant, there's no work. I mostly don't understand why Cv is chosen instead of Cp.
  6. L

    Confusion about work done by a gas - Thermodynamics

    This is chemistry but it's basically physics :D. I used PV = nRT, I get V = 37.44 L. This is fine. So then I have W = P(Vfinal - Vinitial). Vinitial is zero, because there was no hydrogen gas initially. So I get 3.78 kJ. And as the gas expanded from 0 L to 37.44 L, the gas has done positive...
  7. J

    Problem with KE and work equation (perfect rocket in space example)

    Can someone help me by taking a look at the attachment and figuring out where I am making a bad assumption? It's driving me nuts.
  8. Heisenberg7

    Tension Force Problem (applying Work-Kinetic Energy Theorem)

    We know that net work done is equal to the change in kinetic energy, so we write: $$\Delta K = W$$ The tension is acting at angle ##\theta## due to the x axis, so we will only be taking its x component ##T_x = T \cos{\theta}##. Since we can look at this as one dimensional motion (##T_y## does no...
  9. Heisenberg7

    B Work Done By Conservative Forces

    I did classical mechanics a while ago and I was going over some stuff that I wasn't sure if I understood correctly and now I've come over this one. It says that work done by conservative forces is equal to the negative difference in potential energy. Or, ##W_c = - \Delta U##. And I've really...
  10. hraghav

    Calculating work done by a gas

    I have found the answers for T = 848.615K, P = 126137.7705 Pa and change in S = -184.27008 J/K. But my answer for work is not correct and I am not sure where I am making an error. Could someone please help me out with how to calculate work? My steps for work is : We are given: Pi = 101325 Pa Pf...
  11. B

    Textbook 'The Physics of Waves': Calculating Work Done by Force

    Reference textbook “The Physics of Waves” in MIT website: https://ocw.mit.edu/courses/8-03sc-...es-fall-2016/resources/mit8_03scf16_textbook/ Chapter 2 - Section 2.3.1 [Page 45] (see attached file) Question: In the content, it states that we need to use real force and real displacement...
  12. S

    Which movement has the greatest net work done?

    My answer is (D) but the answer key is (C). My reasoning is that the net work for (A), (B) and (C) are all zero since the object moves with constant speed so the resultant force is also zero. What is the mistake in my reasoning? Thanks
  13. A

    I Determining Whether Solution is Vector vs Scalar?

    Ok, so currently, I'm working on problems involving work done by a general variable force. I had a question as far as solving these that ties into other problems as well... I see in some of the worked problems in this textbook, when they take the integral of a force equation to determine the...
  14. highschoolstudent454

    Resistance force changing the velocity of an object

    Mentor note: Moved from a technical section, so is missing the homework template. A Body of mass 1kg moves with a uniform velocity of magnitude 12m/s, a resistance of force of magnitude 6x^2 (newton) where x is the displacement (meter) which the body travels under the action of the resistance...
  15. hello478

    Minimum work done to push the cube over

    my attempt: centre of mass of standing block is at 0.6 and the fallen block is at 0.25 so then change in height = 0.35 and mg = 4000 N so change in gpe = 4000* 0.35 = 1400J but wouldnt we take into account that we only have to apply force till the centre of mass is outside of the block so...
  16. hello478

    How Is Work Calculated on a Conveyor Belt with Friction?

    my answer was c but i didn't understand why the work done in 16m or 12m wont be the greatest? how would we calculate work done in 12 m and 16 m??
  17. hello478

    Power, force, velocity and tension...

    my attempt: i solved it all correct but i dont understand a few things mentioned above... 82.04 * v = 56 so i got v as 0.68 m/s which is correct but i dont understand the concept...
  18. hello478

    How Do Power, Force, and Velocity Interact?

  19. tellmesomething

    Work energy theorem problem -- Block sliding up a curved incline

    I did: Work done by gravity+work done by applied force= KE(final)- KE(initial) Work done by gravity should simply be -mgh =100*5=-500J For work done by applied force we know: W=∫F⋅ds which can also be written as W=∫Fdscos(θ) since F is constant i can take that out W=Fcos(θ)∫ds here since ive...
  20. C

    I Work done by a non constant force over a non constant angle

    I was just wondering if I needed to set up this double integral or not. I've never seen anywhere people setting up double integrals for calculating the work.
  21. amandela

    Work Done and Acceleration (Mistaken Answers?)

    So for Q1, I answered down (towards Earth) but the solution says there is no acceleration there. For Q2, I answered mgh, but the solution says it's mgh/t, which is power, right? I just want to make sure I'm not super confused. Thank you.
  22. Z

    Understanding work in translation and rotation of a magnetic dipole

    these notes on magnetic field from MIT OCW's 8.02 course. There is the following snippet on page 10 At this point it is still not clear to me exactly what a magnetic dipole is. I've done all the calculations for a current-carrying rectangular loop, so I will assume that a magnetic dipole is...
  23. dcmf

    Find net external work of a system w/ 2 objects w/ opposite forces

    A diagram is also provided, which looks like this: I'm not sure what is correct and after doing some digging online I still haven't been able to come to a consensus. I'm currently stuck between one of two possibilities: positive or zero. I'm pretty sure work on A (Wa) and work on B (Wb) are...
  24. Ineedhelpwithphysics

    Sign of Work (Negative or Positive)

    Im confused I did 850*9.8*14.7 = 1.2x10^5 Why is it not negative since the force is opposite to the motion?
  25. Ineedhelpwithphysics

    Work Pulley Problem with constant speed

    For A the 1.2 kg block is being pulled by gravity hence work is done downwards which will make work positive since it's going with the same direction as the force. 1.2 * 9.8 = 11.76 N pulled downwards Work = F*d 11.76*0.75 = 8.82 J The tension is the other force and since the thing is...
  26. Z

    Is the Total Energy Change in a Closed System Always Zero?

    Thus $$\Delta K=-\Delta U + W_{nc}$$ $$W_{nc}=\Delta K+\Delta U=\Delta E_m$$ My question is about the following statement The system is closed. ##\Delta E_{system}## does not necessarily have to be zero. Where does (1) come from?
  27. Klaus3

    I Power done by work on a continuous body isn't the derivative of work?

    The definition of work and power done over a continuous body is: $$ W = \int Tn \cdot u dA + \int b \cdot u dV $$ $$ P = \int Tn \cdot v dA + \int b \cdot v dV $$ ##T## is the stress tensor, ##b## is the body force, ##u## is the displacement vector, ##v## is the velocity, ##n## is the normal...
  28. I_Try_Math

    Work done by friction on a hockey puck

    The answer key claims the answer is -0.9 J. By my math it should be -0.501 J, is that incorrect?
  29. MatinSAR

    Question about the thermodynamic temperature scale

    My first problem is to find the absored and rejected heat. Can I say that it is equal to the work done in an isothermal proccess (##dQ=Pdv##)? My reasoning : We have ##dQ=C_V d\theta + Pdv##. For constant temperature it becomes :$$dQ=Pdv$$
  30. walkeraj

    I The classical concept of work in a QCD context?

    Question: Is it meaningful to think of the repulsion of mutual color charge and the attraction of three different color charge in QCD as being indicative of the classical concept of work taking place? Exactly, how is this explained in the context of three charges needed to elicit the...
  31. clueless_roboticist

    I Work done along a path: how does velocity play into it?

    To boil down the question, if you have a body at rest and apply a constant force, it will accelerate and the work done on it will be F*s (or the integral version of that statement). However, as the body accelerates due to the force, does that mean, per a given time unit, more and more work will...
  32. D

    I Why Substitute Force Magnitude in Spring Work Calculation?

    i'm copying from the book... Hookes Law - F = -kx W = Fdcos∅ since ∅ is 180°, W = -Fd = -Fx W = ∫(-Fxdx) now the book says, from Hookes Law equation "the force magnitude F is kx. Thus, substitution leads to W = ∫(-kxdx)" why are they saying to substitute the magnitude of the force and not the...
  33. P

    Conceptual misunderstanding in applying formula for work done in adiabatic compression

    I was trying to solve a JEE ADVANCED 2020 paper 1 question 13 on thermodynamics. I attempted the problem and got a different answer than they did. I did it in a different way, but I can't find a mistake in both methods. So now I am stuck. I think the problem is in the first step probably using...
  34. M

    A Work done during adiabatic expansion on piston

    Does the work "extracted" from a gas (with the same initial properties) against a piston while expanding change based on the mass of the piston? For example, I have a specific volume of compressed air inside a cylinder with a piston positioned horizontally with stops. The air temperature is...
  35. A

    Is enthalpy just the sum of internal energy and work against external pressure?

    This was the question This is my solution The problem arose after reading this post on PhysicsSE and this answer given So If I remember correct work done is ##-P_{ext}\Delta V## I don't understand why $$\Delta H=\Delta U+(5×4-1×10)L.atm$$ If that answer (the answer on the PSE post) is...
  36. S

    Find the amount of work done against gravity in moving a 150kg rocket

    I started by calculating the energy at the Earths surface, which is just -G(150)(6 x 10^24)/(6400 x 10^3) = -0.9 x 10^10, and calculating the energy at the orbit radius, E = -G(150)(6 x 10^24)/(18000 x 10^3) = -3.4 x 10^9, then doing some subtraction we have -3.4 x 10^9 - (-0.9 x 10^10) = 5600 x...
  37. biomajor

    Would one be pushing as much as their weight?

    Because one is effectively pushing in the same way as a chest press machine, wouldn't the work done be the same?
  38. E

    Where Can I Get Help with My Physics Homework?

    HI, I need help wtih my physics work and dont know where to ask
  39. Z

    Work done during quasi-static, isothermal expansion from P_i to P_f

    I am posting this question after I thought I had easily solved the problem, but then when I checked the back of the book I saw that I was incorrect. Here is what I did. (a) $$W=-\int_{V_i}^{V_f} PdV\tag{1}$$ $$V=V(P,T)-\frac{nRT}{P}\tag{2}$$ $$dV=\left ( \frac{\partial V}{\partial P}\right...
  40. mazia

    Questions Regarding a Cat Going Up a Ramp

    EDIT: I've finally found the solution, so here's what I did. First, calculate Work using the equation: (F-mgsin(theta))*displacement, where F=the force being applied by the push, theta is the angle of the ramp, and displacement is the length of the ramp. Now that you have the value for work...
  41. Z

    I Is it accurate to say work is motion against an opposing force?

    Is the following quote accurate: "The fundamental property in thermodynamics is work: work is done to achieve motion against an opposing force" Specifically, I am asking about the portion after the colon. I am a little confused by the notion of an opposing force. Let's say we are in outer...
  42. A

    Work done during a collision -- Change in Kinetic Energy & change in Momentum

    Hello guys, I need help on this problem, "You throw a ball with a mass of 0.4kg against a brick wall. It hits the wall moving horizontally to the left at 30 m/s and rebounds horizontally to the right at 20m/s. (a) Find the Impulse of the net force on the ball during its collision with the wall...
  43. physicsmaster123

    Work done by gravity on a car rolling down a hill

    I tried E =Fxcos0 but only ended up with 243kJ
  44. E

    Work done by non-conservative force while particle moves in circle

    I have to show that the force is non-conservative, i.e. that work done for the round trip ##\neq 0##. Rearranging the circle equation, I can say that:$$x = \sqrt {R^2 - y^2}$$$$y = \sqrt {R^2 - x^2}$$ Then I have:$$F_x = -\sqrt {R^2 - x^2}$$$$F_y = \sqrt {R^2 - y^2}$$ Now, as I understand it, I...
  45. E

    Signs of work done and delta K.E.

    I'm asking about this with particular reference to the signs of the answers. Here is the question: The answers in the back of the book are: (a) ##1.2 \times 10^4 \text{ N}## (b) ##39 \text{ m}## (c) ##4.7 \times 10^5 \text{ J}## (d) ##4.7 \times 10^5 \text{ J}## Here's a rough sketch of...
  46. aliinuur

    I This gas work violates 2nd law of thermodynamics?

  47. E

    I Confusion on Callen's Maximum Work Theorem

    This question was, effectively, asked here (please refer to that question for additional context); however, I don't think the given answer is correct (or at least complete) despite my having added a bounty and having had a productive discussion with the answerer there. In particular, I don't...
  48. singularcell

    Efficiency and Temperature in Heat Engine Cycles: Approaching Parts B, C, and D

    I didn't have much trouble with part a but I'm struggling with b,c, and d. I considered the efficiency formula for a heat engine e = work done by engine/ qh but i am unsure of how to approach it. for part c) not sure how i can get to Tc without knowing Th for d) my gut is telling me 5/2 but i...
  49. D

    Why doesn't my collision detection between two Area2D objects in Godot 4 work?

    Hi everyone I'd like to detect collisions between a circle and a floor. I have set them up as follows: The script runs fine (it's just a circle falling towards the floor), but nothing happens when the two objects collide. Does anyone know what I've done wrong?Thanks
  50. A

    Why is the work done double its expected value? (conveyer belt)

    The question was this: My calculations show that the answer should be equal to work done on crate to make it reach the same velocity which is equal to 216 J but the answer given is 432 J It is believed that extra energy is needed to overcome friction but friction is an internal force and...
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