What is Work done: Definition and 1000 Discussions

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. A

    Is there any work done by static friction when accelerating a car?

    I'm asking for clarification, but it's my understanding, that of the thread below, and my college physics book Paul A tipler, that when walking or driving a car, the force of friction from the ground does no work. This makes sense in a car becuase the engine/fuel makes the power, driving the...
  2. Reneee

    What is the work done to move this Charge?

    I found two formulas to calculate the work done. One is with this path integral: ## W_{AB}## = W(## r_A,r_B ##)=q* ## \int_{r_A}^{r_B} E*dr ## but here is the one I tried to use: ## W_{AB}## = q*Δ U = q*(## \frac {kQ} {r_A} ## - ## \frac {kQ} {r_B} ## ) Now here's my problem, what are...
  3. polytheneman

    D'Alembert's principle and the work done by constraint forces

    From what I understand, constraint forces do no work because they are perpendicular to the allowed virtual displacements of the system. However, if you consider an unbalanced Atwood machine, in which both masses are accelerating in opposite directions, you'll find that the tension force of the...
  4. BiggestAfrica

    Determining the factors impacting the amount of Work done

    Apologies for my fairly undescriptive title, I'm having trouble coming up with a better one. I'm simply trying to verify whether my understanding of this problem is correct. 1. Pushing the oven up a ramp 4m long would require more force (my reasoning here is based on the results of a lab which...
  5. E

    Two different answers for work done during compression

    I'm having a little trouble with part a) of this question: Since it is stated that the heating is slow, I thought it was reasonable to assume the process is reversible which means that the pressure in both sides should be equal. Consequently, $$W = - \int_{V_{0}}^{V_{1}} P dV = -...
  6. Adesh

    A proof that magnetic forces do no work?

    The proof of magnetic forces do no work is given in Introduction to Electrodynamics by David J. Griffiths like this My problem is why he has replaced d\mathbf{l} with \mathbf{v}dt? This substitution implies that the charged particle was moving with \mathbf{v} only and no force acted on it...
  7. Aletag

    Work done BY the gas in a cyclic thermodynamic process

    Since the assignment asks the work done by the gas, that should be equal to P1*(V2-V1) aka the area under the P1 line. Do I have to subtract the work done to the system or is this the solution already? If so, why do I need P2?
  8. G

    To find the work done in extending a spring

    Hi, Could I please ask where I am going wrong with this very simple question: Here's my answer (units implied): A force of 20 extends the spring by 1/100 and so the Work Done in performing this extension is 20 * 1/100 = 1/ 5 Now, the work done in extending a spring is given by the formula...
  9. brawler234

    Understanding Work Calculations: Clarifying the Role of Forces and Intuition

    A box is moving at a constant velocity. 250N of applied force is acting on a box while 250N of friction is acting on it on the opposite direction. If the box travels 4 meters in the same direction as the applied force, how much work is done? I know that work = force * displacement. Is the...
  10. T

    Who Does the Work When a Ball is Kicked: The Player or the Ball?

    if I shoot the football, the ball will recive a force in specific direction, and it would follow that direction. Does the work is done by me when I shooted the ball, or the ball does work when it moves all the period after my shooting ? the work includes force and displacement , so who does the...
  11. R

    Work done pushing a refrigerator up an Incline

    My teacher told me that answers are F=1200 N for lifting straight up and F= 360 N for using the ramp. I can get the force for lifting straight up by using 1800=F(1.5)cos(0) but I do not understand why for using the ramp the equation 1800 = F(1.5)cos(72.5) does not produce the correct answer.
  12. jisbon

    Calculating the work done in a PV diagram

    So I've been digging this problem for quite some time, and still can't figure out a way to obtain work done in AB. I do understand that work done in AB = area under the graph. However, I can't figure out how to obtain the volume at B. I can't use PV=nRT since I am unable to obtain the...
  13. A

    Power delivered/Internal work done by a car's engine

    Attached is an example from my Tipler physics book of a determining the power delivered from a cars engine. I get the derivation(math), but am having some issues with the understanding/concept. Mainly, isn't Power=Work done/time. If so, they define the system as Car-earth. There is no external...
  14. B

    Is it possible to pull a wagon without wheels?

    Fa = Force applied on the cart by the parent Fax = x component For the force applied on the cart by the parent Fay = y component For the force applied on the cart by the parent Ff = force of friction The method I thought to use was to solve for Facos, Fasin and then use pythagorean's theorem to...
  15. SLTH02

    Work required to attach the bowstring to the bow?

    E final = (1/2)(500)(1.14012)^2 = 324.968 J E initial = (1/2)(500)(0.5)^2 = 62.5 J E final - E initial = 262.468 J
  16. weilam06

    Work done by an electric motor to drive elevator

    https://www.physicsforums.com/attachments/250022 For the part (a), set the tension of the string that pulls the elevator be ##T_1## and the tension that pushes the counterweight be ##T_2##. Then we have the following equations: $$T_1−1200g=1200 \cdot 1.5$$ $$1000g−T_2=1000 \cdot (−1.5)$$ where...
  17. E

    Work done by a tangential force shouldn't be zero

    Applying the dot product to the position vector and force vectors yields the result that the force vector is always at 90 degrees to the position vector, namely that it is directed tangentially. The mark scheme gives the work done as 0, however I don't agree. I believe they got this by finding...
  18. P

    A Can Work Be Defined for a Quantum System with a Time-Dependent Hamiltonian?

    I have a Hamiltonian ##H_{\lambda(t)}##, where ##\lambda (t)## characterizes a time dependent path in parameter space. The parameter is changed in finite time from ##\lambda (t_i)## to ##\lambda(t_f)## . At ##t = t_i## the system is in the intial state ##|\Psi>##. What is the work done on the...
  19. H

    Maximum work done by a body in an external medium

    I am reading Landau's Vol 5 on Statistical Physics and have trouble grasping some concepts in Section 20. If I understand this correctly, the body and the medium are in direct contact and can exchange work and heat while the object can only exchange work with the body. So the minimum...
  20. SebastianRM

    Work done by a constant Force: questions

    I was reviewing physics stuff and I've managed to confuse myself. I was reading about work (principles of ohysics, serway&jewtt): -work is an energy transfer -if work is done on a system and W is positive, energy is transferred to the system; if W us negative, energy is transferred from the...
  21. Kaushik

    Internal forces and the work done by them

    My book states that, '∑Finternal=0∑Finternal=0. But the work done by them may or may not be zero. Why is this true? Edit: How can the work ≠ 0 sometimes?
  22. A

    Elastic Collision: how do the forces and work done make sense?

    Okay, say we have two balls(equal mass and size), 1 and 2. #1 has kinetic energy and #2 is at a standstill, they collide. Ignore all friction, heat, sound losses etc.. Now I know that 1 exerts a force on 2, hence doing work, which in turn uses its kinetic energy up. Therefore, 2 speeds up...
  23. mehmet

    In which circumstances is work done?

    So I have honestly no idea what kind of force is acting on the objects in each case, nor if work is being done. I attempted to identify the force for ‘d’ and I wrote the following: “work is being done and forces acting on the book are balanced, the weight of the book acts downwards and the...
  24. C

    Why is the work done in moving a charge around a circular path zero?

    Work done in moving a charge around the circular path is 0 because the distance between the charges remain the same so there is no potential difference and delta(V)=0 but I thinkk some torque or external force must be required to move the charge around the circula path
  25. Physics lover

    Work done by friction on a two-block system

    I calculated friction = 1N.But since force is more than maximum friction,therefore lower block will not move and a force of 0.5N will be applied to upper block. Therefore its work done by friction will be W=0.5×6=3 J. But answer given is 1 J. Can anyone help.
  26. J

    Work done by varying friction against a constantly moving surface

    Suppose a metal disk is spinning at constant rate of one cycle per second and at a radius of about four inches a small metal block is pressed against the disk. The circumference of the path of the contact is exactly one foot. The coefficient of kinetic friction between the disk and the small...
  27. bottle_shadow

    I Line integral for work done by gravity

    Dear Physics Forums people, My problem lies in understanding how the following line integral, which represents work done by the gravitational force, was calculated Specifically, in the integral after the 2nd = sign, they implicitly used \hat{r}\cdot d\vec{s} = dr I wish to understand what...
  28. Hawkingo

    Why there is a negative sign in the formula of calculating work done?

    $$W = - \int _ { a } ^ { b } \vec { F } \cdot d \vec { r }$$ ( The Force here is referring to the applied force, When moving a positive charge towards another positive charge(stationary) / field In this formula why there is a negative sign in the formula? I am not asking the sign of the total...
  29. M

    Electrostatics: Understanding this "Work Done" Line Integral Question

    I have a quick question about the work done concept here, especially the line integral part of it. So I understand the fact that the work done from getting from point A to B is: \int_{a}^{b} \vec F \cdot d\vec r . However, within the context of electric fields, when we define electrostatic...
  30. Hawkingo

    Why the work done is negative when bringing 2 opposite charges together?

    we know that if the applied force is in the direction of the displacement then work done is positive.But in case of bringing 2 opposite charges from infinite to a certain distance,the work done is negative even the force and the displacement of the charge is in the same direction. From...
  31. M

    Calculating the work done on a motorcycle and rider

    IN PROGRESS
  32. D

    Find the acceleration and the work done by a force pulling a spool?

    I already solved the first part, but according the my book, the answer isn't quite correct. This is what I did. Finally, I ended up with ##a=\frac{F(r-R\cos\alpha)}{Rm(\gamma+1)}##. But according to my book, the answer is ##a=\frac{F(\cos\alpha-\frac{r}{R})}{m(1+\gamma)}##, what am I doing...
  33. Eucliwood

    Work done on an object given a variable mass

    I was just doing some review on my physics lecture and I stumble on the idea of what if there was an object hanging and the cord mass is also included in the weight and it's displaced upward without having velocity nor time hypothetically and the cord change mass. I tried solving it by W = fΔx...
  34. M

    Thermodynamics: Joule / Brayton Cycle - Specific Work Done Question

    Relevant Equations: Specific Work Done: w = \frac{p_2 v_2 - p_1 v_1}{1-\gamma} (1) w = \frac{R}{1-\gamma} \times (T_2 - T_1) (2) Thermal efficiency: \eta _{th} = 1- \frac{1}{r_p ^ {\frac{\gamma - 1}{\gamma}}} = \frac{w_{out} - w_{in}}{q_{in}} = \frac{q_{out} - q_{in}}{q_{in}} (3) Context...
  35. A

    What is the work done on cart by the string?

    Homework Statement A cart of mass M1 = 6 kg is attached to a block of mass M2 = 3 kg by a string that passes over a frictionless pulley. The system is initially at rest and the table is frictionless. After the block has fallen a distance h = 1 m: What is the work Ws done on the cart by the...
  36. solarcat

    Work Done by Gravity and Tension in a Pulley System

    Homework Statement Two blocks are attached to a rope across a pulley. Block A is on the ground and has a mass of 4 kg. Block B is 2 meters above the ground and has a mass of 12 kg. When the blocks are released, block B hits the floor and block A rises to 2 meters; each block has a final speed...
  37. A

    Work done by Hooke's Law? (Calc 2)

    An open tank has the shape of a right circular cone. The tank is 8 feet across the top and 6 feet high. How much work is done in emptying the tank by pumping the water over the top edge? (The weight-density of water is 62.4 pounds per cubic foot.)
  38. Romain Nzebele

    How to calculate the work done

    Homework Statement A sledge loaded with bricks has a total mass of 18.0 kg and is pulled at constant speed by a rope inclined at 20.0° above the horizontal. The sledge moves a distance of 20.0 m on a horizontal surface. The coefficient of kinetic friction between the sledge and surface is...
  39. Burhan Uddin

    Calculating the work done using a line integral

    Homework Statement a) A point charge + q is placed at the origin. By explicitly calculating the relevant line integral, determine how much external work must be done to bring another point charge + q from infinity to the point r2= aŷ ? Consider the difference between external work and work...
  40. Ben Harris

    Work done on a meteor approaching the Earth -- Is it +ve or -ve?

    Hi all, i am trying to figure out if work done by gravity is positive or negative on an approaching meteor, I have used W(1->2) = integral (1->2) F(gravity) dot dr. and got a result of (GMem/r2) - (GMem/r1). Were G is gravity, Me is Earth mass, m is meteor mass and r1, r2 are initial and...
  41. J

    Understanding Work Against Friction in Physics

    When we say the work done against friction, what do we mean by that? This concept is really worrying me.Thanks in advance!
  42. B

    Work done on the system through a piston?

    I am stuck on like the first page of thermal physics. It seems like the signs of the work done on the system are opposite when the volume is expanded and compressed. But when I imagine myself pushing or pulling the piston, I get confused from W = \textbf{F}\cdot \textbf{d} This work will be...
  43. S

    Determining Work Done by a Person on a Luggage

    I'm trying to determine the work done by a person as they pull a luggage up a ramp. The ramp has a height of 5 m and the distance the person walks up is 20 m. The weight of the bag is also 10 kg. I am trying to compare the work done by pulling the luggage up a ramp to carrying an equally heavy...
  44. I

    Does greater acceleration result in more work done?

    Work W done by moving the object with force F for distance s is W = Fs. When I move the same object the same distance but with twice the acceleration, does the work done gets also doubled? By F=ma, doubling the acceleration yields m*2*a = 2F -> 2Fs = 2W. I've mostly read, that if I want to...
  45. R

    Work Done on Charges by a Capacitor and an Inductor

    Homework Statement https://imgur.com/vQRoMjZ Homework EquationsThe Attempt at a Solution I believe that the inductor does negative work on the charges via the electric field (by way of induced EMF). I am unsure about how the capacitor would be doing positive work, though. Can anyone help steer...
  46. B

    Question on the work done while charging up a capacitor.

    So the work done when charging up a capacitor is ##dW=VdQ## However, when we add a charge ##dQ## to the capacitor, ##V## also changes accordingly, so I was wondering why the work done wasn't written as ##dW=VdQ+QdV## (one that also takes into account t he change in ##V##). Thanks in advance.
  47. D

    Understanding the Work-Energy Theorem

    Are the following definitions correct? Work done as energy transferred The energy transferred when the forces between two objects interact Work done by a force Work done = force x distance moved in the direction of force applied (W=F*Δs) Work done by a gas Work done = pressure x change...
  48. C

    Definition of work done by torque

    I' m trying to derive the work done by a torque from W = ∫ F ⋅ ds and I' ve looked up the internet, it said: W = ∫ F ⋅ ds ( since ds = dθ × r ) ---------------------------------------- ( Line 1 ) it can be written as W = ∫ F ⋅ dθ x r this is a vector triple product , thus can also...
  49. kolleamm

    Work done by a force on a spring

    Homework Statement The unstretched length of a spring with 'k' = 250 N/m is 20 cm. A force 'F' is applied to stretch it to a length of 24 cm. How much work was done by 'F'? Answer : 0.2 Nm Homework Equations F = k * delta x Work = F * delta x The Attempt at a Solution [/B] change in spring...
  50. shahrilmajid

    Finding work done by worker pulls

    Homework Statement Homework Equations fd cos theta The Attempt at a Solution Finding the work done. Please anybody help me[/B]
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