What is Work and energy: Definition and 333 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. L

    I Work to move a point charge from infinity to the centre of a charge distribution

    Electrostatic potential $$ \Phi(\vec{r})=k \int \mathrm{d}^{3} r \frac{\rho\left(\vec{r}^{\prime}\right)}{\left|\vec{r}-\vec{r}^{\prime}\right|} (i) $$ with $$ k=\frac{1}{4\pi\epsilon_{0}} $$ in SI units. What work is required to move a point charge q from infinity to the center of the through...
  2. E

    Integral for work done leading to potential energy sign confusion

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  3. Rhdjfgjgj

    To find work done by a variable force

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  4. E

    Bow string problem sign query

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  5. E

    Work done for round trip when force is function of velocity

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  6. A

    I How come you have to use work energy to find v0 of spring?

    If you use work energy, you can get 0.5*k*x^2 = 0.5*m*v^2 to get the velocity if you pulled the spring a distance x. How come you cannot do kx*(delta t) = m*v to get the initial velocity and what would be the delta t value?
  7. Lotto

    B What is meant by "upper limit of work done on Earth"?

    I think that the work is meant to be work done for instance in power stations. Or is it similar to work I do on a body when I lift it for example? But how can we then do that work on our Earth? I just need to understand the task, otherwise I want to solve it myself. The problem involves...
  8. Altoid

    Why does the logic of negative acceleration not always indicate negative work?

    Knowing that negative work occurs when the force applied to an object opposes the direction of displacement, and that the direction of acceleration vector should align with the force vector, I assumed the correct answer was that the indication of negative work comes from negative acceleration...
  9. Argonaut

    Understanding Work-Energy Theorem: Solving an Exercise with Different Solutions

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  10. R

    When is power constant in this scenario?

    Power P = F x v,where F is force and v is velocity, if power remains constant then i think force can not remain constant as it will change the velocity v, but the solution I found is, F = ma, v = at, so, P = F x v = ma x at = ma2 t, after that calculus comes to show that displacement is...
  11. A

    The Work of Friction: Explained in .32m

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  12. MatinSAR

    Work and Energy for a mass sliding down a curved ramp (why does the ramp move?)

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  13. M

    A Calculate work done by a time-dependent pressure to a spherical hole

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  14. M

    B Which Energy Law is the Key to Understanding Work and Conservation?

    Hi, everyone! There are a lot of work energy conservation laws and I get confused which one of them summarizes all of them. Which one I should keep with me and rest should be easy to derive on spot ? 1. ##\Delta E_{mec}=0## 2. ##\Delta E_{mec}=W_{ext}## 3.##\Delta E_{mec} + \Delta...
  15. M

    I Principle of Physics: Derivation of -dU/dx=F

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  16. A

    Work and energy of electromagnetic wave with intensity I

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  17. A

    Finding kinetic energy and initial velocity of a cart over time

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  18. rudransh verma

    B What is the need for the concepts of Work and Energy (KE)?

    What is the need to introduce the concept of work and energy when the motion can be completely understood by the concept of force and acceleration and momentum and velocity and displacement, etc? Why do we need to understand the same thing once again in terms of Work and energy? Also the kinetic...
  19. rudransh verma

    B About verification on Kinetic energy and work

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  20. L

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    Hello, I’ll start by saying I have the answers and the steps to the solutions, but there’s a comprehension disconnect somewhere that I’m trying to figure out. There are two parts to my question but the second one may not apply depending on the answer to the first. I wasn’t sure from the forum...
  21. MattGeo

    I Work Done/Energy Transferred in One Dimensional Collision

    I spend a lot of time thinking about collision problems because for me they are both extremely interesting and often very difficult to grasp when one thinks about them beyond the basics we are taught in introductory or even intermediate university courses. Suppose there is a perfectly elastic...
  22. bluesteels

    Does work = neg or pos change in potential energy?

    u = (9*10^9)(1.61*10^-19)^2 * (1/[3*10^-15 ]- 1/[2*10^-10]) u = 7.68*10^-14 J but here the question. I have been taught that W= -U so shouldn't the answer be negative?? When i look up at the solution all other sources say that the W = U and therefore the answer is in postive.
  23. hayke101

    What is the Conservation of Energy in Spring Work at an Angle?

    ..
  24. L

    B Work and energy: conceptual doubt

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  25. Rikudo

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  26. JackyCheukKi

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  27. Max2020

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  28. Soyuz42

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  29. E

    Why is PV work in thermodynamics so difficult to understand?

    i can't manage to grasp the concept of PV work in thermodynamics, for example we all know that W= integral(F*dx) like here but this says that, at the end, W doesn't really depend on the gas temperature or reversible process crap at the end W is simply a constant, atmospheric pressure is...
  30. mattlfang

    Find the velocity and acceleration of a pulley in a mass-spring system

    This looks like a classical setup but I can't find a solution. We can calculate the energy of the system by looking at the work done by the gravity and the spring. But how do we divide the energy between the kinetic energy of the pulley and the rotation of the pulley?
  31. Like Tony Stark

    Conservative forces vs friction

    Hello I've written that homework statement as an example to illustrate my doubt: How can I tell if a force is conservative or not? I've read that, if the curl of the force is 0, it's conservative. But what about the friction force (##f=\mu N##)? Its curl is also zero, but it's not conservative...
  32. E

    Work & energy VS conservation of angular momentum

    Summary:: Would energy method give us a different answer from conservation of angular momentum? Hello, I do not know how to type equations here. So, I typed my question in Word and attached it here. Please see photos. Note: This question is not a homework. I did not find it in textbooks or...
  33. Rahulx084

    How Does Joule Thomson Expansion Affect Gas Temperature?

    Wikipedia says, Unlike a free expansion , in Joule Thomson expansion work is done causing the change in internal energy. Whether the internal energy increases or decreases is determined by whether work is done on or by the fluid; that is determined by the initial and final states of the...
  34. R

    Variable friction on an inclined plane and maximum velocity

    This problem was from the chapter on Work and Energy so, I thought of using the principle of conservation of mechanical energy. Clearly, the potential energy of the block decreases by mgh (assuming the block has mass m). This energy should have been converted to kinetic energy, but it clearly...
  35. ItsukaKitto

    Maximum height a block reaches after separating from a curved moving ramp

    Diagram attached at the endI personally think there's something wrong with this question, and I'd like if someone can tell me whether it's the question that is wrong or my approach. If I attempt the solution thinking that M should be stationary, the solution is simple. 0 - 1/2 mv^2 = -mgh...
  36. P

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  37. kuruman

    Work is not done by static friction when accelerating a car

    A recent thread posed the question whether work is done by static friction in the case of an accelerating car. Before I had a chance to reply, the thread was closed on the grounds that the subject was "beaten to death". Undaunted, I am determined to deliver the coup de grâce here with a simple...
  38. Prabs3257

    Velocity of an object on an inclined plane

    I used work energy theorem between initial top point and point x along the incline(downwards) i got the expression of v then diffrentiated it to get a maxima but it gives me a wrong ans which is 10/6 but the actual ans is 10/3 please tell me what i did wrong
  39. R

    Projectile Motion : Work and Energy

    The problem is based on a projectile-spring launcher. A ball is loaded into a tube that pushes back a spring and is then launched. The ball was launched straight horizontally not at an angle. I'm trying to find the work done on the ball by the spring. The info I have: Displacement of spring =...
  40. RyanRhino

    Calculate the average force used to drag a box up a ramp

    I first wrote down that 55% = Eout/Ein I also know that W = (Facos20)(4) and I substitute it into the first equation 55% = Eout/[(Facos20)(4)] But I'm missing two variables here. Did i forget something or is the question missing some information?
  41. babaliaris

    Why does voltage = energy/charge?

    When you say ##i = \frac{dq}{dt}## it makes sense since current is the flow of charge over time. But why was voltage defined as ##v = \frac{dw}{dq}## ? What made physicians define it in this way? Is there a mathematical way that can lead to this definition or did they define voltage just on the...
  42. M

    A Motorcyclist crossing a stretch of land

    TA = Ec 0,7*m*g*d = (m*v² /2) 1,4*g*d = (400/12,96) 14*d = (400/12,96) d = 2,2 m No
  43. A

    Work and energy of an object on a ice slope

    Homework Statement: A perfect hemisphere of frictionless ice has radius R=7 meters. Sitting on the top of the ice, motionless, is a box of mass m=7 kg. The box starts to slide to the right, down the sloping surface of the ice. After it has moved by an angle 11 degrees from the top, how much...
  44. A

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    Homework Statement: Hetsut is the foreman of a construction project in ancient Egypt. He needs to move a giant block of stone, of mass 12 metric tons, from the docks to the temple grounds. He can go along the roads by traveling 295 meters east, then 89 meters north. Along the roads, the...
  45. A

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    Homework Statement: A perfect hemisphere of frictionless ice has radius R=6.5 meters. Sitting on the top of the ice, motionless, is a box of mass m=6 kg. The box starts to slide to the right, down the sloping surface of the ice. After it has moved by an angle 20 degrees from the top, how much...
  46. A

    Total work of a directional wind on a mailman

    Homework Statement: Mike the Mailman takes his oath seriously: "Neither snow, nor rain, nor heat, nor gloom of night stays these courageous couriers from the swift completion of their appointed rounds". Even though a blizzard is raging outside, he goes out to deliver the mail. He makes four...
  47. SLTH02

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    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
  48. KingOfDirewolves

    Classical Mechanics Problem Based On Conservation Of Energy

    I have tried using Conservation of Energy but I'm getting incorrect answer.
  49. naji0044

    Inelastic collision problem: Bullet striking a wood block

    Hey everyone I'm struggling on the last part of this assignment. I need to find the total work done by the block and the bullet, when the collision happens. The informations is: mblock=0.3 kg mbullet=0.01 kg vg=700 m/s Height=0.72m The final speed after the collision is vf=22.6 m/s and the...
  50. Don Bori

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    Based on my understanding, Top Tank Refilling Advantage: Atmospheric Pressure Disadvantage: High Head (Requires more distance, thus more Work since W = f x d) Bottom Tank Refilling Advantage: Low Head Disadvantage: High Static Pressure (Requires more Force, thus more Work since W = f x d)...
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