What is Work-energy theorem: Definition and 95 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. T

    Finding the speed before impact using the work-energy theorem

    Homework Statement A blue whale materializes 11.5 km above an airless planet with an acceleration of gravity of 6.85 m/s2. What is the speed of the whale right before impact? Use the work energy theorem. Homework Equations Work energy theorem : W = the change in k W=kf - ko w=1/2(mvf2)...
  2. R

    Work-energy theorem and resistive forces

    Homework Statement A skier slides down a hill, starting from rest at a height of 250m above the bottom of the hill. She skis over an intermediate hill, whose height is 100m above the bottom of the hill. If resistive forces are neglected, what is the speed of the skier a) at the top of the...
  3. R

    Work-energy theorem and Tension Force

    Homework Statement While running at a speed of 5.0m/s Tarzan grabs a 9.0m long vine hanging vertically from a tall tree in the jungle and swings on it. As he swings on the vine, how much work is done on him by the tension force in the vine? To what maximum vertical height can he swing if he...
  4. A

    How Does the Work-Energy Theorem Determine the Initial Speed of a Sledgehammer?

    Homework Statement 490 J of work is done on a pin when a 5 kg sledgehammer strikes it (bringing the sledgehammer to rest). What is the initial speed of the sledge hammer. Homework Equations (Fnet)(d)= (0.5)(m)(velocity final^2 - velocity initial^2) The Attempt at a Solution...
  5. J

    Work-Energy Theorem: Homework Statement & Solution

    Homework Statement A mass m whirls on a frictionless table, held to circular motion by a string which passes though a hole in the table. The string is slowly pulled through the hole so that the radius of the circle changes from l1 to l2. Show that the work done in pulling the string equals...
  6. K

    What is the Work-Energy Theorem for a Skier Crashing into Netting?

    Homework Statement A skier comes crashing into the netting at the bottom of a ski hill. The skier has a mass of 85 kg and is moving at a speed of 65 km/h (234 m/s). (a) How much work is done by the netting while bringing the skier to rest? (b) If the "spring" constant for the netting is...
  7. J

    Calculating Kinetic Energy and Average Force in Projectile Throws

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  8. 1

    Work-energy theorem , not sure what I am doing wrong.

    On an essentially frictionless, horizontal ice rink, a skater moving at 6.0 m/s encounters a rough patch that reduces her speed by 42 \% due to a friction force that is 20 \% of her weight. Use the work-energy theorem to find the length of this rough patch. i do: [.42 * (.5) * (6)^2]/ [.2...
  9. C

    Work-Energy Theorem / Finding The Mass

    Homework Statement It takes 241 kJ of work to accelerate a car from 22.0 m/s to 28.8 m/s. What is the car's mass? W = 241000 J vo = 22.0 m/s vf = 28.8 m/s m = ? Homework Equations KE = 1/2 mv2 W = KEf - KEo The Attempt at a Solution W = KEf - KEo W = 1/2 mvf2 -...
  10. L

    Work-energy theorem problem - I have it close to correct

    1. You are a member of an alpine rescue team and must project a box of supplies, with mass m, up an incline of constant slope angle alpha so that it reaches a stranded skier who is a vertical distance h above the bottom of the incline. The incline is slippery, but there is some friction present...
  11. I

    Derivation of Work-Energy Theorem

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  12. S

    Work-Energy Theorem: Effect of increasing speed vs stopping distance.

    Homework Statement If the speed of a car is increased by 50%, by what factor will the minimum braking distance be increased assuming all else is the same? Ignore the driver's reaction time. Homework Equations W = Fd \cdot cos\theta W = \Delta{E_k} The Attempt at a Solution...
  13. C

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  14. D

    Work-Energy theorem and kinetic energy pertaining to a car

    Homework Statement An 1,100 kg car is traveling 46 km/h on a level road. The brakes are applied long enough to remove 51 kJ of energy. A. What is the final speed of the car? B. How much more kinetic energy must be removed by the brakes to stop the car? C. Is this energy really being...
  15. K

    Work-Energy Theorem: Calculating Work Done on a Sliding Block

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  16. B

    Work-Energy Theorem: Find Force on 16g Bullet Accelerated to 1010m/s

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  17. H

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  18. Q

    Work-Energy Theorem in Inertial Reference Frames

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  19. L

    Work-energy theorem in relation to the velocity of two masses

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  20. N

    Deriving the Work-Energy Theorem: A Calculus Refresher

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  21. D

    Work-Energy Theorem: Bar, Resistor, & Magnetic Field

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  22. W

    Work-energy theorem, no equations

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  23. H

    How to Write Formulas in Physics Forums

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  24. O

    Physics help using Work-Energy theorem

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  25. clope023

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  26. A

    Another work-energy theorem problem

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  27. A

    Homework Problem about the Work-Energy Theorem

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  28. A

    Confusion about the Work-Energy Theorem

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  29. P

    Finding the masses of two blocks in a pulley system using work-energy theorem

    Two blocks with different mass are attached to either end of a light rope that passes over a light, frictionless pulley that is suspended from the ceiling. The masses are released from rest, and the more massive one starts to descend. After this block has descended a distance 1.00m , its speed...
  30. P

    Maximum height reached by a ball using work-energy theorem

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  31. V

    Calculating Stopping Distance Using the Work-Energy Theorem

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  32. D

    KE Work-Energy Theorem Problem question

    I am compleatly lost on these two KE Work-Energy Theorem Problem questions any feedback or help would be greatly appreciated. Thank you in advance... 1.) 47.0 g golf ball is driven from the tee with an initial speed of 52.0 m/s and rises to a height of 25.0 m. (a) Neglect air resistance...
  33. mbrmbrg

    Simple problem using Work-Energy Theorem

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  34. H

    Work-Energy Theorem rock throw Question

    This is the problem: You throw a 20-N rock vertically into the air from ground level. You observe that when it is 15.0 m above the ground, it is traveling at 25.0 m/s upward. Use the work-energy theorem to find a) its speed just as it left the ground; b) its maximum height. I drew a...
  35. S

    Work-Energy Theorem calculations

    Please check to see if my calculations are correct. I am unsure if I used the right values and setup. You throw a rock of weight 21.0 N vertically into the air from ground level. You observe that when it is a height 14.8 m above the ground, it is traveling at a speed of 25.7 m/s upward...
  36. D

    Problems On Work-Energy Theorem

    i have a Qns. given that i have a 2kg mass on Earth and i want to accelerate from 0 m/s to 10 m/s the work done should be : Workdone = 1/2(m)(V)^2 - 1/2(m)(v)^2 = 1/2(2)(10)^2 -0 = 100 J however, if i were to take my frame of reference from let say space i...
  37. J

    What is the Total Work Equation for Changing an Object's Vertical Position?

    Hello, If the work changes the object's velocity through a horizontal distance then: W = delta KE But, what if the work changes the object's vertical position. Then the object's mph changes. Would the Work formula be: W = delta PE? But, if something is moving upwards (y axis) then...
  38. R

    Fallacy in Work-Energy theorem?

    Fallacy in Work-Energy theorem?? Consider a block moving on a floor which has some friction. It is given an initial velocity of 10 m/s and it comes to rest after some time due to friction. Since friction slows the block down, heat is generated, some of which goes into raising the temperature of...
  39. M

    2 Things: Work-Energy theorem and parabolic trajectories.

    Hi, When reading through the work-energy thread, i just got reminded of something bugging me for quite a while. I don't think this is mentioned in the work-energy thread, firstly, how can we show that, \int f \bullet ds = \Delta E also, I've heard that there's a way to show that...
  40. P

    Work-Energy Theorem: Find Final Speed of Ship

    I'm having trouble with where to begin with this question, if anyone has guidance I'd be grateful. A spaceship of mass 5.00 x 10^4 kg is traveling at a speed 1.15 x 10^4m/s in outer space. Except for the force generated by its own engine, no other force acts on the ship. As the engine...
  41. B

    Deriving the work-energy theorem

    In this problem, you will use your prior knowledge to derive one of the most important relationships in mechanics: the work-energy theorem. We will start with a special case: a particle of mass m moving in the x direction at constant acceleration a. During a certain interval of time, the...
  42. P

    Inclined Plane Friction Question

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  43. B

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  44. O

    Tackling the Work-Energy Theorem

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  45. S

    Proton AND Work-Energy Theorem

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