What is Work energy: Definition and 186 Discussions

In physics, energy is the quantitative property that must be transferred to a body or physical system to perform work on the body, or to heat it. Energy is a conserved quantity; the law of conservation of energy states that energy can be converted in form, but not created or destroyed. The unit of measurement in the International System of Units (SI) of energy is the joule, which is the energy transferred to an object by the work of moving it a distance of one metre against a force of one newton.
Common forms of energy include the kinetic energy of a moving object, the potential energy stored by an object's position in a force field (gravitational, electric or magnetic), the elastic energy stored by stretching solid objects, the chemical energy released when a fuel burns, the radiant energy carried by light, and the thermal energy due to an object's temperature.
Mass and energy are closely related. Due to mass–energy equivalence, any object that has mass when stationary (called rest mass) also has an equivalent amount of energy whose form is called rest energy, and any additional energy (of any form) acquired by the object above that rest energy will increase the object's total mass just as it increases its total energy. For example, after heating an object, its increase in energy could be measured as a small increase in mass, with a sensitive enough scale.
Living organisms require energy to stay alive, such as the energy humans get from food. Human civilization requires energy to function, which it gets from energy resources such as fossil fuels, nuclear fuel, or renewable energy. The processes of Earth's climate and ecosystem are driven by the radiant energy Earth receives from the Sun and the geothermal energy contained within the earth.

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  1. R

    Work Energy Theorem Problem

    Homework Statement A weapon fired a 25.8-kg shell with a muzzle speed of 880 m/s. What averageforce acted on the shell?Homework Equations work energy theorem? The Attempt at a Solution Can someone please explain where the 6.00 comes from? Starting from rest, the shell acquired a kinetic...
  2. D

    Block Sliding, might use work energy theorum

    Homework Statement A block of mass 4.2 kg which has an initial velocity of 3.3 m/s at time t=0, slides on a horizontal surface. If a constant friction force of 10.2N is exerted on the block by the surface, what is the acceleration? Homework Equations Work Energy Theorum? .5 m vo2 -...
  3. B

    Mass on an inclined plane w/ FRICTION (use work energy theorem)

    Mass on an inclined plane with friction! There is a mass at the bottom of an inclined plane. It travels with an initial velocity up the inclined plane at an angle θ. There is a coefficient of friction on the ramp. How far up the ramp will the mass go before stopping? What is the speed of the...
  4. N

    Work & Power of Pulley System Homework

    Homework Statement 20 Kg box is raised 0.75m by a constant 25N force applied at the end of the cord if the pulleys are massless, determine work done on the crate. What is the power delivered at the end of the cord at this point? Identical pulley system...
  5. M

    Work energy theorem and gravitational potential energy

    In my book it is talking about conservation forces and work energy theorem. In the book it said... "suppose gravitational potential energy alone does work (constant force) *Then Wnet = Wg ... ending statement Ef = Ei... My question... Does the work energy theorem only deal with...
  6. M

    Work Energy Theorem: Explaining Constant Force and Acceleration

    I posted a picture of the paragraph that I am confused about... The following paragraph says the body accelerates even though the forces are constant... can some one explain why the acceleration is not zero?
  7. J

    Kinetic Energy and Work energy theorem

    Homework Statement A toboggan is initially moving at a constant velocity along a snowy horizontal surface where friction is negligible. When a pulling force is applied parallel to the ground over a certain distance, the kinetic energy increases by 47%. By what percentage would the kinetic...
  8. X

    Work Energy Problem Package released to Spring

    Homework Statement A 2.00 kg package is released on a 53.1 degree incline, 4.00m from a long spring with force constant 120 N/m that is attached at the bottom of the incline. The coefficients of friction between the package and the incline are u{s} = 0.40 and u{k} = 0.20. The mass of the...
  9. M

    Finding the Distance a Mass Travels Up an Incline with Friction

    Homework Statement Hey all! I was given a multi-step problem and solved everything but the last step. Okay so, a mass on a frictionless plane by a string at an angle theta for a distance until it reaches an incline that has friction and is parallel to the string. The problem asks to find the...
  10. V

    What are limitations of work energy theorem ?

    what are the limitation work energy theorem ? this theorem is very helpful in solving questions. But I think there are some limitations of this bold theorem. that is (1) does not define the direction of velocity & (2) does not define spontaneity of process, like say there is an body on the...
  11. O

    The work energy theorem in polar coordinates

    Homework Statement Mass m whirls on a frictionless table, held to circular motion by a string which passes through 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...
  12. R

    Work energy thm and conservation

    Since the change of kinetic energy, K2-K1, ALWAYS equals the integral of F.dr along any path, how can that integral depend upon the path? I realize that the integral is ONLY equal to the change in potential energy (F is the derivative of the potential) at the end points when F is a conservative...
  13. C

    What is the Total Work Done to Reach a Speed of 60 mph?

    if a cars mass is 1604 kg and it is traveling at a speed of 60 mph ...then it's KE is .5 X 1604 X 60 squared = a KE of 2,88,200 J ...now...my question is...using the work energy theorem - what is the TOTAL WORK done to get to a speed of 60 mph ??
  14. D

    Loop-de-loop work energy problem

    Homework Statement A particle of mass m slides along a frictionless track. It starts from rest at height h above the bottom of the loop of radius R. A) what is the minimum value of h(in terms of R) such that the particle completes the loop? If the particle starts from height h=3.5R, find the...
  15. L

    What is the work energy theorem for a car at different distances?

    1. Homework Statement _> Suppose the 2.0kg model car is initially at rest at x=0 and F is the net force acting on it. Use the work energy theorem to find the speed of the car at (a) x=3.0m, (b) x=4.0m, and (c) x=7.0m...
  16. E

    Work energy question. Rock down a hill.

    Work energy question. Fish on a spring. Question has been changed as I figured it out. New one is on fish and springs Homework Statement If a fish is attached to a vertical spring and slowly lowered to its equilibrium position, it is found to stretch the spring by an amount d. If the...
  17. S

    Can the force of 1.50N accelerate a 0.20kg cart to a speed of 6.70m/s?

    Work energy or Newton's law? I have a question and solution(from book) too: Question: A force of 1.50N acts on a 0.20kg cart so as to accelerate it along an air track. The track and force are horizontal and in line. How fast is the cart going after acceleration from rest through 30cm, if...
  18. D

    Please check my work energy required to raise temp by 10degthanks

    Homework Statement You want to heat 1 L of water by 10 ºC, and so you heat it with a mechanical egg beater that is powered by a 1 kg mass on a rope over a pulley. How far does the mass have to descend to supply enough work for heating the water? Homework Equations |work| = mg(x_f - x_0)...
  19. M

    Can speed be a imaginary number validity of work energy theoram in 1D

    Consider mass m_{1}and m_{2}with position vector (from an inertial frame) \overrightarrow{x_{1}} and \overrightarrow{x_{2}} respectively and distance between them be x_{0}. m_{1}\frac{d^{2}}{dt^{2}}\overrightarrow{x_{1}}=\overrightarrow{F} \Rightarrow...
  20. V

    Work Energy Theorem: Find Speed of Block Descend

    Hello :) Homework Statement <Q>A wheel of moment of inertia "I" and radius "R" is free to rotate about its center as shown in figure http://s1102.photobucket.com/albums/g448/vissh/?action=view&current=pulley-1.jpg" .A string is wrapped over its rim and a block of mass m is attached to the...
  21. J

    Work Energy Theorem Question, Relatively Easy

    Homework Statement The left side of the figure shows a light (`massless') spring of length 0.320 m in its relaxed position. It is compressed to 74.0 percent of its relaxed length, and a mass M= 0.230 kg is placed on top and released from rest (shown on the right). The mass then travels...
  22. M

    I dont understand some part of work energy theorem

    Homework Statement \SigmaW = \DeltaE so if we have an elevator getting pulled by force equal to it's weight making it move with a constant velocity then the net force is equal to 0 so the sigma W is equal to zero but isn't there non zero change in potential energy ? Homework Equations...
  23. K

    Work, Energy, Power Homework: A Lift of Mass 400kg & 70kg Person

    Homework Statement g= 9.8 Nkg-1 Q. A lift has a mass of 400 kg. A man of mass 70 kg stands on a weighing machine fixed to the floor of the lift. Four seconds after starting from rest the lift has reached its maximum speed and has risen 5 m. a) What...
  24. A

    What is the speed of a block pulled by a spring with friction?

    Homework Statement A block with mass 1.50 kg is attached as shown to a spring with a force constant of 456.0 N/m. The coefficient of kinetic friction between the block and the surface on which it slides is 0.210. The block is pulled 3.60 cm to the right of its equilibrium position and then...
  25. D

    What is the minimum stopping distance of a truck with locked brakes?

    Homework Statement A truck is moving on a level street with speed Vo when the brakes lock (the tires don't roll and skid). Find the minimum stopping distance of the truck in terms of Vo, g, and the coefficient of kinetic friction μk. Homework Equations W = F*d W = K2-K1 K = 1/2mv2...
  26. Z

    Work Energy Momentum Homework: Find Plank Velocity After Block Departs

    Homework Statement A plank of mass 5 kg is placed on a frictionless horizontal plane. Further a block of mass 1 kg is placed over the plank. A massless spring of natural length 2 m is fixed to the plank by its one end. The other end of the spring is compressed by the block by half of...
  27. H

    Calculating Daily Operating Cost for a Motor at Rated Load

    Homework Statement a motor operates at its rated load of 10 hp for 8.0 h a day. its efficiency is 87 percent. what is the daily cost of operation if electric energy costs 5 cents/kWh? Homework Equations The Attempt at a Solution i convert 10 hp to watts 10 x 746=7460 watts...
  28. M

    Work Energy Problem Homework: Find v'B, W & in eV

    Homework Statement A particle has a mass of m = 1.8*10^-5 kg and a charge of q = +3.25*10^-5 C. It is released from point A with an initial speed of v'A (v of A) = 3.5 m/sec and accelerates until it reaches point B. The charged particle moves along a straight line and does not rotate. The...
  29. M

    Work energy principle vs Newtons law

    Why do we choose to use the work energy principle instead of the Newtons law to analyze the motion of a system of rigid bodies?
  30. R

    How Fast Will Ms. Reach the Top of the Ramp?

    Your cat "Ms." (mass 7.00 kg) is trying to make it to the top of a frictionless ramp 2.00 m long and inclined upward at 30.0 degrees above the horizontal. Since the poor cat can't get any traction on the ramp, you push her up the entire length of the ramp by exerting a constant 100 N force...
  31. C

    Work energy theorem for falling object

    Homework Statement A branch falls from the top of a 95.0 m tall redwood tree, starting from rest. How fast is it moving when it reaches the ground?Homework Equations Total work = \Delta KThe Attempt at a Solution I have no idea how to do this problem.
  32. P

    Work-Energy Theorem: Determine Distance for 73kg Skier w/ 4.2m/s

    Homework Statement A 73kg skier coasts up a hill inclined at 9.3 degree's to the horizontal. Friction is negligible. Use work-energy theorm to determine how far along the hill the skier slides before stopping, if the intial speed at the bottom is 4.2m/s To find the distance I know i need...
  33. S

    Work Energy: block w/ friction

    Homework Statement Find the initial speed of a block which travels 10m along a horizontal surface if \mu= 0.40 between the block and the surface before stopping. Homework Equations \SigmaW = 1/2mvB2 - 1/2mvA2 + mgyB - mgyA \SigmaW = W * dAB * cos (W, dAB) f = \mu* N N=...
  34. P

    Work Energy and Power Question

    Thanks in advance, I just want to double check! Homework Statement A 5 kg ball is tossed into the air with an initial velocity of 6m/s. a) What is the kinetic energy of the ball? b) If the power delivered to the ball was 100watts, over what time interval did the power...
  35. M

    Work energy theorem vs Newton's 2nd law and kinematics

    So i have this general question. When the situation arises in which there are velocity and/or position depended forces acting on an object, would it be generally easier to use the work energy theorem or Newton's 2nd law & kinematics in predicting the motion? My guts tell me it will be work...
  36. H

    Pulley and Work Energy problems

    Well, I took a Physics midterm this morning, I had a bit of trouble with a few of the questions and was hoping I could get some help here. I did what I thought to be right, but as I really only started picking up physics concepts just recently, I am a bit weary on if I actually computed them...
  37. P

    Work energy theorem involving a pulley

    So, yeah, my midterm is on friday, and i can't even figure out how to do questions like these because i just don't understand the way my prof teaches and the textbook has nothing relevant. So far, I've used work energy theorem to find the work done to stop the block. then i wrote out a net work...
  38. I

    Velocity from Kinetic energy and work energy theorem

    1. A 0.066 kg arrow is fired horizontally. The bowstring exerts an average force of 50 N on the arrow over a distance of 0.95 m. With what speed does the arrow leave the bow? 2. Work Energy Theorem = change in kinetic energy = (1/2*mass*Final Velocity^2)-(1/2*mass*Initial Velocity^2)...
  39. I

    Work Energy Theorem, Kinetic Energy, and Tension

    1. A rescue helicopter lifts a 90 kg person straight up by means of a cable. The person has an upward acceleration of 0.70 m/s2 and is lifted from rest through a distance of 10 m. What is the tension in the cable and how much work is done by the tension in the cable? Use the work-energy theorem...
  40. G

    Solve Work Energy Power Homework with Friction Present

    Homework Statement Hi Guys I have a problem when working with conservation of energy and work energy theorem. I have a problem when friction is present in the system. I don't know when to add the friction to get the total work done or when to minus the friction to find work done. Will...
  41. G

    Understanding the Work-Energy Theorem

    Hi I just can't seem to grasp this concept! Anyone have like a small tutorial for this section or notes on it? Im in grade 12.. For example a question from the exam... A person skis down a 20m long snow slope which makes an angle of 25degrees with the horizontal. The total mass...
  42. N

    Work energy theorem to find the maximum compression

    Homework Statement A 6.0 kg box moving at 3.0 m/s on a horizontal, frictionless surface runs into a light spring of force constant 75N/cm. Use the work energy theorem to find the maximum compression of the spring. Homework Equations Wtotal=K2-K1 k1=1/2mv^2=27 J The Attempt at a...
  43. N

    Momentum and work energy question

    Homework Statement Two kids on roller skates push off each other in opposite directions. One weighs 36kg and he rolls 15m before stopping, the other weighs 24kg, how far does he move before stopping? Assume they experience the same frictional force by the path - assume friction force is...
  44. I

    Work Energy Theorem and Circular Motion

    Homework Statement A car is coasting without friction toward a hill of height 'h' and radius of curvature 'r'. What initial speed will result in the car's wheels just losing contact with the roadway as the car crests the hill? Homework Equations Kinetic Energy = (1/2)(m)(v^2) Potential...
  45. C

    Understanding the Net Work Energy Concept: Defining F in Work Equations"

    I've been confused about work. I know it equals F x D. But what is F? Is it the netforce or the force being exerted. If you are pushing up an object up an incline with 200N and the opposing force of gravity + friction is 100N for D=20m. Is Work, Fnet x D or 200N x D? Does the same apply...
  46. J

    Work Energy Theorem of a sled

    I don't really have any clue on this problem... Homework Statement A sled with mass 8.00 kg moves in a straight line on a frictionless horizontal surface. At one point in its path, its speed is 4.00 m/s. after it has traveled 2.5m beyond this point its speed is 6.00 m/s. Use the work...
  47. N

    Work Energy Theorem of a fired arrow

    Homework Statement A 0.065 kg arrow is fired horizontally. The bowstring exerts an average force of 70 N on the arrow over a distance of 0.90 m. With what speed does the arrow leave the bow? Homework Equations W=Fd W=KE final - KE initial KE= 1/2mv^2 The Attempt at a Solution...
  48. B

    Finding Velocity Using Work-Energy Principle

    Homework Statement The system of two blocks is released from rest. The pulley is a solid cylinder of mass 5.00kg and radius of 10.0cm. MA is 10.0kg and MB is 20.0kg. The coefficient of kinetic friction between MA and the table is 0.2. Find the velocity of the blocks after moving 3.00m...
  49. T

    Calculating the Length of a Rough Patch Using the Work-Energy Principle

    Homework Statement On an essentially frictionless horizontal ice-skating rink, a skater moving at 3.00 m/s encounters a rough patch that reduces her speed by 45.0 % to a friction force that is 25.0 % of her weight. Use the work-energy principle to find the length of the rough patch...
  50. S

    Work energy theorem formula

    Homework Statement a 20N force acts on a 2 KG object for 3 seconds. If the object was initally at rest, How much work was done in this process? I am having problems working this formula with no distance involved W = Fd Homework Equations The Attempt at a Solution
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