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Understanding Work and Power in Physics

Learn about the concepts of work and power in physics, including their definitions, equations, and units. Explore sample problems and calculations to deepen your understanding.

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Understanding Work and Power in Physics

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  1. Work is only done by a force on an object if the force causes the object to move in the direction of the force. Objects that are at rest may have many forces acting on them, but no work is done if there is no movement.

  2. Work, by definition, is the product of the force exerted on an object and the distance the object moves in the direction of the force. W=F·d Work is a scalar quantity.

  3. The SI unit of work is the Joule, named in honor of James Prescott Joule. One Joule, J, of work is the work done when 1.0 N of force is applied through a distance of 1.0 m.

  4. If the force and displacement are not in the exact same direction, then Work = Fd(cosq), where q is the angle between the force direction and displacement direction. F =40 N d = 3.0 m The work done in moving the block 3.0 m to the right by the 40 N force at an angle of 35 to the horizontal is ... W = Fd(cosq) = (40N)(3.0 m)(cos 35°) = 98 J

  5. The force must be applied in the same direction as the displacement to be considered work. • Ex. Is a man carrying a heavy box doing work on the box while moving forward? NO!

  6. Power, by definition, is the time rate of doing work; or the time rate transfer of energy. P=W/t Power is ascalarquantity.

  7. The SI unit of power is the Watt, named in honor of James Watt. One Watt, W, of power is the power achieved when 1.0 J of work is done or 1.0 J of energy is transferred in a time of 1.0 s.

  8. Sample Work Problems: W = F d • How much work is done if a force of 15 N is used to push a box horizontally a distance of 4 m? • W = (15) (4) W = 60 J • 2) If it takes 20 J of work to lift a 10 N object, how far was it lifted? • 20 = (10) (d) d = 2 m • Bill does 100 J of work to push a box 5 m. What force was used? • 100 = F (5) F = 20 N

  9. Sample Power Problems: P = W • t • If you do 50 J of work in 5 seconds, what is your power output? • P = 50/5 P = 10 W • 2) If your power output is 100 W, how long did it take you to do 200 J of work? • 100 = 200/t t = 2 sec • If your power output is 250 W, how much work did you do in 5 seconds? • 250 = W/5 W = 1250 J

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