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Work and Power

Work and Power. The word “WORK”. What does it mean in everyday life? “Work” has a specific definition in science. Remember: Newton’s 1 st Law. An object only begins moving when an unbalanced force acts on it. Net force > 0. Work.

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Work and Power

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  1. Work and Power

  2. The word “WORK” • What does it mean in everyday life? • “Work” has a specific definition in science.

  3. Remember: Newton’s 1st Law • An object only begins moving when an unbalanced force acts on it. • Net force > 0

  4. Work • Work is done when a force acts on an object in the direction the object moves. • Work = force x distance • Work requires motion • If there is no movement, no work is done.

  5. Force must act in SAME DIRECTION as motion • Any part of a force that does not act in the direction of motion does no work on an object. • Examples

  6. Which ones require work? • Ronnie pushes on a wall until he is exhausted. • Timara knocks a book off the table and it falls to the ground. • A rocket accelerates through space. • Doneisha carries her books to class.

  7. Question: • You carry two heavy grocery bags upstairs to your kitchen. • Will you do more work on the bags if you carry them up one at a time? • Explain.

  8. Units of Work • Remember: units of force? • Newton • Remember: units of distance? • meter Work = Force x Distance Work = N x m N x m = 1 Joule

  9. JOULE(J) is the unit for Work

  10. Calculating Work A weightlifter lifts a 1800-N barbell over his head. He has to lift it for 2.0 m. What amount of work is done? F = 1800 N D = 2.0 m W = F x D W = 1800 N x 2.0 m W = 3600 Nm = 3600 J

  11. Practice How much work does a 25-Newton force do to lift a potted plant from the floor to a shelf 1.5 meters high?

  12. Practice A factory machine uses 2600-Newtons of force to move a robotic arm 3.5 meters. How much work does the machine perform?

  13. Power • What does it mean in everyday life? • “Power” also has a specific meaning in science.

  14. Power • The rate of doing work. • How FAST can work get done? • To get work done faster, more power is required. Work Time Power =

  15. Which has more power: • A person with a snow shovel? • A snow blower? • A truck with a snow plow?

  16. Predicting • Two cars have the same weight, but Car A has an engine that provides twice the power of Car B. • Which car can make it to the top of a mountain pass first? • Which car does more work to reach the top?

  17. Units of Power • Remember: units of Work • Joule • Remember: units of Time • Seconds Work Time Power = Joule second = 1 Watt Power =

  18. Watt(W) is theunit for Power.

  19. Example: • 40-Watt light bulb • Requires 40 joules of work for every second that it is lit • Same power as lifting your textbook a height of 1 meter in half a second.

  20. Calculating Power You exert a vertical force of 72 Newtonsto lift a box to a height of 1.0 meter in a time of 2.0 seconds. How much power is used to lift the box? Force x Distance Time Work Time F = 72 N D = 1.0 m T = 2.0 s Power = = 72 N x 1.0 m 2.0 s 72 J 2.0 s Power = = Power = 36 J/s = 36 Watts

  21. Practice • Deshawna ice skates 70 m in 60 seconds with a force of 10 N. How much work did she do? How much Power did she have? • Darane runs up the NLCP stairs in 3 seconds. She uses a force of 20 N and the stairs are 4 meters tall. How much work did she do? How much Power did she have?

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