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Work and Power

Work and Power. Chapter 5!! The End of the ROAD is near…..!!. WORK. Work : a transfer of energy that occurs when a force makes an object move over a distance a. Two requirements for work : 1. Applying a force to an object HAS to

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Work and Power

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  1. Work and Power Chapter 5!! The End of the ROAD is near…..!!

  2. WORK • Work: a transfer of energy that occurs when a force makes an object move over a distance a. Two requirements for work: 1. Applying a force to an object HAS to make it move; if it doesn’t move no work is done 2. The distance the object moves MUST be in the same direction as the force applied to the object

  3. WORK NO WORK WORK Direction of applied force Direction of applied force Direction of motion Direction of motion Carrying a Box Lifting a Box

  4. What’s Work? • A scientist delivers a speech to an audience of his peers. • A body builder lifts 350 pounds above his head. • A mother carries her baby from room to room. • A father pushes a baby in a carriage. • A woman carries a 20 kg grocery bag to her car.

  5. What’s Work? • A scientist delivers a speech to an audience of his peers. NO • A body builder lifts 350 pounds above his head. YES • A mother carries her baby from room to room. NO • A father pushes a baby in a carriage. YES • A woman carries a 20 kg grocery bag to her car. NO

  6. WORK b. Equation: Work = Force x distance c. Unit: Joule (J) = N x m W F d

  7. Practice Calculations • The force needed to lift an object is equal in size to the gravitational force on the object. How much work is done in lifting an object with a mass of 5 kg a vertical distance of 2.0 m? • A lawn mower is pushed with a force of 80 N. If 12,000 J of work are done in mowing a lawn, what is the total distance the lawn mower was pushed? m = 5 kg d = 2.0 m W = ? How much work if the object is then carried a horizontal distance of 2m? W = F x d W = 49 N x 2.0 m None! Force applied is not in the same direction as movement W = 98 J F = 5 kg x 9.8 m/s/s = 49 N d = W F F = 80 N W = 12,000 J d = ? d = 150 m d = 12,000J 80 N

  8. POWER W P t

  9. POWER • 1000 watts = 1 kilowatt • How does a light bulb get its power rating? a 50 w light bulb does 50 J of work per second e. 1 horsepower= 750 watts

  10. Check for Understanding • Two students, Ben and Bonnie, are in the weightlifting room. Bonnie bench-presses the 50 kg barbell (a distance of 0.6m) 10 times in 1 minute. Ben bench-presses the 50 kg barbell the same distance 10 times in 10 seconds. • Which student does the most work? • Which student delivers the most power?

  11. Check for Understanding • Ben and Bonnie do the same amount of work. They apply the same force to lift the barbell the same distance. • Ben is more powerful since he does the same amount of work in less time.

  12. Practice Calculations • In lifting a baby from a crib, 50 J of work are done. How much power is needed if the baby is lifted in 2 s? • The power produced by an electric motor is 500 W. How long will it take the motor to do 10,000 J of work? 3. How much work can be done by a 150 horsepower engine in 1 minute? (HINT: Use dimensional analysis to convert horsepower to watts and be careful with the time) P = W t P = 25 w P = 50 J 2 s W = 50 J t = 2 s P = ? t = 20 s t = W P t = 10,000 J 500 W P = 500 W W = 10,000 J t = ? W = ? P = 150 horsepower t = 60 s W = P x t W = 112,500 w x 60 s x 750 w = 112500 w 1 horsepower W = 6,750,000 J

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