2008 Work
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2008 Work Presentation Transcript

  • 1.
    • Notes: Work and Power
    • H: Review Worksheet 5.1
  • 2. Work and Power April 6, 2009
  • 3. Objectives
    • Recognize the difference between the scientific and ordinary definitions of work.
    • Identify where work is being performed in a variety of situations.
    • Define and calculate work and power.
  • 4. Work
    • Work is the transfer of energy through motion.
    • In order for work to take place, a force must cause an object to move in the direction of the force.
  • 5. 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?
    No No No Yes Yes
  • 6.
    • There are two factors to keep in mind when deciding when work is being done:
    • something has to move
    • the motion must be in the direction of the applied force.
  • 7. Work is done on the books when they are being lifted, but no work is done on them when they are being held or carried horizontally.
  • 8.  
  • 9. Direction of Force Direction of Motion
  • 10. Direction of motion Direction of Force
  • 11. Formula for work
    • Work = Force x Distance
    • One newton-meter is equal to one joule
    • So, the unit of work is a joule
  • 12. Power
    • Power is the rate at which work is done.
    • Power = Work * /Time
    • * (force x distance)
    • The unit of power is the watt.
  • 13.
    • 1 horsepower = 550 ft lbs / sec or 746 Watts
  • 14. Check for Understanding
    • Two physics students, Ben and Bonnie, are in the weightlifting room. Bonnie lifts the 50 kg barbell over her head 10 times in one minute; Ben lifts the barbell the same distance over his head 10 times in 10 seconds.
    • Which student does the most work?
    • Which student delivers the most power?
  • 15.
    • Ben and Bonnie do the same amount of work; they apply the same force to lift the same barbell the same distance above their heads.
    • Ben is the most powerful since he does the same work in less time.
  • 16.
    • How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s a distance of 10 meters in 5 seconds?
    • Force = Mass x Acceleration
    • Work = Force x Distance
    • Power = Work/Time
  • 17.
    • How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s a distance of 10 meters in 5 seconds?
    • Force = Mass x Acceleration
    • Force = 10 kg x 2 m/s/s
    • Force = 20 N
    • Work = Force x Distance
    • Work = 20 N x 10 m
    • Work = 200 Joules
    • Power = Work / Time
    • Power = 200 J / 5 s
    • Power = 40 watts