14.1 Work and Power

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14.1Work and Power
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Work

• Product of distance and the force in the
  direction an object moves.
• Work is done on an object when a force is applied
  to the object and the object moves a certain
  distance.

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Work- Quantified

• Work Force Distance
• W Fd
• The force in this equation is assumed to be a
  constant force applied to an object
• The force used in this equation is in the same
  direction as the displacement of the object

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The Joule

• The SI unit for work
• 1 J 1 Nm
• The Joule is named for James Joule (1818-1889), a
  British physicist interested in the study of the
  efficiency of electric motors

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Joules

• 1 kilojoule 1000 joules
• (1 kJ 1000 J)
• 1 megajoule 1,000,000 joules
• (1 MJ 1,000,000 J)

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Calculate Work

• A 35 N force is applied to a box and the box
  moves a distance of 10 m
• W Fd (35 N)(10 m) 350 J

35 N
10 m
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Calculate Work

• How much work is done when I push the wall with a
  force of 200 N?
• If a force is applied to an object and the object
  does not move, then d 0 and
• W (200 N)(0 m) 0

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Power

• The quantity work does not account for the time
  it takes to do a certain task
• But there is physical difference between pushing
  a heavy couch 10 m in 10 seconds as opposed to 10
  minutes
• Power Rate of doing work

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Power

• Power work done
• time interval
• P W/t
• Watt The SI unit for power
• 1 W 1 J/s

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The Watt

• Named after James Watt (1736-1819) who developed
  the steam engine in the eighteenth century
• 1 kilowatt 1000 watts
• (1 kW 1000 W)
• 1 megawatt 1,000,000 watts
• (1 MW 1,000,000 W)

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Power

• Horsepower (hp) Common unit of power, equal to
  about 746 W
• 1 hp 0.746 kW or 1 hp 746 W

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Vehicle Power Ratings

• Lawn mower 5 hp
• Ford Escort 110 hp
• Toyota 4Runner 270 hp
• Ferrari 355 F1 375 hp
• 747 500,000 hp

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Calculate Work

• A 35 N force is applied to a box and the box
  moves a distance of 10 m
• W Fd (31.7 N)(10 m) 317 J

35 N
T 25
31.7 N
10 m