Work = Force which causes movement

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Chapter 3

• Work Force which causes movement
• work is a form of energy
• W Fd SI unit Joules (J)
• 1 J 1 Nm 1 kgm2
• s2
• Force to lift objects Weight

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• Examples
• 1. A horizontal force of 80 N is used to move a
  20 kg crate across a level floor. How much work
  is done to move the crate 5 m?

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• 2. How much work is needed to raise a
• 110 kg load of bricks 12 m above the ground to
  the roof of a building?

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• II. Simple Machines
• Make work easier by trading Force for distance
• (Longer distance Less Force needed
• for same work)
• Inclined Plane
• longer less steep less force needed
• Examples Ramps, Stairs
• B. Wedge two inclined planes back to back
• longer thinner less force needed
• Examples Ax, Chisel, Knives,
• Can opener, Zipper

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• C. Screw inclined plane wrapped around a
  cylinder
• longer threads closer together
• less force needed
• Examples Screws, Bolts, Jar lids,
• Faucets, Ice augers
• D. Lever
• longer distance to hand than to object
• less force needed
• Examples Prybar, Rake, Broom,
• Scissors, Door Hinges,
• Bottle Opener

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• E. Wheel Axle
• wheel radiusgtaxle radius less force needed
• Examples Steering wheel, Door knob,
• Eggbeater, Screwdriver
• F. Pulley
• more rope sections less force needed
• Examples Block Tackle, Crane,
• Window blinds, Flagpole

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• III. Power rate of work done
• P Work
• t
• SI unit Watts (W)
• 1 W 1 J/s 1 Nm 1 kgm2
• s s3

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• Examples
• 1. A weightlifter raises a 90 kg barbell from the
  floor to a height of 2.2 m in 0.60 s. What was
  his power output?

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• 2. An 80 kg mountaineer climbs a 3000 m mountain
  in 10 hours. What is his average power output
  during the climb?

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• IV. Energy the ability to do work
• SI unit Joules (J)
• A. Kinetic Energy the energy of motion
• KE ½ mv2

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• Examples
• 1. What is the speed of an 800 kg car whose
  kinetic energy is 250 kJ?

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• 2. A 1 kg salmon is hooked by a fisherman and it
  swims off at 2 m/s. The fisherman stops the
  salmon in 50 cm by braking his reel. How much
  force does the fishing line exert on the fish?

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• B. Potential Energy stored energy
• PE mgh
• C. Law of Conservation of Energy
• energy is not created or destroyed,
• only changes from one form to another
• Work done PE lost KE gained

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• Examples
• 1. A 70 kg athlete runs up the stairs from the
  ground floor of the Empire State building to its
  102nd floor, which has a height of
• 370 m. How much potential energy does he have
  at the top?

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• 2. If it took 25 minutes for the athlete to climb
  to the 102nd floor, how much power did he use?

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• 3. A 3 kg stone is dropped from a height of 100
  m. What is its velocity when it is 50 m from the
  ground?

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• Quiz Next Week
• Name 6 types of simple machines give an example
  of each
• Know SI units for work, energy power
• Know the different forms of energy