Work and Power

1
Work and Power
2
What does it mean to do work?

• In science, you do work when you exert a force on
  an object that causes the object to move a
  distance.
• Ex. You push a child on a swing
• Ex. You lift a bag of groceries

3
No work is done!

• In order to do work on an object, the object must
  move some distance as a result of the force.
• The force must be exerted in the same direction
  as the objects motion.
• In the drawing at right, the person is pushing on
  a wall. Because the wall is not moving, no work
  is done.

4
How much work is done?

• The amount of work done on any object is found by
  multiplying the force times the distance.
• Example, You exert a force of 20N to push a desk
  10m. How much work is done?
• Work force x distance
• Work 20N x 10m
• Work 200 Nm or 200 J

5
The Joule

• The joule was named in honor of James Prescott
  Joule a physicist in the 1800s.
• One joule (J) is the amount of work you do when
  you use a force of 1N to move an object 1 meter.

6
Power

• Power is the rate at which work is done.
• Power is calculated by dividing the amount of
  work done by the amount of time taken to do it.
• Power work / time and is measured in watts.
• If a force of 8000N is needed to lift a beam 75 m
  in 30 s then the amount power done by the crane
  is found from the work done and the time.
• Power 8000N x 75 m / 30 s 20,000 watts
• 
  
  
  

7
James Watt

• The watt is named in honor of James Watt who
  invented the steam engine.
• A watt is a small unit of power. 1000 watts
  equals one kilowatt (kW). A washing machine uses
  about one kilowatt an hour when it is running.
• James Watt also introduced the word horsepower.
  He compared his steam engine to the work of a
  horse hauling coal.
• One horsepower is the amt. of work done by a
  horse to lift a 33,000-pound weight a distance of
  one foot in one minute.
• One horsepower equals 746 watts.

8
How do you make work easier?

• Work is made easier by machines.
• A machine changes the amount of force you exert,
  the distance over which you exert the force, or
  the direction of the force.
• All modern machines are based on six simple
  machines.

9
(No Transcript)
10
Examples of Compound Machines
11
Inclined Plane

• The inclined plane makes work easier by spreading
  out the effort over a greater distance (the
  ramp), rather than trying to lift it.

12
Wedge

• This wedge acts to split the wood. Instead of
  requiring a large force, the wedge multiplies the
  small force to do the job.

13
Wheel and axle

• The larger the wheel, the less force required to
  move the load.
• Notice the force is moving in the same direction
  as the load.

14
Pulley

• The larger the wheel, the less force required to
  move the load (just like the wheel and axle).
• The load and force move in opposite directions.

15
Lever

• All levers have two parts the bar and the
  fulcrum.
• The longer the lever, the less force needed to
  move the load.
• The load and force move in opposite directions.
• There are three classes of levers first, second
  and third class.

16
Classes of Levers

• First class the fulcrum can be moved closer or
  further away from the effort, and either multiply
  the force you apply or the distance. Think
  seesaw. Move fulcrum closer to heavier person,
  then you multiply your force, and the seesaw
  balances!
• Second class always multiply force. Think
  wheelbarrow.
• Third class Multiply distance but do not change
  the direction of the force. Think a rake
  sure makes getting all those leaves much easier!

17
Screw

• The screw consists of an inclined plane and a
  post.
• The longer the inclined plane, the less force
  required to move the load.
• Force and load move in the same direction.

18
Engineering feats!

• Machines have helped to create many of the
  worlds most beautiful and useful constructions.

19
Great Pyramid of Giza, 2550 B.C.

• Workers used wooden wedges to cut 2.3 million
  blocks of stone.
• The wedges were driven into cracks in the rock,
  which split the rock.
• Workers hauled the blocks up inclined planes to
  the top of the pyramids walls.

20
Theatre at Epidaurus, Greece500 B.C.

• The Greeks used a crane powered by pulleys to
  lift the stone blocks to build the theatre.
• The crane was also used to lower actors to the
  stage during performances.

21
Yingxian Pagoda, China1056 A.D.

• Slanted wooden beams called ang were used as
  levers to build up the roof of this pagoda.
• The of weight of the center of the roof presses
  down on one end of the beam. The other end
  swings up to support the outer edge of the roof.

22
Notre Dame Cathedral, Paris

• Many new inventions were needed to construct
  these enormous buildings.
• They needed to support the heavy roof, and
  stained glass windows.
• The wheelbarrow was invented to move materials
  around.
• Cranes, winches and steeplejacks were used to
  raise construction materials.

23
Empire State Building, NYC, NY

• It took one year and 45 days to complete the
  building.
• Large cranes were required to raise the steel
  girders to the higher floors.
• A railway car was built at to move materials at
  the site. This was more efficient than a
  wheelbarrow as it held eight times as much
  material.

24
The Chunnel, United Kingdom to France

• At a cost of 22 billion dollars, the tunnel
  connecting the UK and France is the most
  expensive construction project in the world.
• A tunnel-boring device was used to move the
  material out of the way while maintaining the
  wall structures.

25
Sydney Opera House

• It took three tower cranes to complete
  construction of the sails.
• Two mechanical stage lifts move scenery and props
  for performances.

26
Examples of Compound Machines
27
(No Transcript)