Work/Power/Machines

1
Work/Power/Machines

• Adapted from Prentice Hall Motion, Forces and
  Energy
• By Jim Barnaby

2
???Work???

• What is work?
•  
• List answers
•  
• Which of your suggestions require energy and
  which do not?

3
Work/Energy

• All work requires energy
• Work and energy are the same thing

4
Work Defined

• Work
•   When force moves an object
• Work force ? Distance
• W F D m a D
•   Unit joule (j)
•   1 j 1 N m

5
Work Problems

• 1 joule work done to lift a ¼ lb hamburger (1
  N) 1 meter
•  
• Austin lifts a 200 N box 4 meters. How much
  work did he do?

6
Work Problems

• Chase lifts a 100 kg (220 lbs) barbell 2 meters.
  How much work did he do?
•  
• Caitlin pushes and pushes on a loaded shopping
  cart for 2 hours with 100 N of force. The
  shopping cart does not move. How much work did
  Caitlin do?

7
Power Defined

• Power
•   Rate at which work is done
•  
• Power Work F D m a D
• Time t t

8
Power Rangers

• What is the unit of power?
• What?
• What?  
• Unit watt
• 1 watt 1 j/s
•   Unit 1 j/s 1 N m/s 1 watt (w)

9
Power in our life

• Compare cost of appliance to power. In store
  look at appliances (garbage disposals, blow
  driers, heaters, driers, leaf blowersetc) and
  compare power in watts to cost.

10
Power Humor

• What did the light bulb say to his mom?
• I wuv you watts!
• Who is the most powerful teacher at the Web?
• Ms. Watts
• A 100 watt light bulb does 100 j of work each
  second

11
Power Problems

• Korey does 3000 j of work in 6 seconds. What
  was his power?

12
Power Problems

• I shovel 10,000 kg (11 tons) of dirt and move
  the pile 50 meters in 6 hours. A bulldozer moves
  the same pile back in 30 minutes.
•   Who does the most work?
• Same
•   Who is the most powerful? Why?
•   Bulldozer--- less time

13
Power Problems

• Danielle exerts 40 N of force to move an object
  2 m in 4 seconds. What was her power?
•  
• Charles bench presses 100 kg (220 lbs) 0.5 m for
  20 reps in 20 seconds. What was the power of
  this impressive man of steel?

14
Power Problems

• Nigel exerts 1000 N of force to move an object
  600 meters in 1 minute. What was the power of
  this remarkable man?
•  
• Solve study guide problems
• 15-22, 25-33

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Energy

• Energy
•   Ability to do work
•   Ability of something to cause change
•   Measured in joules (same as work)
•   Energy Work

16
Potential Energy

• Potential Energy (PE)
•   Energy of position (stored energy)
•   (Example rock on a cliff, battery, stretched
  rubber band, food)
•  
• Work is done on object to gain PE)

17
Kinetic Energy

• Kinetic Energy (KE)
•   Energy of motion
•   Object in motion has the ability to do work
•  
• (Example bowling ball hits pins, hammer hits
  nail, rubber band shoots paper)

18
Energy Conservation

• Energy is conserved
•   Energy in Energy out
•   Work in Work out
•  
• Momentum in Momentum out

19
Rubber Band Energy

• Does a rubber band have energy to do work?
• (un-stretched rubber bands have no energy to do
  work)
•   Stretch rubber bandDoes it have energy to do
  work?
• (stretched rubber bands have energy to do work)
•  

20
Rubber Band Energy

• Where is the energy when the rubber band is
  stretched?
• (it is stored in the rubber band, Elastic PE)
•  
• What happens to the energy when the rubber band
  is released?
• (it is converted into motion -- PE converts
  into KE)

21
Rubber Band Energy

• Rubber band can do work on paper wad.
•  
• What happens if stretched farther?

22
Main Idea

1. People have depended on machines for thousands of
   years to make their lives easier and more
   enjoyable
2. Most machines are complex mechanical systems that
   are designed to perform some overall function.
3. A complex mechanical system is made up of many
   subsystems composed of simple machines.

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Mechanical System

• Mechanical System
•  
• A machine composed of more than one simple
  machine

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Machines impact lives

• Q If machines, convenience items and fast food
  make our lives easier (we no longer hunt or grow
  our own food), why do people have less time than
  ever before in history?
• See word document for machines in order (next
  slide has answers)

25
Chronological order

• 1. Weighing scales 3500 BC
• 2. Gears 100 BC
• 3. Mechanical Clocks 1300
• 4. Steam locomotive 1804

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Chronological order

• 5. Electric Motor 1830
• 6. Internal Combustion engine 1860
• 7. Motorcycle 1885
• 8. Gasoline powered lawn mower 1902

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Chronological order

• 9. Blender 1923
• 10. Automatic clothes washing machine
• 1937
• 11. Jet aircraft 1939
• 12. CD player 1979

28
Machines

• Machine
•   A device that helps you do work
•   All machines are made up of one or more simple
  machines
•   Machines make work easier by changing the size
  or direction of force

29
Machines/Energy

• Machines obey the law of Conservation of Energy
•  
• Energy or Energy or Friction
• Work into a work out of losses
• Machine a machine

30
Efficiency

• Efficiency
• Comparison of work output to work input
•  
• All machines have friction losses
•  
• 100 efficient machine (no friction or heat
  losses) does not exist

31
Mechanical Advantage

• Mechanical Advantage (MA)
•  
• How many times easier a machine makes your work
•  
• Number of times a machine multiplies effort
  force
•  
• No Unit for MA

32
Mechanical Advantage

• You want a machine with a MA of 10 instead of 5,
  as work will be 10 times easier rather than 5
  times easier
•  
• MA FR Resistance force
• FE Effort force
•  
• MA DE Effort Distance
• DR Resistance Distance

33
Mechanical Advantage

• If we apply 20N of force to move a 60N object
• Solve problem on board
• No unit on the number for MA

34
Forces on Machines

• Two forces involved with machines
•   Effort Force (FE)
•   Force you put on a machine
•   Resistance Force (FR)
•   Force the machine is working against (often
  the weight of the object)

35
Distances/Machines

• Effort Distance (DE)
•   Distance of the effort force
•  
• Resistance Distance (DR)
•   Distance the object moves
• See word document (solve problems)

36
Lever Law

• Lever law
•   Work in work out
•  
• F1D1 F2D2
•  
• An 800N man is 2 meters from the fulcrum of a
  teeter-totter. How far away must a 400N child
  sit in order to balance?

37
Mechanical Advantage

• A person pushes a crowbar down 2 m with 200N of
  force. The crowbar raises the box 0.3 m. What
  is the MA?

38
Six simple machines

• 6 simple machines
•   Inclined Plane
• Wedge
• Screw
• Lever
• Wheel and axle
• Pulley

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Inclined Plane

• Every day my adorable husband lifts 3000 pounds
  5 feet. He does this 2, 4 or 6 times a day,
  depending on the day. What is he doing?
•  
• Driving his car up the driveway

40
Inclined Plane

• Inclined Plane
• A slanted surface used to move an object from
  low to high or high to low position.
•  
• Examples driveway, ramp, stairs, road
•  

41
Inclined Plane MAs

• Demo with inclined plane, spring scale and
  weight
• FR weight of object (lift with spring scale)
• FE spring scale drag
• DE length of ramp
• DR height of ramp
• Slide object and do Force MAs (MA FR/FE)

42
Inclined Plane MAs

• How will MA vary with height?
•   Steeper inclined plane less MA
•   Less steep inclined plane large MA
  (easier)
•  
• Example Climb a mountain - steep sections
  harder to climb

43
Inclined Plane Lab

• Solve Inclined Plane problems in word document
• Demo lab. Groups of 3 4 do lab and turn it
  in.
• Review lab

44
Inclined Plane Review

• Quick review of inclined plane
•   See word document for quality graphic and
  formulas
• How will MA vary with height for an inclined
  plane?
•  
• Steeper inclined plane less MA, more difficult
  to use
•  
• Less steep inclined plane larger MA, easier to
  use

45
Wedge

• Wedge
•   An inclined plane that moves
•  
• Examples knife, axe, door stoop, wood
  splitting wedge..
•  
• How does sharpening a knife help it do more
  work?

46
Screw

• Screw
•   An inclined plane wrapped around a cylinder
•   Examples spiral staircase, mountain road,
  C-clamp, bolts
•   Each time a screw turns, it moves a definite
  distance up or down.
•  
• Depends on the distance between threads
• Draw on chalkboard

47
Lever

• Lever
•   A bar that rotates on a fixed point called a
  fulcrum
•   Fulcrum
•   The fixed point a lever rotates around
• MAs can be very large, depends on leverage

48
Lever MA

• Theoretically, if you have a long and rigid
  enough lever you could lift anything (Webber)
•  
• MA DE Effort arm length
• DR Resistance arm length
• Use word document for all 3 classes of levers
  and examples

49
Lever Demos

• Watch Eureka video on levers
• Demo lever activity Students calculate MA and
  list class of lever
• Demo lever law with meter stick and weights
• Wheel and axle demos

50
Wheel and Axle

• Wheel and Axle
•   A lever that moves in a circle turning another
  circle
•  
• Examples screw driver, socket wrench, faucet
  knob, steering wheel, pencil sharpener, door
  knob

51
Wheel and Axle MA

• Calculate MA of wheel and axle
•  
• MA DE Radius of wheel
• DR Radius of axle
• See word documents

52
Advantages of a Machine

• What are the two advantages of a machine?
• They change the size and direction of effort
  force

53
Pulleys

• Pulley
•   A lever that rotates around a fixed point
•   2 benefits of a pulley
•  1.     Change the direction of force
• 2. Provide Mechanical Advantage
• Bag the technology and go back to the chalk
  board like our ancestors

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Example Problems

• Example problems
• George weighs 700 N and is 5 meters from the
  fulcrum on a teeter-totter. How far must Martha
  be from the fulcrum to balance if she weighs 200
  N?
•   F1D1 F2D2
•   A 46 N force is used to lift a 192 N weight.
  What is the MA?

56
Example Problems

• A lever is used to lift a 2000 N weight with an
  effort force of 50 N. What is the MA of the bar?
•  
• What is the MA of a ramp 30 meters long and 4
  meters high?

57
Example Problems

• A woman pushes a crow bar down 3 meters to pry a
  spike out 0.15 meters. What was the MA?
•   Create and solve 8 MA problems (4 with distance
  and 4 with force) and 2 lever law problems
  (teeter-totter). Turn this in for easy points.

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