WorkPowerMachines

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

15
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

• People have depended on machines for thousands of
  years to make their lives easier and more
  enjoyable
• Most machines are complex mechanical systems that
  are designed to perform some overall function.
• 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

24
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

26
Chronological order

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

27
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

39
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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