Chapter 14 Work, Power and Simple Machines

1
Chapter 14Work, Power and Simple Machines
2
Questions to think about before

• What does work mean to you??? 
• List some examples of work

3
Is this work???
4
Work Science

• Now...think about work in terms of science...it
  probably means something very different than what
  you listed above.

5
(No Transcript)
6
14.1 Work and Power

• What is work?
• Recall...From Chapter 12
• Question How does an stationary object begin
  moving?

7
Answer

• Answer When an unbalanced force acts on it.
• Work the product of force and distance
• Work is done when a force acts on an object in
  the direction the object moves.

8
Is work being done?
9
Work Requires Motion

• Question Does a weight lifter do work on the
  barbell to lift it over his head?

10
Answer yes, force is up and barbell moves up
11
Stationary Objects

• Question Is the weight lifter doing work while
  he holds the barbell stationary over his head? 

12
ANSWER

• Answer NO, the barbell is stationary
• For a force to do work on an object, some of the
  force must act in the same direction as the
  object moves.  If there is NO movement, NO work
  is done!!!

13
Work Depends on Direction

• The amount of work done on an object, if any,
  depends on the direction of the force and the
  direction of the movement.
• A force does not have to act entirely in the
  direction of movement to do work.

14
Is work being done?
15
Is work being done????

• The force acts upward and to the right.
• The suitcase only moves to the right.
• Any part of a force that does not act in the
  direction of motion does NO work on an object

16
Calculating Work

• Work Force x Distance
• Units of Work
• SI unit for force is newtons
• SI unit for distance is meters

17
JOULE

• The SI unit for work is newton-meter or the JOULE
  (J)   
• When a force of 1 newton moves an object 1 meter
  in the direction of the force, 1 joule of work is
  done.

18
Practice Problem

• Imagine the weight lifter. The weight lifter
  lifts a 1600 newton barbell over his head. 
  Assume the barbell is lifted to a height of 2.0
  meters.  What is the work done?
• Work Force x Distance

19
Practice Problem Answered

• Work 1600 N x 2.0 m
• Work 3200 N m 3200 J

20
What is Power?

• Power the RATE of doing work
• Doing work at a faster rate requires more power. 
  To increase power, you can increase the amount of
  work done in a given time, or you can do a given
  amount of work in less time

21
Q Does a person shoveling snow do work?
22

• Answer YES, because the shovel is moving in the
  same direction as the force being applied

23
Q Does a snow blower do work?
24

• Answer YES, but because the snow blower does the
  work in less time it has more POWER!!!

25
Calculating Power

• Power Work / Time
• Work is in joules (J)
• Time is in seconds (s)
• The SI unit for POWER is the watt (W) one joule
  per second
• Thus, a 40-watt light bulb requires 40 joules
  each second that it is lit.

26
Practice Problem

• You exert a vertical force of 72 newtons to lift
  a box to a height of 1.0 meter in a time of 2.0
  seconds.  How much power is used to lift   the
  box?

27
Practice Problem Answered

• Power work / time
• OR can be written as
• Power (Force x Distance) / Time
• (72 N x 1.0 m)/ 2.0 s 36 J/s 36 Watts

28
James Watt and Horsepower
29
Horsepower

• Horsepower (hp) common unit for power.  One
  horsepower is equal to about 746 watts. 
• FYI...Interesting side note Horsepower is
  literally based on the power output of a very
  strong horse!!!

30
14.2 Work and Machines

• Machine a device that changes a force
• Machines make work easier to do. They can
• Change the size of the force needed
• The direction of a force
• The distance over which the force acts
• However
• They cant do work for us!

31
Increasing a force
32

• Ex a car jack
• Each rotation of the jack applies a small force
  over a large distance and the car is lifted a
  small distance
• Tradeoff total distance traveled is much
  greater

33
Increasing Distance
34

• Ex oars of a boat
• You move oars a small distance and the end in the
  water moves a large distance
• Tradeoff increased travel of the oar requires
  you to exert a greater force

35
Changing Direction
36

• Ex pulley
• You pull down on the rope and the load moves up

37
Work Input

• Because of friction, the work done by a machine
  is always less than the work done on the machine!
• The force you exert on the machine is called
  input force. The distance the input force acts
  through is called the input distance.
• Work input IF X ID

38
Work Output

• The force exerted by the machine is called the
  output force. The distance the output force is
  exerted through is the output distance.
• Work output OF X OD

39
14.3 Mechanical Advantage
40

• Mechanical Advantage the number of times that
  the machine increase an input force
• AMA load force/effort force
• Q Using a lever, a person is able to lift a 100N
  object using only 20N of force. Calculate the MA
  of this machine

41

• A AMA 100N/20N 5
• In other words, this machine has multiplied the
  effort force 5 times.

42

• Ideal Mechanical Advantage MA without friction
• IMA Input Distance/Output Distance
• Q A woman drives her car onto a ramp. She
  drives 1.8 meters along the ramp to raise it 0.3m
  off the ground. Calculate IMA

43

• A IMA 1.8m/0.3m 6

44
Efficiency

• Because some of the work input to a machine is
  always used to overcome friction, the work output
  is always less.
• Efficiency the percentage of the work input
  that becomes work output
• Always less than 100 due to friction
• Efficiency W.output/W. input X 100

45
14.4 Simple Machines

• The six types of simple machines are
• Lever
• Wheel and axle
• Inclined plane
• Wedge
• Screw
• Pulley

46
Lever
47
3 classes of levers
48
Wheel and axle
49
Inclined Plane
50
Wedge
51
Screw
52
Pulley
53
(No Transcript)
54
Chapter 15 Energy
55
15.1 Energy and Its Forms

• What is Energy?
• Energy- the ability to do work
• Energy is transferred by a force moving an object
  through a distance

56
Work Energy

• Energy is closely related to work
• Work is a transfer of energy
• When work is done on an object, energy is
  transferred to that object
• Both are typically measured in joules (J) 

57
Types of Energy

• Energy can be classified as two general types
• kinetic energy
• potential energy. 

58
Kinetic Energy
59
Kinetic Energy

• Kinetic energy - (KE) the energy of motion
• The kinetic energy of any moving object depends
  on two things
• Mass of the object
• Speed of the object
• To calculate the KE of an object, use the
  following formula 
• KE ½ mv2

60
KE ½mv2

• Notice that doubling the mass doubles the KE
• But, if you double the speed you quadruple the
  KE!

61
Practice Problem

• A 70kg man is walking at a speed of 2m/s.
  Calculate his KE.
• Show your work!

62
Practice Problem Solved

• KE ½ 70kg x (2m/s)2
• KE 35kg x 4m/s 140J

63
Potential energy
64
Potential Energy

• Potential energy energy that is stored as a
  result of position or shape
• Energy that is stored has the ability to do
  work! 
• There are two types of potential energy
• Gravitational potential energy and
• Elastic potential energy 

65
GPE

• Gravitational potential energy depends on an
  objects mass, height, and acceleration due to
  gravity. 
• GPE m x g x h or GPE w x h
• m mass (kg)
• g acceleration due to gravity (m/s/s)
• h height (m)
• Remember m x g w (N)

66
GPECalculate the GPE in the picture below
Show your work here
67

• 75kg x 9.8 m/s/s x 4m 2940 J

68
Practice Problem

• A diver at the top of a 10 m high platform has a
  weight of 490N. Calculate GPE

69
Practice Problem Solved

• GPE 490N x 10m 4900J

70
Elastic Potential Energy

• Elastic potential energy the PE of an object
  that is stretched or compressed.
• Something is said to be elastic if it springs
  back to its original shape after being stretched
  or compressed
• Example rubber band, basketball 

71
EPE
72
Mechanical Energy

• Mechanical energy- the energy associated with the
  motion and position of everyday objects
• The sum of an objects PE and KE

73
Further Classification of Energy

• Energy can be potential or kinetic, but it can be
  further classified into different types of
  energy
• Thermal energy     
• Electrical energy          
• Nuclear energy
• Chemical Energy
• Electromagnetic Energy 

74
Thermal Energy
75
Thermal Energy

• Thermal energy- the total potential and kinetic
  energy of all the microscopic particles in an
  object
• When atoms move faster thermal energy increases
  causing the object to become warmer 
• Ex sun, hot beverage, lava, feverish skin

76
Chemical Energy
77
Chemical Energy

• Chemical energy- energy stored in chemical bonds.
  
• When the bonds are broken and new bonds form, the
  released energy can do work
• Examples
• fuel like gasoline
• Food
• Any chemical fuel stores energy

78
Electrical Energy
79
Electrical Energy

• Electrical energy- energy associated with moving
  electric charges  (electrons)
• Electric charges exert forces that do work
• Examples
• electricity
• lightning 
• Electric fence

80
Electromagnetic Energy
81
Electromagnetic Energy

• Electromagnetic energy- energy that travels
  through space in the form of waves
• Can travel long distances through air and space
• Often used for communication
• Examples
• visible light
• x-rays 
• radio waves

82
Nuclear Energy
83
Nuclear Energy

• Nuclear energy- energy stored in atomic nuclei
• Fission- release of energy by splitting nuclei
• Fusion- release of energy when less massive
  nuclei combine to form a more massive nuclei
• Example heat and light from the sun (fusion),
  nuclear power (fission), nuclear bombs (both)

84
(No Transcript)
85
15.2 Conversion and Conservation of Energy
86
Conversion

• Energy can be converted from one form to another
• Energy conversion the process of changing
  energy from one form into another

87
Example a wind-up toy converts PE into KE when
it unwinds
88
(No Transcript)
89
Energy Conservation

• As one form of energy converts into another form
  the total energy remains the same!!!
• The law of conservation of energy states that
  energy can NOT be created or destroyed.

90
Energy Conservation

• Question Why do you slow down after you stop
  pedaling your bike?
• Where did the bikes KE go?

91
Energy Conservation

• Answer Friction!
• Since we do not live in a frictionless world, we
  have to take it into consideration
• The work done by this frictional force changes KE
  into thermal energy.
• When the energy lost to frictional forces is
  accounted for all energy is conserved!

92
GPE to KE
The gravitational PE of an object is converted to
the KE of motion as the object falls.
93
(No Transcript)
94
Pendulum Conversions
95
Bouncing ball
96
Energy Conversion Calculations

• When friction is small enough to be ignored, an
  objects mechanical energy does not change.
• Remember mechanical energy is the TOTAL KE and
  TOTAL PE of an object
• Mechanical Energy KE PE

97
Energy is Conserved

• The total mechanical energy at the beginning of
  the conversion must equal the total mechanical
  energy at the end!
• (KE PE)beginning (KE PE)end

98
(No Transcript)
99
Practice Problem

• At a construction site, a 1.5kg brick is dropped
  from rest and hits the ground at a speed of 26
  m/s. Assuming air resistance can be ignored,
  calculate the GPE of the brick before it was
  dropped.

100
Practice Problem Answered

• (KE PE)beg (KE PE)end
• (½ x 1.5kg x (26m/s)2 0)end (0 PE)beg
• 507 J PE

101
Tying it all in to Nuclear Chemistry

• Nuclear Chemistry Connection/Review 
• Remember Einsteins equation? E mc2
• This equation says that energy and mass are
  equivalent and can be converted into each other.
   

102
Nuclear Chemistry

• In other words, energy is released as matter is
  destroyed and matter can be created from energy.
• Remember the law of conservation of mass was
  modified to account for this, and says that mass
  and energy together are always conserved.