1. Chapter 14 Work, 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.

6. 14.1: Work and Power What is work? Recall...From Chapter 12
Question: How does an unmoving 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 can’t 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. 14.3 Mechanical Advantage

38. Mechanical Advantage = the number of times that the machine increase an input force
MA = 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

39. A: AMA = 100/20 = 5
In other words, this machine has multiplied the effort force 5 times.

40. 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

41. A: IMA = 1.8m/0.3m = 6

42. 14.4 Simple Machines
The six types of simple machines are:
Lever
Wheel and axle
Inclined plane
Wedge
Screw
Pulley

43. Lever

44. 3 classes of levers

45. Wheel and axle

46. Inclined Plane

47. Wedge

48. Screw

49. Pulley

51. Chapter 15 Energy

52. 15.1 Energy and Its Forms

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

54. 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) 

55. Types of Energy Energy can be classified as two general types:
kinetic energy
potential energy. 

56. Kinetic Energy

57. 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

58. KE = ½mv2 Notice that doubling the mass doubles the KE
But, if you double the speed you quadruple the KE!

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

60. Practice Problem Solved
KE = ½ 70kg x (2m/s)2
KE = 35kg x 4m/s = 140J

61. Potential energy

62. 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 

63. GPE
Gravitational potential energy depends on an object’s 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
h= height 
Remember m x g = w (N)

64. GPE Calculate the GPE in the picture below
Show your work here:

66. 75kg x 9.8 m/s/s x 4m = 2940 J

67. Practice Problem
A diver at the top of a 10 m high platform has a mass of 50kg. Calculate GPE

68. Practice Problem Solved
GPE = 50kg x 9.8m/s2 x 10m = 4900J

69. 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 

70. EPE

71. Mechanical Energy
Mechanical energy- the energy associated with the motion and position of everyday objects
The sum of an object’s PE and KE

72. 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 

73. Thermal Energy

74. 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 

75. Chemical Energy

76. 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

77. Electrical Energy

78. Electrical Energy
Electrical energy- energy associated with moving electric charges 
Electric charges exert forces that do work
Examples:
electricity
lightning 

79. Electromagnetic Energy

80. 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

81. Nuclear Energy

82. 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 

84. 15.2 Conversion and Conservation of Energy

85. Conversion Energy can be converted from one form to another
Energy conversion = the process of changing energy from one form into another

86. Example: a wind-up toy converts PE into KE when it unwinds

88. 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.

89. Energy Conservation
Question: Why do you slow down after you stop pedaling your bike?
Where did the bike’s KE go?

90. 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!

91. GPE to KE
The gravitational PE of an object is converted to the KE of motion as the object falls.

93. Pendulum Conversions

94. Bouncing ball

95. Energy Conversion Calculations
When friction is small enough to be ignored, an object’s mechanical energy does not change.
Remember: mechanical energy is the TOTAL KE and TOTAL PE of an object
Mechanical Energy = KE + PE

96. 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

97. 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.

98. 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

99. Tying it all in to Nuclear Chemistry
Nuclear Chemistry Connection/Review: 
Remember Einstein’s equation? E = mc2
This equation says that energy and mass are equivalent and can be converted into each other.  

100. 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.