1. Work, Power, and Machines
Physical Science – Unit 7
Chapter 9

2. Work What is work? Examples:
Work is the quantity of energy transferred by a force when it is applied to a body and causes that body to move in the direction of the force.
Examples:
Weightlifter raises a barbell over his/her head
Using a hammer
Running up a ramp

3. Work Work in simple terms:
Transfer of energy that occurs when a force makes an object move
The object must move for work to be done
The motion of the object must be in the same direction as the applied force

4. Work The formula for work: Measured in Joules (J)
Work = force x distance
W = F x d
Measured in Joules (J)
Because work is calculated as force times distance, it is measured in units of newtons times meters (N●m)
1 N●m = 1 J = 1 kg●m2/s2
They are all equal and interchangeable!
James Joule - English scientist and inventor

5. Work
1 J of work is done when 1N of force is applied over a distance of 1 m.
kJ = kilojoules = thousands of joules
MJ = Megajoules = millions of joules

6. Practice problem
A father lifts his daughter repeatedly in the air. How much work does he do with each lift, assuming he lifts her 2.0 m and exerts an average force of 190 N?
W = F x d

7. Practice problem
A father lifts his daughter repeatedly in the air. How much work does he do with each lift, assuming he lifts her 2.0 m and exerts an average force of 190 N?
W = F x d
W = 190 N x 2.0 m
= 380 N●m = 380 J

8. Practice problems
A mover is moving about 200 boxes a day. How much work is he doing with each box, assuming he lifts each 10 m with a force of 250 N.

9. Practice problems
A mover is moves about 200 boxes a day. How much work is he doing with each box, assuming he lifts each 10 m with a force of 250 N.
W = F x d
= 250 N x 10 m
= 2,500 N●m = 2,500 J

10. Practice problems
A box with a mass of 3.2 kg is pushed m across a floor with an acceleration of 3.2 m/s2. How much work is done on the box?
What do you need to calculate first?????

11. Practice problems
A box with a mass of 3.2 kg is pushed m across a floor with an acceleration of 3.2 m/s2. How much work is done on the box?
F = ma
= 3.2 kg x 3.2 m/s2
= 10.2 kg● m/s2 = 10.2 N

12. Practice problems
A box with a mass of 3.2 kg is pushed m across a floor with an acceleration of 3.2 m/s2. How much work is done on the box?
F = ma
= 3.2 kg x 3.2 m/s2
= 10.2 kg● m/s2 = 10.2 N
W = F x d
= 10.2 N x m
= 6.80 N●m = 6.80 J

13. Practice problems
Get a calculator

14. Page 54 asks for distance……
W= F x D

15. #1
.6 m

16. #2
.6m

17. #3
2.6m

18. #4
2.398m

19. P55 asks for force
W= F x D

20. #5 N

21. #6
27N

22. #7 N

23. #8
95 454N

24. P56 asks for W………thank goodness
W= F x D

25. #9 J

26. #10
3.2 x 106 J

27. #11 J

28. #12 J

29. How about a harder one….
#18

30. #18
Calculate force first, then work J

31. Power Power is a quantity that measures the rate at which work is done
It is the relationship between work and time
If two objects do the same amount of work, but one does it in less time. The faster one has more power.
Rate at which work is done or how much work is done in a certain amount of time

32. Power Formula for power: SI units for power – watts (W)
Power = work
time
P = W/t
SI units for power – watts (W)
1 kW – Kilowatt = 1000 watts
1 MW – Megawatt= 1 million watts

33. Power
A watt is the amount of power required to do 1 J of work in 1 s. (Reference – the power you need to lift an apple over your head in 1 s)
Named for James Watt who developed the steam engine in the 18th century.

34. Practice problems
A weight lifter does 686 J of work on a weight that he lifts in 3.1 seconds. What is the power with which he lifts the weight?
P = W/t

35. Practice problems
A weight lifter does 686 J of work on a weight that he lifts in 3.1 seconds. What is the power with which he lifts the weight?
P = W = 686 J
t s
221 J/s = 221 W

36. Practice problems
How much energy is wasted by a 60 W bulb if the bulb is left on over an 8 hours night?
P = W t

37. Practice problems
How much energy is wasted by a 60 W bulb if the bulb is left on over an 8 hours night?
P = W t
1st convert 8 hr to seconds
8 hr (60 min/1hr)(60 sec/1min) = sec

38. Practice problems
How much energy is wasted by a 60 W bulb if the bulb is left on over an 8 hours night?
P = W t
1st convert 8 hr to seconds
8 hr (60 min/1hr)(60 sec/1min) = sec
2nd calculate for energy
W = P x t
= 60 W x sec
= 1.7 x 107 J

39. Practice problems

40. P58 asks for work
P= W/t

41. #1
412.5J

42. #2 J

43. #3 J

44. #4
1.17 x 1010J

45. P59 asks for time

46. #5
sec

47. #6
456.14sec

48. #7
1 500sec

49. #8
4.5sec

50. P60 asks for power

51. #9
5 x 108 watts

52. #10
2.75 x 1010

53. Do 11 & 12

54. #11
300sec

55. #12 J

56. Machines and Mechanical Advantage
Which is easier… lifting a car yourself or using a jack?
Which requires more work?
Using a jack may be easier but does not require less work.
It does allow you to apply less force at any given moment.

57. What is a machine? A device that makes doing work easier… is a machine
Machines increase the applied force and/or change the distance/direction of the applied force to make the work easier
They can only use what you provide!

58. If machines cannot make work, why use them?
Why use machines?
If machines cannot make work, why use them?
Same amount of work can be done by applying a small force over a long distance as opposed to a large force over a small distance.

59. Effort and Resistance
Machines help move things that resist being moved
Force applied to the machine is effort force (aka: Input force)
Force applied by the machine is resistance force (aka: Load, output force)

60. Mechanical Advantage
Mechanical advantage is a quantity that measures how much a machine multiplies force or distance
Defined as the ratio between output force and input force

61. Mechanical Advantage Mechanical advantage = output force input force
Mechanical advantage= input distance
output distance

62. Machines Lever Pulley Wheel & Axle Inclined Plane Screw Wedge
Simple Machines
Lever
Pulley
Wheel & Axle
Inclined Plane
Screw
Wedge

63. The Lever family
Lever
a rigid bar that is free to pivot about a fixed point, or fulcrum
Force is transferred from one part of the arm to another.
“Give me a place to stand and I will move the Earth.”
– Archimedes
Engraving from Mechanics Magazine, London, 1824
Resistance
arm
Effort arm
Fulcrum

65. Lever First Class Lever Most common type Fulcrum in middle
can increase force, distance, or neither
changes direction of force

66. Lever Second Class Lever always increases force
Resistance/load in middle

67. Lever
Third Class Levers
always increases distance
Effort in middle

69. Pulley
Pulley
grooved wheel with a rope or chain running along the groove
a “flexible first-class lever” or modified lever F Le Lr

70. Pulley Ideal Mechanical Advantage (IMA) IMA = 0 IMA = 1 IMA = 2
equal to the number of supporting ropes
IMA = 0
IMA = 1
IMA = 2

71. does not increase force
Pulley
Fixed Pulley
IMA = 1
does not increase force
changes direction of force

72. doesn’t change direction
Pulley
Movable Pulley
IMA = 2
increases force
doesn’t change direction

73. Pulley Block & Tackle combination of fixed & movable pulleys
increases force (IMA = 4)
may or may not change direction

74. Wheel and Axle Wheel and Axle
two wheels of different sizes that rotate together
a pair of “rotating levers”
When the wheel is turned so so is the axle
Wheel
Axle

75. Wheel and Axle Wheel and Axle
Bigger the difference in size between the two wheels= greater MA
Wheel
Axle

76. What is an inclined plane?
A sloping surface, such as a ramp.
An inclined plane can be used to alter the effort and distance involved in doing work, such as lifting loads.
The trade-off is that an object must be moved a longer distance than if it was lifted straight up, but less force is needed.

77. What is an inclined plane?
MA=Length/Height

78. Incline Plane Family A wedge is a modified incline plane
Example ax blade for splitting wood
It turns a downward force into two forces directed out to the sides

79. Incline Plane Family A screw looks like a spiral incline plane.
It is actually an incline plane wrapped around a cylinder
Examples include a spiral staircase and jar lids

81. Practice problems
A roofer needs to get a stack of shingles onto a roof. Pulling the shingles up manually used 1549 N of force. Using a system of pulleys requires 446 N. What is the mechanical advantage?
Mechanical advantage = output force
input force

82. Practice problems
A roofer needs to get a stack of shingles onto a roof. Pulling the shingles up manually used 1549 N of force. Using a system of pulleys requires 446 N. What is the mechanical advantage?
Mechanical advantage = output force
input force
= 1549 N = 3.47
446 N

83. Practice problems
Asks for output force (N)
Asks for input distance (cm)

84. #1
444.4N

85. #2
11cm

86. #3
3 675N

87. Asks for output distance (cm)
Asks for input force (N)
Asks for output distance (m)

88. #4
3/0.85= 3.52

89. #5
2220/.0893= N

90. #6
1.57/12.5=0.1256m

91. Solve for MA in #7 & #8

92. #7
3.28

93. #8
23.99

94. Compound Machines
Compound machines are machines made of more than one simple machine
Example include a pair of scissors has 2 first class levers joined with a common fulcrum; each lever arm has a wedge that cuts into the paper

95. Energy is the ability to cause changes.
It is measured in Joules or kg●m/s2
When work is done on an object, energy is given off

96. Energy 5 main forms of energy: Mechanical – associated with motion
Heat – internal motion of atoms
Chemical – the energy required to bond atoms together
Electromagnetic – movement of electric charges
Nuclear – released when nuclei of atoms fuse or split

97. Mechanical Energy
Mechanical Energy is the sum of the kinetic and potential energy of a large-scale objects in a system
Nonmechanical energy is the energy that lies at the level of atoms and does not affect motion on a large scale

98. Those 5 forms of energy can be classified into one of two states:
Potential energy – stored energy
Kinetic energy – energy in motion

99. Kinetic Energy The energy of motion
An object must have mass and be moving to possess kinetic energy
The greater the mass or velocity--- the greater the kinetic energy
Formula:
KE = ½ mv2
m = mass
v = velocity

100. Kinetic Energy
Atoms and molecules are in constant motion and therefore have kinetic energy
As they collide then the kinetic energy is transferred from one to another

101. Practice problem
A sprinter runs at a forward velocity of 10.9 m/s. If the sprinter has a mass of 72.5 kg. What is their kinetic energy?
KE = ½ mv2

102. Practice problem
A sprinter runs at a forward velocity of 10.9 m/s. If the sprinter has a mass of 72.5 kg. What is their kinetic energy?
KE = ½ mv2
= ½ (72.5 kg) (10.9 m/s)2
= .5 x 72.5 x
= kg●m/s = J

103. Potential Energy
The stored energy that a body possesses because of its position.
Examples: chemical energy in fuel or food or an elevated book because it has the potential to fall.
Potential energy due to elevated potential is called gravitational potential energy (GPE).

104. Potential energy Formula: PE = mgh m = mass g = gravity (9.8 m/s2)
h = height

105. Practice problems
A pear is hanging from a pear tree. The pear is 3.5 m above the ground and has a mass of 0.14 kg. What is the pear’s gravitational potential energy?
PE = mgh

106. Practice problems
A pear is hanging from a pear tree. The pear is 3.5 m above the ground and has a mass of 0.14 kg. What is the pear’s gravitational potential energy?
PE = mgh
= .14 kg x 9.8 m/s2 x 3.5 m
= 4.8 kg●m2/s2 = 4.8 J

107. Conservation of Energy
Energy is almost always converted into another form of energy
One most common conversion is changing from potential energy to kinetic energy or the reverse.

108. Conservation of Energy
The transfer of energy from one object to the next is a conversion of energy.
The law of conservation of energy states that all energy can neither be created or destroyed; it is just converted into another form.

109. Conservation of Energy
Energy conversions occur without a loss or gain in energy
Therefore…. KE = PE

110. Energy Transformations

111. Conservation of Energy
Amount of energy the machines transfers to the object cannot be greater than energy you put in
Some energy is change to heat by friction
An ideal machine would have no friction so energy in = energy out

112. Efficiency
Efficiency is a measure of how much work put into a machine is changed to useful work output by the machine
Not all work done by a machine is useful therefore we look at the efficiency of the machine

113. Efficiency Formula for Efficiency (Work output / Work input) X 100
Efficiency = useful work output x 100
work input
Efficiency is always less than 100% because no machine has zero friction or 100% efficiency
Lubricants can make a machine more efficient by reducing friction
Oil
Grease

114. Practice Problem
What is the efficiency of a machine if 55.3 J of work are done on the machine, but only 14.3 J of work are done by the machine?
Efficiency = useful work output
work input

115. Practice Problem
What is the efficiency of a machine if 55.3 J of work are done on the machine, but only 14.3 J of work are done by the machine?
Efficiency = useful work output
work input
= 14.3 J x 100 = 25.9 %
55.3 J

116. Perpetual Motion Machines
Perpetual motions machines are machines designed to keep going forever without any input of energy
It is not possible because we have not been able to have a machine with a complete absence of friction!

117. Newman’s machine
Joseph Newman, claimed it would produce mechanical power exceeding the electrical power being supplied to it

118. Take out your homework

119. Power, Work and Force I 6.48W 6 692J 5.76S 112.93 kg 7.11W 16.4S 9.45W

120. Work and Power I 20J 5 900J 14 000J 50m 5.10m 50W 13W 18 000J 100J 60W
115N in 15m(1725J
20kg lift=1960J
80%
500J
over 490J
What do you think?
25%