energy lecture slide 1 energy conservation. energy can be thought of as the capacity for doing work...
TRANSCRIPT
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Energy Lecture Slide 1
Energy Conservation
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Energy Conservation
• Energy can be thought of as the capacity for doing work
• Energy may be transformed from one type of energy to another.– Kinetic, potential, “heat”, work
• Energy can be neither created nor destroyed
Energy Lecture Slide 2
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Energy Lecture Slide 3
Energy Conservation• The total energy of a system is
constant.
• That is, the sum of the PE + KE + other types of energy = constant
• If PE decreases, KE increases … and vice versa
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Energy Lecture Slide 4
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Energy Lecture Slide 5
Pendulum Energy
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Pendulum Energy
Energy Lecture Slide 6
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Pendulum Answers
• A: h = 0.306 m (6 J = 2 kg *9.8 m/s/s * h)• B: h = 0.153 m (3 J = 2 kg *9.8 m/s/s * h)• C: v = 1.73 m/s (3 J = 0.5 * 2 kg * v2)• D: h = 0 m (0 J = 2 kg * 9.8 m/s/s*h)• E: v = 2.45 m/s (6 J = 0.5 * 2 kg * v2)• F: h = 0.306 m (6 J = 2 kg * 9.8 m/s/s * h)
Energy Lecture Slide 7
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Roller Coaster Energy
Energy Lecture Slide 8
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Energy Lecture Slide 9
Roller Coaster Energy
• First hill must be highest• What impact does friction have on the
height of the successive hills?
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Roller Coaster
Energy Lecture Slide 10
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Ski Jumper Energy
Energy Lecture Slide 11
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Energy Lecture Slide 12
Simple Machines
• If no friction, then• Work Input = Work Output
• Inclined plane
• Pulley
• Lever
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Why Use a Machine?
• If there is no friction,– Work done on the machine (work input) is
equal to the work done by the machine (work output)
• and … when there is friction,– work input > than work output
• So why do we use machines?
Energy Lecture Slide 13
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Energy Lecture Slide 14
Inclined Plane
• Output work = work done if you didn’t use the machine– Output work = (weight of load)(vertical height)
• Input work = work done using machine– Input work = (force exerted)(length of plane)
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Inclined Plane Problem
Energy Lecture Slide 15
• What work would be done by the man if he lifted the block vertically upward 3m?
• How does this relate to the change in potential energy of the block?
The mass of the block of ice is 50 kg.
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Inclined Plane Problem
Energy Lecture Slide 16
• What work would be done by the man if he slid the block along the incline to the top?
• What force would the man have to use to move the block at a constant speed along the ramp?
The mass of the block of ice is 50 kg.
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Ramps & Energy Conservation
• What is the speed of the block at the bottom of each of the 5 m high ramps? Ignore friction. Use g = 10 m/s/s.
Energy Lecture Slide 17
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Energy Lecture Slide 18
Pulley
• Mechanical Advantage of Single Pulley is 1• Single pulley changes direction of force
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Pulley Systems
Energy Lecture Slide 19
Compound Pulleys can multiply force.
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Window Washer• The window washer
has a mass of 75 kg and the platform plus the bucket and squeegee has a mass of 25 kg.
• What force does the washer have to exert to lift himself and the platform?
Energy Lecture Slide 20
• What length of rope will he have to pull in order to lift himself 12 m?
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Energy Lecture Slide 21
Lever
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Energy Lecture Slide 22
Mechanical Advantage
• Ideal Mechanical Advantage
Forcewithout machineAMA
Forcewithmachine
Distance
Distance
withmachineIMA
without machine
AMAefficiency
IMA
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Energy Lecture Slide 23
Efficiency• Because of friction,
– work output < work input– Efficiency < 100%– Efficiency can also be found by
WorkInput
WorkOutputEfficiency
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Energy Practice 1• Both the KE and PE of a block freely sliding
down a ramp are shown below only at the bottom position in the sketch. Fill in the missing values for the other positions
Energy Lecture Slide 24
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Energy Practice 2
Energy Lecture Slide 25
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Energy Practice 3
Energy Lecture Slide 26
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Energy Practice 4
Energy Lecture Slide 27