Chapter 6 Work and Energy. Forms of Energy Mechanical Kinetic, gravitational Thermal Microscopic...
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Transcript of Chapter 6 Work and Energy. Forms of Energy Mechanical Kinetic, gravitational Thermal Microscopic...
![Page 1: Chapter 6 Work and Energy. Forms of Energy Mechanical Kinetic, gravitational Thermal Microscopic mechanical Electromagnetic Nuclear Energy is conserved!](https://reader035.fdocuments.us/reader035/viewer/2022062300/56649d755503460f94a564d2/html5/thumbnails/1.jpg)
Chapter 6Work and Energy
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Forms of Energy
• Mechanical
• Kinetic, gravitational
• Thermal
• Microscopic mechanical
• Electromagnetic
• Nuclear Energy is conserved!
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Bowling ball video Conservation of energy
• https://www.youtube.com/watch?v=mhIOylZMg6Q
• Conservation of energy
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Roller Coaster Conservation of Energy
• https://www.youtube.com/watch?v=LrRdKmjhOgw
![Page 5: Chapter 6 Work and Energy. Forms of Energy Mechanical Kinetic, gravitational Thermal Microscopic mechanical Electromagnetic Nuclear Energy is conserved!](https://reader035.fdocuments.us/reader035/viewer/2022062300/56649d755503460f94a564d2/html5/thumbnails/5.jpg)
Work examples
• Pushing a heavy suitcase requires a lot of energy or WORK. The heavier the suitcase and the longer distance you push it, the more work is done.
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Units of Work and Energy
SI unit = Joule1 J = 1 Nm = 1 kgm2/s2
W F x
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Example 1: Work
• You push a 20kg box a distance of 2.5 meters. How much work is done?
• You measured the amount of work you put in after pulling bag of potatoes to be 50 J. You know that the potatoes weigh 10 kg and you accelerated 0.8 m/s. What was the distance you traveled.
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Work• Relates force to change in energy
• Scalar quantity
• Independent of time
W rF (
rx f
rxi )
Fx cos
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Example 2: Work
• A boat is put into the water from a ramp that makes an angle of 60 degrees. The boat slides a distance of 5.0 meters down the ramp. The boat weighs 4900 kg. How much work does gravity do on the boat?
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How about you are pushing something and it doesn’t
move?• Are you doing any work?
• Why or why not?
• Think of the equation…
• Why do you get tired?
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Work can be positive or negative
• Work is positive if the force has a component that is in the direction of motion.
• Work is negative if the force has a component that is opposite to the direction of motion.
• Work is zero if there is no motion.
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Work can be positive or negative
• Man does positive work lifting box
• Man does negative work
lowering box
• Gravity does positive work when box lowers
• Gravity does negative work when box is raised
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The work done by separate forces can be summed
• W total=W1+W2+W3….
• Since the work can be positive or negative, it is possible for the work to be zero (or below) even though individual amounts of work are nonzero and positive.
• Pg. 195 in the textbook
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Section 6.2 and 6.3 Work and Energy
• Key terms:
• Kinetic energy
• Potential energy
• Mechanical energy
• Elastic energy
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Kinetic energy
• Energy due to motion of an object. When you go down a hill you are creating more kinetic energy as the car speeds up.
• Measured in Joules (J)
• Kinetic energy is increased if mass or velocity increases.
• Think of a child vs. a large adult sliding to first base..
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Kinetic Energy
Same units as work
Remember the Eq. of motion
Multiply both sides by m,
1
2mv f
2 1
2mvi
2 max
KE f KEi Fx
v f2
2vi2
2ax
KE 1
2mv2
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Example 3A skater of mass 60 kg has an initial velocity of 12 m/s. He slides on ice where the frictional force is 36 N. How far will the skater slide before he stops?
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Work and kinetic energy are related
• Total work done on an object is equal to the change in its kinetic energy
• 0.5(m)(vf)2 – 0.5(m)(vi)2 =Work total
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Example 4
• How much work is required for a 74 kg sprinter to accelerate from rest to a speed of 2.2 m/s?
• Vi=0
• Vf=2.2m/s
• M=74kg
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Potential EnergyIf force depends on distance,
For gravity (near Earth’s surface)
PE Fx
PE mgh
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Example 5
• Find the potential energy of a 60kg person standing on a building that is 10meters high?
• A candy bar has a nutritional value of 880,000J. If a 80kg mountain climber eats a bar and converts all the energy to potential energy (not realistic) how much altitude can the climber gain?
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Potential Energy of Spring
PE=-Fx
x
F
PE 1
2(kx)x
PE 1
2kx2
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Springs (Hooke’s Law)
Proportional to displacement from equilibrium
F kx