Ch' 15 Energy

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Ch. 15 - Energy

• I. Energy and Work
• Energy
• Conservation of Energy
• Energy Resources

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A. Energy
internal motion of particles
ENERGY
motion of objects
changes in the nucleus
kinetic potential
motion of electric charges
bonding of atoms
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• Energy vs. Work
• Work is a transfer of energy
• Work and energy are usually measured in joules
  (J)
• Ex carrying a backpack up a flight of stairs

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A. Energy

• Kinetic Energy (KE)
• energy in the form of motion
• depends on mass and velocity

80 km/h truck
50 km/h motorcycle
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• Kinetic energy can be calculated using the
  following formula
• KE ½ mv2
• KE Kinetic energy in Joules
• (1 Joule 1 kg-m2/s2)
• m mass in kg
• v speed in m/s

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• A 70.0 kg man is walking at a speed of 2.0 m/s.
  What is his kinetic energy?

GIVEN m 70.0 kg v 2.0 m/s KE ?
WORK KE ½ mv2 KE ½ (70.0 kg)(2.0 m/s)2 KE
140 J
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• A 1400 kg car is moving at a speed of 25 m/s.
  How much kinetic energy does the car have?

GIVEN m 1400 kg v 25 m/s KE
WORK KE ½ mv2 KE (1400 kg)(25 m/s)2 KE
440,000 J
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• A 50.0 kg cheetah has a kinetic energy of 18,000
  J. How fast is the cheetah running?

GIVEN m 50.0 kg v KE 18,000 J
WORK KE ½ mv2 18,000 J ½ (50.0 kg) v2
18,000J / 25.0 kg v2 720 J/ kg v2 V 27
m/s
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A. Energy

• Potential Energy (PE)
• stored energy
• depends on position or configuration of an object

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• Gravitational Potential Energy
• Depends on objects height
• Increases as object is raised to a higher level

• Which boulder has greater gravitational PE?

• What other ways can an object store energy?

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• Gravitational Potential Energy
• PE mgh
• PE potential energy (J)
• m mass (kg)
• g acceleration due to gravity (m/s2)
• h height (m)

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• Calculate the potential energy of a diver at the
  top of a 10.0 meter high diving platform whose
  mass is 50.0 kg.

GIVEN m 50 kg h 10.0 m g 9.8 m/s2 PE
WORK PE mgh PE (50.0 kg)(9.8 m/s2)(10.0m) PE
4900 J
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• Elastic Potential Energy
• The potential energy of an object that is
  stretched or compressed
• Greater when object is stretched further

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C. Conservation of Energy

• Energy Conversions
• Energy can be converted from one form to another.
• EX
• wind up toy elastic PE ? KE
• battery chemical ? electrical
• mechanical ? thermal
• chemical ? thermal

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C. Conservation of Energy

• Law of Conservation of Energy
• Energy may change forms, but it cannot be created
  or destroyed under ordinary conditions.

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C. Conservation of Energy

• Question
• A car moving at 30 m/s has 100,000 J of kinetic
  energy. The driver steps on the breaks and
  brings the car to a stop. Where did the energy
  go?

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C. Conservation of Energy

• Answer
• 100,000 J of kinetic energy was converted to
  100,000 J of thermal energy
• Friction (a force) did work on the car as it
  brought it to a stop over a distance.
• The work transferred energy from one form to
  another.

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C. Conservation of Energy

• Mechanical ? Thermal

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C. Conservation of Energy

• PE ? KE