Kinetic and Potential Energy

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Kinetic and Potential Energy
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Energy and Work are closely related

• Work is a transfer of Energy
• SI unit for work is Joules (J)
• Energy is the capacity to do Work
• We recognize it by the changes it causes
• We use energy to carry a backpack upstairs.
• (work)

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Kinds of Energy

• 1. Potential Energy(PE)
• the energy that an object has because of its
  position, shape or condition.

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Potential Energy of Position

• PE depends on an objects mass, height, and the
  acceleration due to gravity.

PE mgh
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Kinds of Potential Energy

• Potential Energy of Position (Gravitational PE)
• Figure 11, page 392 the apple has potential
  energy that results from the gravitational
  attraction between the apple and earth

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Kinds of Potential Energy

• Elastic Potential Energy
• The PE of an object that is stretched or
  compressed
• Ex- stretching a rubber band or bouncing a
  basketball

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Calculations with PE mgh

• Ex. calculations- page 393
• Units for PE Joules (J)
• A joule is a kg-m2 / s2

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Practice Problems

• Calculate the gravitational potential energy in
  the following systems
• A car with a mass of 120 kg at the top of a 42 m
  hill
• A 65 kg climber on top of Mt Everest (8800 m
  high)
• A 0.52 kg bird flying at an altitude of 550 m

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A car with a mass of 120 kg at the top of a 42 m
hill

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• PEmgh(120kg)(9.8m/s2)(42m)4.9x104J

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a 65 kg climber on top of Mount Everest (8800m
high)

• PE(65kg)(9.8m/s2)(8800m)5.6x106J

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a 0.52kg bird flying at an altitude of 550m

• PE(0.52kg)(9.8m/s2)(550m)2.8x103J

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Another Problem

• A science student holds a 55g egg out a window.
  Just before the student releases the egg, the egg
  has 8.0 J of gravitational potential energy with
  respect to the ground. How far is the students
  arm from the ground (in meters)?

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Answer

• h PE/mg 8.0J/(0.055kg)(9.8m/s2)15m

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Another Problem

• A diver has 3400J of gravitational potential
  energy after stepping up onto a diving platform
  that is 6.0m above the water. What is the divers
  mass in kilograms?

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Answer

• mPE/gh(3400J)/(9.8m/s2)(6.0m)58kg

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Kinetic Energy (KE)

• The energy of a moving object due to the objects
  motion
• The KE of any moving object depends on its mass
  and velocity
• See math skills pg 395
• Note if you double the mass, it doubles the
  KE. If you double the velocity, it will
  QUADRUPLE the KE

KE ½ mv 2
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Practice Problem

• Calculate the kinetic energy in joules of a
  1500kg car moving at the following speeds
• i. 29m/s
• ii. 18m/s
• iii. 42km/hr
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• A 35kg child has 190J of kinetic energy after
  sledding down a hill. What is the childs speed
  in meters per second at the bottom of the hill?

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Answers

• Calculate the kinetic energy in joules of a
  1500kg car moving at the following speeds
• i. 29m/s KE1/2mv2(1/2)(1500kg)(29m/s)26.3x105J
  
• ii. 18m/s
• KE(1/2)(1500kg)(18m/s)22.4x105J

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If the speed was 42 km/hr

• 42 km/s 11.66666 m/s
• ½ (1500 kg) (11.7m/s)2
• (750 kg) (136 m2/s2)
• 1.0 x 10 5 joules

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Answer

• A 35kg child has 190J of kinetic energy after
  sledding down a hill. What is the childs speed
  in meters per second at the bottom of the hill?