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Kinetic and Potential Energy

Kinetic and Potential Energy. 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). Kinds of Energy.

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Kinetic and Potential Energy

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  1. Kinetic and Potential Energy

  2. 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)

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

  4. Potential Energy of Position • PE depends on an objects mass, height, and the acceleration due to gravity. PE= mgh

  5. 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

  6. 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

  7. Calculations with PE= mgh • Ex. calculations- page 393 • Units for PE: Joules (J) • A joule is a kg-m2 / s2

  8. 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

  9. A car with a mass of 120 kg at the top of a 42 m hill PE=mgh=(120kg)(9.8m/s2)(42m)=4.9x104J

  10. a 65 kg climber on top of Mount Everest (8800m high) PE=(65kg)(9.8m/s2)(8800m)=5.6x106J

  11. a 0.52kg bird flying at an altitude of 550m PE=(0.52kg)(9.8m/s2)(550m)=2.8x103J

  12. 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 student’s arm from the ground (in meters)?

  13. Answer • h= PE/mg = 8.0J/(0.055kg)(9.8m/s2)=15m

  14. 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 diver’s mass in kilograms?

  15. Answer • m=PE/gh=(3400J)/(9.8m/s2)(6.0m)=58kg

  16. Kinetic Energy (KE) • The energy of a moving object due to the object’s 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 mv2

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

  18. Answers • Calculate the kinetic energy in joules of a 1500kg car moving at the following speeds: • i. 29m/s KE=1/2mv2=(1/2)(1500kg)(29m/s)2=6.3x105J • ii. 18m/s KE=(1/2)(1500kg)(18m/s)2=2.4x105J

  19. 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

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

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