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Kinetic and P otential Energy

Kinetic and P otential 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 walk upstairs . (work). Kinds of Energy.

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Kinetic and P otential 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) • Energyis the capacity to do Work • We recognize it by the changes it causes We use energy to walk 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 EXTENSION- • 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/hr = 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?

  21. One more distinction about Energy Types Mechanical vsNonmechanical

  22. Mechanical vsNonmechanical Energy (write somewhere on your notes- no blanks!) • Mechanical Energy- Mechanical Energy is the energy that an object has due to is motion or position, this can be either kinetic energy or potential energy. * Hint: If something has mechanical energy you can see it move (or you would see it move if it only has potential energy)

  23. Non-mechanical Energy- All the other types • Exs- • Chemical • Light • Thermal (Heat) • Nuclear • Electromagnetic Hint: You cannot usually “see” non-mechanical energies

  24. Work • W= F x D • Work= Force x Displacement • Units- in joules • Work Energy Theorem- work is equal to the change in energy • W= DE

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