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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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Ex. calculations- page 393 Units for PE: Joules (J) – A joule is a kg-m 2 / s 2 Calculations with PE= mgh

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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 PE=mgh=(1200kg)(9.8m/s 2 )(42m)=4.9x10 5 J

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a 65 kg climber on top of Mount Everest (8800m high) PE=(65kg)(9.8m/s 2 )(8800m)=5.6x10 6 J

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a 0.52kg bird flying at an altitude of 550m PE=(0.52kg)(9.8m/s 2 )(550m)=2.8x10 3 J

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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/s 2 )=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 m=PE/gh=(3400J)/(9.8m/s 2 )(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/s 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 KE=1/2mv 2 =(1/2)(1500kg)(29m/s) 2 =6.3x10 5 J ii. 18m/s KE=(1/2)(1500kg)(18m/s) 2 =2.4x10 5 J ii. 42m/s KE=(1/2)(1500kg)(42m/s) 2 =

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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?

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