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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**

PE=mgh=(1200kg)(9.8m/s2)(42m)=4.9x105J

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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 student’s 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 diver’s mass in kilograms?

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

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**KE= ½ mv 2 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 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 child’s 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=1/2mv2=(1/2)(1500kg)(29m/s)2=6.3x105J ii. 18m/s KE=(1/2)(1500kg)(18m/s)2=2.4x105J 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 child’s speed in meters per second at the bottom of the hill?

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WORK AND ENERGY. POTENTIAL ENERGY An object can store energy as the result of its position Potential energy is the stored energy possessed by an object.

WORK AND ENERGY. POTENTIAL ENERGY An object can store energy as the result of its position Potential energy is the stored energy possessed by an object.

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