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More EnergyMore EnergyMore EnergyMore Energyand someand some

example problemsexample problems

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Quick Review• Energy is what causes work to be done• Kinetic Energy

– KE= ½ mv2

• Potential Energy– PE=mgh

• Conservation of Energy– ΔKE=-ΔPE

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Conservation of Energy if system is not closed

• Conservation of Energy can be used to solve problem even without a closed system

• The energy lost from the system goes into Work

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Conservative and non-Conservative

forces• Conservation

Forces have energy conserved

• Gravity• Potential Energy• ΔPE=-ΔKE

• Non-Conservative forces have energy turn into Work

• Friction• ΔPE=W=Fdor

• ΔKE=W=Fd

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Find the Kinetic Energy• A) 50 kg bowling ball with a

velocity of 20 m/s• B) A 9.11x10-31 kg electron with a

velocity of 3.00x107 m/s• C)A 1 kg ice block with a velocity

of 1 m/s

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Find the Potential Energy

• A) 50 kg bowling ball 20 meters high

• B) A 9.11x10-31 kg electron with a height of 155 meters

• C)A 1 kg ice block with a height of 5565 m

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Spring Potential Energy • PEelastic=½kx2

– k-spring constant– Ex: A block on a vertical spring with a

k= 5.00 x 103 N/m and is pushed downward, compressing the spring 0.100 m. What is the potential energy of the spring?

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Conservation of Energy Ex: Problems

• A) 50 kg bowling ball with a velocity of 20 m/s is thrown into the air. At what height will it stop?

• B) 50 kg bowling ball is 20 meters high. What is the velocity just before it hits the ground?

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– Ex: A block on a vertical spring with a k= 5.00 x 103 N/m and is pushed downward, compressing the spring 0.100 m. If the block has a mass of .25kg:A. What is the potential energy of the

spring?B. How fast is the block moving when

released from the spring?C. What is the maximum height reached

by the block?

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Some energy lost to friction

• A 65 kg refrigerator slides down 10. meters along a driveway against a force of friction of 12N. If the refrigerator ends .60 meters below its original starting point what is the final velocity?

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