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Physical Science Coach Kelsoe Pages 453–459 S ECTION 15–2: E NERGY C ONVERSION AND C ONSERVATION

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O BJECTIVES Describe conversions of energy from one form to another. State and apply the law of conservation of energy. Analyze how energy is conserved in conversions between kinetic energy and potential energy and solve equations that equate initial energy to final energy. Describe the relationship between energy and mass and calculate how much energy is equivalent to a given mass.

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E NERGY C ONVERSION Energy can be converted from one form to another. The process of changing energy from one form to another is energy conversion. These conversions happen very frequently. Light bulbs convert electrical energy into thermal energy and electromagnetic energy.

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C ONSERVATION OF E NERGY When energy changes from one form to another, the total energy remains unchanged even though many energy conversions may occur. The law of conservation of energy states that energy cannot be created or destroyed. According to the law of conservation of energy, energy can be converted from one form to another, but you will always finish with what you started with in a closed system.

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E NERGY C ONVERSIONS One of the most common energy conversions is between potential energy and kinetic energy. The gravitational potential energy of an object is converted to the kinetic energy of motion as the object falls. Sea gulls use energy conversions to eat. They cant break open the shells of oysters, so they pick them up, fly to a high altitude, and drop the oyster over the rocks. The potential energy of the oyster at its height is gradually converted to all kinetic energy as it hits the rocks.

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E NERGY C ONVERSION IN P ENDULUMS Christiaan Huygens, a Dutch scientist, was the first person to use a pendulum in a clock. The time it takes a pendulum to swing back and forth once is precisely related to its length. At the highest point of its swing, the pendulum has nothing but gravitational potential energy. As it reaches the lowest part of its swing, the gravitational potential energy converts to kinetic energy. As it swings back upward, the kinetic energy is converted to gravitational potential energy again.

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E NERGY C ONVERSION C ALCULATIONS When friction is small enough to be ignored, and no mechanical energy is added to a system, then the systems mechanical energy does not change. Total mechanical energy is equal to the total kinetic energy (KE) plus the total potential energy (PE). Mechanical Energy = KE + PE The conservation of mechanical energy says that mechanical energy remains constant during any process. (KE + PE) beginning = (KE + PE) end

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S AMPLE P ROBLEM – C ONSERVATION OF M ECHANICAL E NERGY At a construction site, a 1.50-kg brick is dropped from rest and hits the ground at a speed of 26.0 m/s. Assuming air resistance can be ignored, calculate the gravitational potential energy of the brick before it was dropped. Given:Mass: 1.50 kg Speed: 26.0 m/s Beginning KE: 0 J Ending PE: 0 J

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S AMPLE P ROBLEM – C ONSERVATION OF M ECHANICAL E NERGY At a construction site, a 1.50-kg brick is dropped from rest and hits the ground at a speed of 26.0 m/s. Assuming air resistance can be ignored, calculate the gravitational potential energy of the brick before it was dropped. Solve:(KE + PE) beginning = (KE + PE) end (0 J + PE) beginning = (KE + 0 J) end (PE) beginning = (KE) end Ending KE: ½mv 2 = ½(1.50 kg)(26.0 m/s)2 = 507 J Beginning PE: 507 J

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E NERGY AND M ASS Albert Einstein developed his special theory of relativity in This theory included the now- famous E = mc 2. E = energy, m = mass, c = speed of light Einsteins equation says that energy and mass are equivalent and can be converted into each other. In other words, energy is released as matter is destroyed, and matter can be created from energy.

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V OCABULARY Energy conversion

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