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Energy and Work Energy and Work DESCRIBE THE CONSERVATION OF ENERGY AND HOW ENERGY RELATES TO WORK. Terms and Definitions Potential Energy The energy that an object has because of position Kinetic Energy The energy of an object due to the object’s motion Work Transfer of energy to an object by the application of a force that causes the object to move in the direction of the force Power A quantity that measures the rate at which work is done Equations Variable Equation Potential Energy Kinetic Energy Work Power PE PE = mgh MKS Units Joules (J) English Units ft x lbs KE KE=½mv2 J ft x lbs W P W = Fd P = W/t J Watts (W) ft x lbs ft x lbs/s Sample Problem Problem An 0.125 kg apple is hanging on a tree 3 meters high. How much potential energy does the apple have? The apple falls. How much kinetic energy will it have when it hits the ground? Answer m = 0.125 kg g = -9.8 m/s2 d = -3 m PE = ? PE = mgh PE = (.125)(-9.8)(-3) PE = 3.675 J KE = 3.675 J Sample Problem Problem Answer A crane uses an average F = 5,200 N force of 5,200 N to lift a girder 25 m. How much work does the crane do on the girder? How much power was needed if it took 50 seconds to lift the girder? d = 25 m W=? W = Fd W = (5,200)(25) W = 130,000 J t = 50 s P = W/t P = 130,000/50 P = 2600 W