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Energy, Work, Power, and the Law of Conservation of Mechanical Energy You are studying the most critical and looked out for issue in todays world. Energy • The ability of a physical system to do work on another physical system. • The ability to move matter/energy • Energy cannot be created or destroyed • S.I. Units: joules, kilowatt-hours, kilocalories • E = mc2 joules Forms of Energy • • • • • • • • • • • Matter (E=mc²) Thermal energy (thermal energy in transit is called heat) Chemical Energy Electric Energy Radiant Energy (the energy of electromagnetic radiation, aka light) Nuclear energy Magnetic energy Elastic energy Sound energy Mechanical energy (This is where we study…just a small bit) Luminous Energy Mechanical Energy • Sum of Potential (U) and Kinetic energy (K) of a system • Emechanical = U + K • If forces acting on a system are “conservative”, Emechanical is conserved – E.g.: gravity, ideal spring, magnetic, and electric forces • If forces acting on a system are “nonconservative”, Emechanical can be lost as heat. – E.g.: friction and air resistence Potential Energy (U) • Energy of a body or system due to it is position or arrangement of particles • E.g.: Gravitational, elastic, chemical, nuclear, electrostatic, and magnetic. • Ugrav = mgh = kg * m/s2 * m = J Kinetic Energy (K) • The energy of an object due to its motion • K = ½ mv2 = kg * m2/s2 = J Work • A force acting in a straight line through a distance • W = F * d = Fd cos Θ (where Θ is the angle between F and the displacement vector) = N * m = kg * m/s2 * m = joules • Work-Energy Theorem: W = ∆K = K2 – K1 = ½ m (v22 – v12) Power • The rate at which work is performed or energy is converted • Pavg = ∆W/ ∆t = J/s = Watt • 1 Horsepower = 745.7 Watts A kilogram of TNT contains 4.184 x 106 J of chemical energy. If a NASCAR Sprint Cup car has a mass of 3200lbs, how many kg worth of TNT equivalent would it possess in kinetic energy while travelling at 210 miles per hour? • V = 210 mi/h = 93.9 m/s (308 ft/s, FYI) • m = 3200 lbs = 1454.5 kg • K = 1/2mv2 = 1/2 (1454.5 kg)(93.9 m/s)2 = 6.4 x 106 J • 6.4 x 106 J / 4.184 x 106 J = 1.53 kg (~3.4lbs)TNT Equivalent If 2g of matter were converted into energy, for how many years could you power a 100W light bulb? • • • • • • • E=mc2 = .002kg * (3.0 x 108 m/s)2 = 1.8 x 1014 J Since Pavg = ∆Work/ ∆t t = Work/Pavg = 1.8 x 1014 J / 100 Watts = 1.8 x 1012 s = 57,077 years