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lecture chapter 8

# lecture chapter 8 - Chapter 8 Potential Energy Potential...

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13 Chapter 8. Potential Energy Potential energy is energy stored in gravity, spring… Potential energy, labeled as U, or PE. SI unit: J A ball with initial velocity v 0 raised h, Work done by gravity: Wg = mg * h*cos(180) = - mg h Gravity force did negative work. Change of kinetic energy K = Kf –Ki = 0 – ½ m v 0 2 = Wg = - mg h Lost Kinetic Energy of amount mg h Q: Where did those energy go? A: It was converted to potential energy. Potential energy gain (mg h) during the rising part Energy conservation: Energy cannot be created or destroyed. It only change forms. When Energy seems to be created/ destroyed, it actually is being converted from/to other forms, U = m g y ; y is the height above “ground”

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14 Gravitational potential energy: PE or U, is due to being under gravitational influence of another mass. (Earth). U = m g y ; y is the height above “ground” Potential Energy stored in a string: U= ½ k x 2 ; k :Hooke’s constant, which shows the stiffness of the spring. k here is not kinetic energy K, Hooke’s constant lower case k. Conservative forces : force does zero total work on a closed path. Related Potential Energy does not change, when it goes back to initial position. Spring force and gravity force are the only conservative forces you will see in this semester. Work done by gravity and spring forces converts energy between PE and KE.
15 Mechanical Energy: E Sum of Potential Energy and Kinetic Energy. E = K + U or E = KE + PE Mechanical Energy conservation: When only Spring force and/or gravity force does work, K+U stay constant. Ei=Ef If no friction , no resistance (air, ice, not water…) and no external forces and energy source does work , (no human, animal, motor, lift, push…) You can happily and safely have Ki + Ui =Kf +Uf for any two points during the motion.

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lecture chapter 8 - Chapter 8 Potential Energy Potential...

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