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Mechanical Energy Conservative Forces Conservation of Mechanical Energy

Mechanical Energy Conservative Forces Conservation of Mechanical Energy. Conserved Quantities. When a quantity is conserved, it remains constant The amount of a conserved quantity is the same at the beginning of a situation as it is at the end of the situation. Conservation of Energy.

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Mechanical Energy Conservative Forces Conservation of Mechanical Energy

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  1. Mechanical EnergyConservative ForcesConservation of Mechanical Energy

  2. Conserved Quantities • When a quantity is conserved, it remains constant • The amount of a conserved quantity is the same at the beginning of a situation as it is at the end of the situation

  3. Conservation of Energy • The TOTAL amount of energy in a closed system will remain constant • CLOSED SYSTEM: Nothing in or out – no interaction with the surroundings • TOTAL ENERGY: all kinds of energy (kinetic, PEE, PEG, chemical, heat, etc.)

  4. Mechanical Energy • Mechanical Energy is the sum of kinetic and potential energy • ME = KE + PE • ME = KE + PEG + PEE • “Useful” energy • “Can the energy break the window?”

  5. Conservative Forces • Gravity and elastic forces • Conservative forces DO NOT change the amount of mechanical energy of a system • Energy is converted from one form to another, but the total amount of mechanical energy remains the same

  6. Conservation of Mechanical Energy • When there are only conservative forces acting on an object (gravity, springs), MECHANICAL ENERGY IS CONSERVED • MEi = MEf • KEi + PEGi + PEEi = KEf + PEGf + PEEf

  7. Conservative ForcesConservation of Mechanical Energy PEG decreases & KE increases, but total ME remains constant (in the absence of air friction)!

  8. 10000 J 5000 J Time (s) Conservation of ME & Falling Objects

  9. Conservation of ME & Pendulums PEG = mgh KE = 0 KE = ½ mv2 PEG = 0

  10. Is Mechanical Energy Conserved Here?

  11. Nonconservative Forces • When nonconservative forces act on a system, mechanical energy IS NOT conserved • WNC = ΔME • This is the Work-Energy Theorem

  12. Friction • Friction does negative work on the system, so the mechanical energy is decreased • While ME is not conserved when nonconservative forces act on an object, TOTAL ENERGY is always conserved • Total energy would include heat energy • Non-Conservative forces turn ME into non-ME (usually heat), but the total amount of energy stays the same

  13. Friction & Pendulums

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