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Energy Exchanges

Energy Exchanges. Conservation of Energy. Things to consider…. Work Is a transfer of energy. When an object DOES work, it “loses” energy by transferring it elsewhere. When an object has work DONE on it, it gains or loses an amount of energy equal to the work done.

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Energy Exchanges

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  1. Energy Exchanges Conservation of Energy

  2. Things to consider… • Work • Is a transfer of energy. • When an object DOES work, it “loses” energy by transferring it elsewhere. • When an object has work DONE on it, it gains or loses an amount of energy equal to the work done. • Work done on an object can be stored as gravitational or elastic potential energy

  3. More things to consider… • Potential Energy is STORED ENERGY • Gravitational Potential Energy = mgh • Elastic Potential Energy = ½ kx2 • Kinetic Energy is ENERGY OF MOTION • KE = ½ mv2 • Both work and energy are measured in Joules. • The rate at which work is done (or energy is exchanged) is called Power, and is measured in Watts.

  4. Work/Energy Theorem and Conservation of Energy • W = ΔKE • For CONSERVATIVE FORCES… • Wg= ΔGPE • PE1 + KE1 = PE2 + KE2 • More generally… • PE1 + KE1 = PE2 + KE2 + Wfriction • Wfrictionis the energy transformed by friction into thermal energy.

  5. Set up the problem • Scenario #1 • A bowling ball is lifted up onto a rack for storage. • Statement: • The work done to lift the bowling ball is equal to the grav. Potential energy it has in the rack. • Equation: • Work = GPE or Fd = mgh

  6. Set up the problem • Scenario #2 • A bowling ball is dropped onto the ground. • Statement: • The gravitational potential energy it has is converted to kinetic energy as it falls. At the bottom all GPE has been converted to KE. • Equation: • PE1 + KE1 = PE2 + KE2 or PE1 = KE2

  7. Set up the problem • Scenario #3 • A dart is loaded into a toy gun by pushing against a spring (horizontally) • Statement: • Work done is stored as elastic potential energy in the spring. • Equation: • Work = EPE or Fd = ½ kx2

  8. Set up the problem • Scenario #4 • The trigger releases the dart from the spring-loaded toy gun (horizontally) • Statement: • The elastic potential energy of the spring is converted to kinetic energy. • Equation: • EPE = KE or ½ kx2 = ½ mv2

  9. Set up the problem • Scenario #5 • Instead of aiming horizontally, the toy gun is aimed straight up and the dart leaves the gun and rises as high as it can go. • Statement: • The elastic potential energy of the spring is converted to kinetic energy (and work to lift the dart) but ultimately the energy is converted to gravitational potential energy. • Equation: • EPE = GPE or ½ kx2 = mgh

  10. Set up the problem • Scenario #6 • A diver makes a running leap off a diving platform and lands in the water. • Statement: • The kinetic energy and gravitational potential energy of the diver is converted to kinetic energy as the diver falls. Upon landing, all the energy has been transformed into kinetic energy. • Equation: • PE1 + KE1 = PE2 + KE2 or PE1 + KE1 = KE2

  11. Set up the problem • Scenario #7 • A car’s engine stalls and it rolls to a stop. • Statement: • Friction does work on the car to reduce it’s kinetic energy to zero. • Equation: • W = ΔKE or Fd = ΔKE or…KE1 = KE2 + Wf

  12. Set up the problem • Scenario #8 • After being kicked, a football sails through the goal posts. • Statement: • Taking the point of release after the kick as zero height, the football exchanges some of its kinetic energy for potential energy. • Equation: • PE1 + KE1 = PE2 + KE2 or KE1 = PE2 + KE2

  13. Set up the problem • Scenario #9 • A child slides down a playground slide. The velocity at the bottom is measured. How might you go about determining what work was “lost” (that is, turned into thermal energy by friction). • Statement: • The potential energy at the top of the slide is converted to kinetic energy at the bottom. The expected energy can be compared to the actual energy evidenced by the speed. The difference is the answer. • Equation: • PE1 + KE1 = PE2 + KE2 + Wfriction or PE1 = KE2+ Wfriction

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