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Energy:

Energy:. The Capacity to Effect Change. Presentation 2003 R. McDermott. Energy is all Around Us:. It causes changes in velocity – kinetic energy It causes rearrangement – potential energy It stretches and compresses – elastic energy It causes heating – dissipated energy. Energy is Energy!.

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Energy:

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  1. Energy: The Capacity to Effect Change Presentation 2003 R. McDermott

  2. Energy is all Around Us: • It causes changes in velocity – kinetic energy • It causes rearrangement – potential energy • It stretches and compresses – elastic energy • It causes heating – dissipated energy

  3. Energy is Energy! • Don’t be confused; all energy is the same, the only difference is the change that energy produces. • Energy is like water; pouring it into different containers may make it look different, but it really isn’t.

  4. Kinetic Energy – Energy of Motion • Energy that causes motion is called kinetic energy: • Translational: linear motion • Rotational: spinning, tumbling • Vibrational: back and forth • Translational KE = ½ MV2

  5. What is the kinetic energy of a 2000kg car moving at 40m/s? Answer: 1,600,000 J Example:

  6. Comments: • Kinetic energy is directly proportional to mass; doubling the mass doubles the KE • Kinetic energy is directly proportional to the square of the velocity; doubling velocity quadruples the KE • Velocity is a larger factor in determining KE than is mass

  7. Gravitational PE – Energy of Height • Energy stored in gravitational field due to separation of mass and Earth • Changes the field geometry • We (erroneously) say that energy is stored in the lifted object, but it is actually in the field • Gravitational PE = MgH

  8. How much potential energy is stored when a 100kg man climbs to the top of a 1000m peak? Answer: 981,000 J Example:

  9. Comments: • Gravitational potential energy is directly proportional to mass and height; doubling either one doubles the PE • Gravitational potential energy is also directly proportional to the gravitational field strength; traveling to a planet with a higher ‘g’ would cause higher PE values for a given height.

  10. Elastic Energy – Energy of a Spring • Stretch or Compression • Energy stored in spring • A type of potential energy • Spring PE = ½ kX2

  11. How much elastic energy is stored in a spring (k= 8 N/m) that is compressed by 0.05 m? Answer: 0.01 J Example:

  12. Comments: • Elastic potential energy for a spring is directly proportional to the spring strength (k); doubling k doubles the PE • Elastic potential energy is also directly proportional to the square of the stretch or compression; doubling quadruples the PE • Stretch/compression is a larger factor in determining PE than is the value of k

  13. Dissipated Energy – “Lost”Energy • Non-isolated system • Energy dispersed into air, ground, etc. • Heat, sound, light, etc • Friction is a common cause • Collisions also lead to dissipated energy

  14. A 2000 kg car moving at 20 m/s brakes to a stop. How much heat is produced in the brakes? Answer: KElost = Heat Heat = 400,000 J Example:

  15. Units • Working from PE = MgH, energy must have units of kg-m2/s2 • But using F = Ma, we see that this is must also be equal to a N-m • However, energy is assigned a derived unit, the Joule, which is equal to the units above • All SI energy units are given in Joules • Energy is a scalar

  16. Energy is Constant – Isolated System • Under normal conditions, energy cannot be created or destroyed • An isolated system/object (no outside interactions) has a fixed amount of energy • Although the change that energy produces (the “form of energy”) may change, the amount of energy in the system does not

  17. Conservation of Energy • In an isolated system, the total energy is constant though it may change “form” • A falling object gradually “converts” PE to KE • Releasing a spring “converts” PE to KE • In a non-isolated system, total energy can include dissipated energy to maintain a constant total • A braking car converts KE to heat • In these cases, total energy before = total energy after

  18. Conservation

  19. Energy and Position

  20. Solving Conservation Problems: • Identify the system/object • Earth is always part of the system • Identify initial energy “forms” • Identify final energy “forms” • Set total initial energy equal to total final energy • Solve

  21. As a coaster falls, PE is converted into KE Total PE + KE (mechanical energy) is constant PE lost = KE gained MgH = ½ MV2 V = (2gH) Roller Coaster – Energy Conservation

  22. A 2000 kg roller coaster car moves to the top of a 100m hill and then falls. Assuming it started at rest, how fast will it be moving when it reaches the bottom of the hill? Answer: MgH = ½ MV2 V = 44.3 m/s Example:

  23. A 2000 kg roller coaster car moves to the top of a 100m hill and then falls. How fast will the car be moving when it is halfway down? Answer: 31.3 m/s It’s the KE that is half, not the velocity! Follow-up:

  24. A 2.0 kg block falls 10 m in compressing a spring (k = 10 N/m). What was the compression of the spring? Answer: MgH = ½ kX2 X = 6.3 m Spring Example:

  25. Acknowledgements • Animations courtesy of Tom Henderson, Glenbrook South High School, Illinois • Artwork courtesy of Dr. Phil Dauber

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