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ENERGY AND WORK

ENERGY AND WORK. Chapter 7. Energy: The ability to do work. Types – Kinetic – energy of motion Mechanical – done by machines Potential – stored energy (gravitational or elastic/flexed solids) Chemical – found in the bonds between atoms. Types cont.

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ENERGY AND WORK

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  1. ENERGY ANDWORK Chapter 7

  2. Energy: The ability to do work Types – • Kinetic – energy of motion • Mechanical – done by machines • Potential – stored energy (gravitational or elastic/flexed solids) • Chemical – found in the bonds between atoms

  3. Types cont. • Thermal – heat or internal kinetic energy • Sound – vibrations in matter carried on waves • Light – electromagnetic radiation • Electrical – moving and static electrical charges • Nuclear – found within the nucleus of atoms

  4. Mechanical Energy and Work • Work – a force exerted during a distance. W = F*d • Work and Energy… You must have energy to do work. • Unit is Joules (J).

  5. 230N 1 m Mechanical Work A weightlifter attempting to lift 230N (about 50 lbs) must pull with a force equal to the weight of the barbell to get it off the ground. At the top of his lift, he has raised the bars 1 meter. How much work has he done? W = F*d W = 230N * 1m W = 230 What unit do we use? Joules J

  6. 230N 2.5m Cont. • How much work to raise the bar over his head to the height of 2.5 m? * • W= 230N*2.5m • W= 575J • If we doubled the weight to 460N, what happens to the amount of work? It would double as well! Directly Proportional *If his arms were long enough!

  7. F distance Work and Vectors (direction) Work is a vector, which means it has a direction. Work can only be done if the force and distance directions are the same. NO WORK DONE!!!

  8. Power • Amount of work done per the amount of time it takes to do it. Work done (J) Power = Time (s) Unit of power = J/s = 1 watt (W) 1 kilowatt (kW) = 1000 W

  9. Law of Conservation of Energy • 1st Law of Thermodynamics • Energy can be changed from one form to another, but it can NOT be destroyed. • When a change of energy occurs, the final energy is equal to the beginning energy. • The ultimate source of ALL energy on Earth?

  10. Conservation of Energy: bow and arrow • The bow stores potential energy in its bent limbs when drawn. • This energy is transformed into kinetic energy as the arrow is released. • PE of the bow = KE of arrow • PE & KE = work the archer had to use to draw the bow

  11. Where does the energy go??? • Even though the arrow is now at rest (no KE) and the bow is not drawn (no PE), NO ENERGY HAS BEEN DESTROYED! • Where did it go?... • The target did “Work” to the arrow by bringing it to rest with friction… a force which was exerted over a distance (the depth the arrow entered the target) • Whenever energy is “lost” it is ALWAYS converted to HEAT.

  12. Potential Energy: Stored! • Gravitational • PE g = mass * gravity * height • Higher an object = ____PE • Greater the mass = ____PE • Greater gravity = ____PE • Elastic • PE elastic= distance stretched x weight • Greater the weight = ____PE • Greater the distance stretched = ____PE

  13. Kinetic Energy: Motion! • KE = ½ mass * velocity2 • The more massive the object = ____KE • The faster the object = ____KE • Why does velocity have a greater role in KE than mass? The mass is cut in half but the velocity is squared!

  14. Other thoughts • What would happen to the depth of penetration if the arrow were moving faster? • What happens to the speed of the arrow when shot from a “stiffer” (harder to draw) bow? • What happens to the speed of the arrow if it weighs more? • What happens to the speed of the arrow if it is longer? (bow pushes it for a longer distance)

  15. Law of Conservation of Energy Again!!! • Energy can be changed from one form to another, but it can NOT be destroyed. • This means that when a change of energy occurs, the final energy is equal to the beginning energy.

  16. Law of Conservation of Energy – YET AGAIN!!! • KEi + PEi = KEf + PEf • In other words, the sum total of KE +PE before and after some kind of transformation MUST be equal.

  17. Conversions meters • Distance has to be in ______. • Force has to be in ______. • How do you change kg to N? • Time has to be in _____. • Mass has to be in _____. • Velocity has to be in _____. • How do you change W to kW? Newtons x10 seconds kilograms m/s /103

  18. Law of Thermodynamics - Entropy • Entropy – a change of a usuable form of energy into an unusable form… HEAT (thermal energy) • Whenever energy is “lost” it is ALWAYS converted in to HEAT.

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