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Ch. 8 Energy

Ch. 8 Energy. Physical Science. What IS energy??. List 5 things that have or exert energy. . Energy is the “stuff” that allows things to do WORK. WORK is the product of a constant force and distance through which an object is moved. Work = force x distance w = F d

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Ch. 8 Energy

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  1. Ch. 8 Energy Physical Science

  2. What IS energy?? List 5 things that have or exert energy.

  3. Energy is the “stuff” that allows things to do WORK. WORK is the product of a constant force and distance through which an object is moved. • Work= force x distance • w = F d • units - joules (a Newton-meter) • Joule is pronounced JOOL (rhymes with cool)

  4. Examples: • Ex.) Lift a book with 25 N of force through a distance of 2 m. • w = fd = (25N) x (2m) = 50 J • Non-ex.) Hold a book with 25 N of force and walk 2 m. • w = 0 • Why is work zero? Because the distance must be in the same direction as the force!

  5. 2 categories of work: • Work done to change the speed of an object. • Work done against another force.

  6. Work transfers energy… • …and power is how long it takes to do work!!! • Power= work done/time interval • Or P=w/t • Unit is the Watt (joule per second) • Ex.) Lift a 25N book 2m in 5 seconds • 50 J / 5 seconds = 10 J/s = 10 Watts • Ex2.) Lift the same book in 1 second. • p = 50 J / 1 second = 50 Watts

  7. MECHANICAL ENERGY • TYPE 2: energy due to the movement of something. • Kinetic Energy • KE=1/2 mv2 • Or… Fd=1/2mv2 • Ex.) throwing a ball, stopping a car • TYPE 1: energy due to the position of something. • Potential Energy • PE= mgh • Ex.) stretched rubber band, chemical energy in food (molecules)

  8. Mechanical Energy= KE + PE

  9. Ski jumper- notice the total energy equals 50,000 Joules at all times… its just transferred!

  10. Conservation of Energy • Energy can not be created or destroyed, it can only be transformed into one form or another. • If no external forces act on a system, the TOTAL energy of the system will remain constant.

  11. Efficiency • The efficiency of a machine tells how much of the energy that goes into a machine actually does useful work. • It is usually expressed as a percent. • Efficiency= Useful work done/Energy input= 100% • Or • Efficiency= Actual MA/Theoretical MA

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