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Journal #51 1/6/2010

Journal #51 1/6/2010 . Calculate the answers to the following problems. Use scientific notation and sig figs in your answers. Journal #51 Solutions. Excerpts from Ch 10-11. Work, Power, and Energy. Work .

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Journal #51 1/6/2010

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  1. Journal #51 1/6/2010 • Calculate the answers to the following problems. Use scientific notation and sig figs in your answers.

  2. Journal #51 Solutions

  3. Excerpts from Ch 10-11 Work, Power, and Energy

  4. Work • Work in the scientific sense: The work done on an object is equal to the product of the force exerted in the direction of motion, times the object’s displacement • Work is a vector quantity! • Work = Force x distance

  5. More on Work… • To lift an object requires work, but to hold an object without moving it (by definition) does not require work. • The unit of joule comes from the name of a British physicist named James Prescott Joule.

  6. Work = Force|| x Distance • Only the component of force parallel to the displacement does work • In this case, the distance is the magnitude of the displacement.

  7. x

  8. Practice Problems - work

  9. Example Problem #2 Solution A.) B.)

  10. Example Problem #3 Solution A.) B.)

  11. Power • Power is equal to the amount of work done per unit time. • The unit for power is the Joule/second which is also called a Watt named after the Scottish inventor, James Watt

  12. What is horsepower? • 1 horsepower = 746 Watts • The horsepower was originally defined to compare the output of steam engines with the power of draft horses • Horsepower is not a recognized SI unit but is still widely used in many applications (such as cars).

  13. Homework – Work and Power • P. 278 at back of chapter 10 #52-59 You must show left-hand line-up (aka - define your terms!), formulas used, all of your work, then circle your final answers!

  14. Journal #52 1/7/10 • Order the speed of the rollercoaster (from fastest to slowest) at the positions labeled. If you believe the speed would be the same at two points, indicate that as well. E F A D B C

  15. Journal #52 Solution • Fastest to slowest: C, F, B, A & D same, then E is slowest. • What rule can we make up? E F A D B C

  16. Making the graph

  17. Energy…the BIG picture • The Universe is made up of matter and energy. • Energy is the “mover” of matter. • There are many forms of energy. • Conversions from one form of energy to another continually occur. • Energy cannot be created or destroyed.

  18. Forms of energy • There are many forms of energy – chemical, mechanical, thermal, solar, electrical, kinetic, potential… • All are measured in Joules, the same units as work!

  19. Potential Energy • Potential energy (symbol PE) is stored chemical energy or energy of position. • Types of PE: elastic, gravitational and chemical.

  20. Gravitational Potential • Gravitational Potential Energy can be found by multiplying the mass (in kilograms) of an object, by the acceleration due to gravity, and the height (in meters) GPE = mgh

  21. Kinetic Energy • Kinetic energy (symbol KE) is energy of motion. Kinetic energy depends on both mass and velocity

  22. Mechanical Energy • Energy possessed by an object due to its motion or its stored energy of position. • It can be either potential energy or kinetic energy, or a combination of both • All forms of energy are measured in joules (J).

  23. Mechanical Energy Conversions …total energy is constant

  24. Work-Energy Theorem • Relates chapter 10 Work and 11 Energy • Note that Kinetic Energy (KE) of a moving object = the work an object can do while it is being brought to rest • Work = change in KE; if no change in KE, no work is done

  25. Calculate speed at positions B,C and D.

  26. More Energy Transfer • High speed winds are used to do work on the blades of a turbine at the so-called wind farm. • Mech. Energy from the air gives the air particles the ability to apply a force to the blades. • As the blades spin, their energy is subsequently converted into electrical energy (a non-mechanical form of energy) and supplied to homes and industries in order to run electrical appliances.

  27. Energy Transfer... • This diagram shows that the boys potential energy is changing. Explain what is happening, in other words how is the energy changing?

  28. Law of Conservation of Energy • Energy cannot be created or destroyed. • Energy can be transformed from one form to another, but the total amount of energy never changes. • Ex: We can go from PE to KE to PE and back again…think about coaster or pendulum

  29. What happens to energy? • The energy we often think of as lost is not truly lost but is just converted into forms that may no longer be useful to us – often transferred to thermal energy or heat…Ex: burning log in a fireplace; gasoline in an engine; electric light bulb

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