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Understanding Work, Power, Energy, and Efficiency

This chapter covers the concepts of work, power, mechanical energy, potential energy, kinetic energy, work-energy theorem, conservation of energy, simple machines, and efficiency.

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Understanding Work, Power, Energy, and Efficiency

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  1. Notes: Chapter 9 EnergyObjectives • Define and describe work (9.1) • Define and describe power (9.2) • Define mechanical energy. (9.3) • Define potential energy. (9.4) • Define kinetic energy and describe work-energy theorem. (9.5) • State the law of conservation of energy. (9.6) • Describe simple machines and mechanical advantage. (9.7) • Explain why no machine can have an efficiency of 100%. (9.8) • Describe the role of energy in living organisms. (9.9)

  2. 9.1 Work • Work is done when a force acts on an object and the object moves in the direction of the force. • Work is the product of the net force working on an object and the distance through which the object moves. What are the units for force?

  3. Work falls into two categories: • To move an object against a force. • To change the speed of an object. • Work changes the energy state of an object.

  4. 9.2 Power • Power is the rate at which work is done. • Power equals to the amount of work done divided by the time interval during which the work is done. What is the unit for power?

  5. Question • If a forklift is replaced with a new forklift that has twice the power, how much greater a load can it lift in the same amount of time? If it lifts the same load, how much faster can it operate?

  6. 9.3 Mechanical Energy • Energy is the property of an object or system that measures its ability to do work. • Energy due to the position or movement of something is referred to as mechanical energy. • Two forms: • Potential energy • Kinetic energy

  7. 9.4 Potential Energy • Potential energy is stored energy related to an object’s relative position that has the ability to do work. • Three types of potential energy: • Elastic potential energy • Chemical potential energy • Gravitational potential energy

  8. The amount of gravitational potential energy possessed by an elevated object is equal to the work done against gravity to lift it. • Height is the distance above some arbitrarily chosen reference level. • A position below this level gives the PEg that is negative with respect to the reference point.

  9. Questions • How much work is done on a 100-N boulder that you carry horizontally across a 10 m room? How much potential energy does it gain? • A.) How much work is done on a 100 N boulder when you lift it 1 m?B.) What a power is expended if you lift the boulder a distance of 1 m in a time of 1 s?C.) What is the GPE of the boulder in the lifted position?

  10. 9.5 Kinetic Energy • A moving object has the ability to do work. • Kinetic energy is the energy of motion. • The KE of an object is equal to the work required to bring to its speed from rest, or the work the object can do while being brought to rest.

  11. Question • When the brakes of a motorcycle traveling 60 km/h become locked, how much farther will the motorcycle skid than if it travels at 20 km/h?

  12. 9.6 Work-Energy Theorem • Work-energy theorem – whenever work is done energy changes.

  13. A car that has TWICE the KE than another car would require twice the work to stop it. If the braking friction were the same in both cases, how much more stopping distance would the car with twice the KE need?

  14. Kinetic energy can take several forms: • Heat • Light • Sound • Electricity

  15. 9.7 Conservation of Energy • The law of conservation of energy states that energy cannot be created or destroyed. It can be transformed from one form of energy to another but the total energy never changes.

  16. When it appears that some energy is lost from the system, it was not destroyed. The unaccounted energy was given off as heat.

  17. Conservation of mechanical energy • The total mechanical energy of a system remains the same before during and after energy is transformed.

  18. Questions • A 45 kg mass is dropped from a height of 10.0 m. How fast is it going when it gets to a height of 5.0 m? How much work did gravity have to do to get this mass up to this speed? • How fast would you have to jump in order to reach a height of 0.6096 m (2 ft)?

  19. 9.8 Machines • Machine – a device used to multiply forces or simply change the direction of forces, or both. • Transfers energy from one place to another or transforms it from one form to another. • Cannot do more work than what is put into it. That is, it cannot create energy. • Basic idea: Work in = Work out

  20. Lever – a simple machine made of a bar that pivots about a fixed point called the fulcrum.

  21. Mechanical advantage (MA) – ratio of output force to input force for a machine. It’s the number of time the machine multiplies force.

  22. Three kinds of levers:

  23. Pulley – a type of lever that is a wheel with a groove in its rim, which can be used to change the direction of a force exerted on a rope. A pulley or system of pulleys can also multiply force. • General rule for pulleys: the number of ropes supporting the load equals to the mechanical advantage (ME).

  24. 9.9 Efficiency • Ratio of useful energy output to total energy input. Or, also, the percentage of work input that is converted to work output. • Lower the efficiency the, the greater is the energy wasted to heat loss.

  25. Inclined plane – a simple machine sometimes called a ramp.

  26. 9.10 Energy for Life • There is more energy stored in the molecules of food than in the reaction products (CO2 and H2O) after the food has been metabolized. • Process is called respiration. • Energy enters an ecosystem when photoautotrophs capture energy from the sun to convert carbon dioxide and water into sugars. • Process is called photosynthesis.

  27. 9.11 Sources of Energy • The sun is the source of practically all our energy here on earth. • Fossil fuels: petroleum, coal, and natural gas • Exceptions: nuclear energy and geothermal energy • Solar power • Photovoltaic (solar) cells: direct transformation to electricity • Wind: Indirect. Caused by uneven heating of earths surface. • Running water: Indirect. Caused by evaporation, condensation, and precipitation at higher elevations

  28. Fuels cells: the opposite reaction of electrolysis. Hydrogen and oxygen are recombined to form water. The process can be used to generate an electric current. • Nuclear and geothermal energy • Most concentrated form in uranium and plutonium. • Earth’s interior is kept hot by nuclear reaction. Basic set up for geothermal power generation.

  29. Power demand for an exponentially growing human population. • Reached 7 billion in October 2011 • Pros and cons of fossil fuels • Pros and cons of nuclear power • Pros and cons of wind and hydropower • Pros and cons of solar

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