7.1 Work: The Scientific Definition

7.1 Work: The Scientific Definition

  • Define nonconservative forces and explain how they affect mechanical energy.
  • The principle of conserve of energy can be applied by treating conservative forces in terms of their potential energy and non conservative forces in terms of their work.
  • The efficiency of an energy conversion process is the fraction left as useful energy or work, rather than being transformed into thermal energy.
  • Power is calculated by calculating changes in energy over time.
  • The cost of energy and power consumption are examined.
  • Explain the human body's consumption of energy when at rest and when engaged in activities that do useful work.
  • The inevitable conversion of energy to less useful forms makes it necessary to conserve energy resources.
  • In everyday events and scientific phenomena, energy plays an essential role.
    • There are many forms of energy, from that provided by our food, to the energy we use to run our cars, to the sunlight that warms us on the beach.
    • Someone with an energetic personality can be an example of what people call energy that isn't scientific.
    • One of the most important concepts of physics is the fact that energy has many interesting forms and is involved in almost all phenomena.
    • The total amount of energy in the universe is unchanging.
    • Energy can change forms, but it can't disappear without a trace.
    • One of the physical quantities we say is conserved is energy.
  • Even though scientists discovered new forms of energy, it has always been found to apply.
  • One of the major building blocks of modern civilization is energy.
    • Economic growth is hampered by energy resources.
    • The world's use of energy resources, especially oil, continues to grow, with ominous consequences.
    • We will look at the world's energy use patterns at the end of the chapter.
  • There is a scientific definition of energy.
    • There are many forms of energy and it is conserved.
    • We begin the chapter with a discussion of work.
    • Work is related to energy and how it moves from one system to another.
  • Writing an exam or carrying a heavy load on level ground are not work as defined by a scientist.
    • The relationship between work and energy is revealed by the scientific definition of work.
  • To do work in the scientific sense, a force must be exerted and there must be displacement in the direction of the force.
  • To find the work done on a system that undergoes motion that is not one-way or that is in two or three dimensions, we divide the motion into one-way one-dimensional segments and add up the work done over each segment.
  • The work done on a system by a constant force is the product of the component of the force in the direction of motion.
  • The force in the direction of motion is the component.
    • The briefcase has no energy in it.
    • It is possible to use the energy from the briefcase to do work.
    • The work done on the briefcase by the generator is negative because it is in opposite directions.
  • There must be force in the direction of the motion for work to be done.
  • There are two ways to understand this energy transfer.
    • The briefcase's weight is thought to give the generator energy.
    • The generator does negative work on the briefcase, thus removing energy from it.
    • The drawing shows the force of the generator on the briefcase and the displacement downward.
  • Both work and energy have the same units.
    • The units are force times distance from the definition of work.
    • A small apple is lifted by one joule, which is not a large amount of energy.
  • Compare this person's average daily intake of food energy to the amount of work they do.
    • The amount of heat needed to warm 1 g of water by is equivalent to the amount of food needed.
  • The problem can be solved by substituting the given values into the definition of work done on a system.
    • The work is unknown because the force, angle, and displacement are given.
  • The work in joules to kilo calories yields.
  • This is a small fraction of what the person consumes.
    • The energy that is released from the consumption of food is not used to work.
    • Less than 10% of our food energy is used to do work and more than 90% is converted to thermal energy or stored as chemical energy in fat.