6.0 - Introduction
The use of energy has played an important role in defining much of human history.
Fire warmed and protected our ancestors. Coal powered the Industrial Revolution.
Gasoline enabled the proliferation of the automobile, and electricity led to indoor
lighting, then radio, television and the computer. The enormous energy unleashed
by splitting the atom was a major factor in ending World War II. Today, businesses
involved in technology or media may garner more newspaper headlines, but energy
is a larger industry.
Humankind has long studied how to harness and transform energy. Early machines
used the energy of flowing water to set wheels spinning to mill grain. Machines
designed during the Industrial Revolution used energy unleashed by burning coal to
create the steam that drove textile looms and locomotives. Today, we still use these
same energy sources í water and coal í but often we transform the energy into
electric energy.
Scientists continue to study energy sources and ways to store energy. Today,
environmental concerns have led to increased research in areas including atomic
fusion and hydrogen fuel cells. Even as scientists are working to develop new
energy technologies, there is renewed interest in some ancient energy sources: the Sun and the wind. They too can provide clean energy via
photovoltaic cells and wind turbines.
Why is energy so important? Because humankind uses it to do work. It no longer requires as much human labor to plow fields, to travel, or to
entertain ourselves. We can tap into other energy sources to serve those needs.
This chapter is an introduction to work and energy. It appears in the mechanics section of the textbook, because we focus here on what is
called mechanical energy, energy arising from the motion of particles and objects, and energy due to the force of gravity. Work and energy also
are major topics in thermodynamics, a topic covered later. Thermodynamics adds the topic of heat to the discussion. We will only mention heat
briefly in this chapter.
Whatever the source and ultimate use of energy, certain fundamental principles always apply. This chapter begins your study of those
principles, and the simulation to the right is your first opportunity to experiment with them.
Your mission in the simulation is to get the car over the hill on the right and around a curve that is beyond the hill. You do this by dragging the
car up the hill on the left and releasing it. If you do not drag it high enough, it will fail to make it over the hill. If you drag it too high, it will fly off
the curve after the hill. The height of the car is shown in a gauge in the simulation.
Only the force of gravity is factored into this simulation; the forces of friction and air resistance are ignored. In this chapter, we consider only the
kinetic energy due to the object moving as a whole and ignore rotational energy, such as the energy of the car wheels due to their rotational
motion. (Taxes, title and dealer prep are also not factored into the simulation; contact your local dealership for any other additional restrictions
or limitations.)
Make some predictions before you try the simulation. If you release the car at a higher point, will its speed at the bottom of the hill be greater,
the same or less? How high will you have to drag the car to have it just reach the summit of the other hill: to the same height, higher or lower?
You can use PAUSE to see the car’s speed more readily at any point.
When you use this simulation, you are experimenting with some of the key principles of this chapter. You are doing work on the car as you drag
it up the hill, and that increases the car’s energy. That energy, called potential energy, is transformed into kinetic energy as the car moves
down the hill. Energy is conserved as the car moves down the hill. It may change forms from potential energy to kinetic energy, but as the car
moves on the track, its total energy remains constant.
6.1 - Work
Work: The product of displacement and the
force in the direction of displacement.
You may think of work as homework, or as labor done to earn money, or as exercise in
a demanding workout.
But physicists have a different definition of work. To them, work equals the component
of force exerted on an object along the direction of the object’s displacement, times the
object’s displacement.
When the force on an object is in the same direction as the displacement, the
magnitude of the force and the object’s displacement can be multiplied together to
calculate the work done by the force. In Concept 1, a woman is shown pushing a crate
so that all her force is applied in the same direction as the crate’s motion.
Work
Product of force and displacement