Energy is one of the most critical concepts in physics, and it is in fact one of the most important concepts underlying all the sciences. It is a hard-won concept, though: many brilliant women and men wrestled with its existence, definition, and properties. Energy turns out to be a difficult concept to pin down, as energy takes many forms. Be prepared throughout this course to continually revise your ideas of what energy is.
pp. 110–111. Work. This chapter starts off with a bang. Instead of easing you into the concepts, it lays an important one out right at the beginning.
The section begins by comparing work to impulse. Impulse we saw was a force applied over a period of time. Work is a force applied over a distance.
One point the book does not make here deserves reinforcement early and often: work is a scalar quantity, not a vector. The definition of work involves multiplying two vectors together; we haven’t learned how to do that yet. Work is a vector dot product, which is not explained in the textbook. (Fear not: it will be explained in class.)
This section distinguishes between work done against a force and work done to accelerate an object. The distinction being drawn here really is between (1) when the force you are interested in is merely one contributor to the net force, and (2) when it composes the whole of the net force. When you slide a heavy box across the floor at a constant velocity, the net force on the box is zero. However, you are still doing work on the box!
Sometimes it is useful to define the net work done on an object. This is found by adding together the work done by each force acting on the object. The net work done on an object is exactly equal to its change in kinetic energy (defined later).
In the example of pushing a heavy box so that it slides at a constant velocity, the net work done on the box is zero. Your push does non-zero work on the box. Because the direction of your push is the same as the motion of the box, the work your push does has a positive sign. The force of friction that equally opposes your pushing force also does non-zero work on the box. Its direction, however, is opposite that of the box’s motion, so the work done by friction has a negative sign. Since the pushing and frictional forces have equal and opposite components in the direction of the box’s motion, the work done by friction is exactly opposite the work done by the push. So, they add to zero, just as they should.
pp. 111–112. Power. In physics we like to relate changes to the time they take to happen. Power is the rate of doing work.
Recommmended workbook exercise: p. 35, questions 1–3.
p. 112. Mechanical Energy. This subsection tries to define energy in words. Don’t worry if it seems fairly nebulous, especially the stuff in the footnote on page 112. It should make more sense as you continue.
pp. 113–114. “Potential Energy”. This section is important, and (I think) written clearly.
p. 114. “Kinetic Energy.”
pp. 115–116. “Work-Energy Theorem”. This subsection begins with an overly simplified statement of the theorem, but rounds it out nicely as it goes. What the work-energy theorem basically states is: if work is done on an otherwise isolated system, the total energy of the system changes by exactly the amount of the work done. The work done on the system can be either positive or negative; if it is negative, it means that the system’s total energy decreases.
Skip pp. 117–121 for now. Do read:
pp. 121–123. Comparison of Kinetic Energy and Momentum. This is good stuff to get some understanding of the distinction between these two crucial, related, but different quantities.
I’ll also call your attention to Figure 7.20 on page 122. I have never done this demonstration, but I have been informed that it is quite painful even though the nails don’t actually penetrate the skin. Physics teachers are dedicated!
Recommmended workbook exercise: p. 40. Keep in mind time and distance, impulse and work!
p. 123. Energy for Life. This section is short, but not crucial for this class. It may be of interest to anyone interested in life sciences or environmental sciences.
pp. 123–125. Sources of Energy. We humans, especially ones living in industrialized nations, each use a tremendous amount of energy. This section very briefly describes the challenge awaiting us in the near future—of how to feed that hunger.
We do not define momentum in terms of impulse—so why do we define energy in terms of work?
Was any part of this reading difficult? If so, what was it, and what made it difficult?
If you didn’t find any part of the reading difficult, how well do you feel you understand the concept of energy?
Copyright © 2008, Richard Barrans
Revised: 11 January 2010. Maintained by Richard Barrans.
URL: http://www.barransclass.com/astr1070/rguides/P1050F10rg_02-09.html