Reading Guide
for February 11

Chapter 7: Energy

p. 117. Conservation of Energy.  This concept follows from the work-energy theorem.  It has been questioned and tested many times, but has so far always proven to be reliable.

p. 118. box “Energy and Technology.”  Read this if you want.  It makes good points.

Recommmended workbook exercise:  p. 37.

pp. 118–119. Machines.  This section explains how simple machines can put out much more force than is put into them.  It’s always a trade-off between force and distance.  That’s right!  The amount of work a simple machine does is equal to the amount of work done on it.  It just does it with a larger force over a shorter distance or a smaller force over a longer distance.

Recommmended workbook exercise:  p. 35, question 4;  p. 38.

pp. 120–121. Efficiency.  OK, so a machine doesn’t necessarily put out the same amount of work as was put into it.  A machine may lose energy to friction or other types of losses that we’ll discuss later in the semester.  Bear in mind that when we say energy is “lost,” it is not destroyed: it just is no longer in a form that we can use.

• What is the general formula for efficiency?

Chapter 4: Newton’s Second Law of Motion

pp. 59–61. Friction.  You should have already read this, but I recommend you go back and skim it again.  Friction and its cousin drag are the main reasons that energy often appears not to be conserved in many processes.  In fact, energy always is conserved in all processes.  Friction and drag convert some of the kinetic energy of the interacting substances into heat.  Heat, for its part, is something we will address in more detail later in the course.  For now, you should know that heat is energy.

As you read this section, think about:

• What does it mean that “the direction of the friction force is always in a direction opposing motion”?
• What happens between interacting surfaces to make friction oppose motion?
• When you push a sliding crate along the floor at a constant speed, are you doing work on the crate?
• When you push a sliding crate along the floor at a constant speed, is friction doing work on the crate?
• What transformations of energy occur when you push a sliding crate along the floor at constant speed?
• What transformations of energy occur when a parachute falls through the air at a constant speed?

Reflection

So far, you have been told that momentum is conserved and that energy is conserved.  Which law do you think would be easiest to test: conservation of momentum, or conservation of energy?  Why?