3.1 Introduction to Work and Energy

Key ideas:

• Work and Energy are a second way to analyze the motion of an object.

• Work and Energy are scalar quantities.  That means easier math!

Definitions:

• Work = force x distance.

  • Notice distance, not displacement.  This is a scalar quantity.

  • The unit of work is the Joule, where one Joule = 1 Newton meter.

  • This is not very scientific, but it's the way I think of it:  Work means pushing things around.

• Energy is the ability to do work.  If an object has energy it can push things around.

  • Energy is also measured in Joules

  • In general, an object has energy from two sources:

    • Potential energy -- energy due to position and force

    • Kinetic energy -- energy due to motion

Notes:

After a predictable, if difficult march through the physics of motion and Newton's Laws, a lot of students find work and energy to be confusing topics.  The truth is that mathematically, they're actually easier!  The problem is conceptual.  You won't find specific formulas to deal with specific situations anymore.  You will need to learn to keep track of what kind of energy is at each point in the problem, and you will need to create your own equations based on that energy accounting.  This is the spot where students get lost and go running back to the "identify variables and plug into equations" mode that they were used to.  Try not to do that!  Here's why:

The concepts of work and energy -- developed from Newton's Laws -- are actually the foundation of most of the rest of the physics that we will study.  The two types of energy -- potential and kinetic -- are lurking underneath almost every other type of energy that you can talk about.  Believe it or not, it has taken us five weeks of intense study just to get to the point where we can learn the building blocks of all of physics.  But here we are.

A little bit of rambling about conservation laws:

If you think about it, most of our study of physics has been about describing change.  Position changes.  Motion changes.  Force changes motion.  Conservation laws tell us what does NOT change, and in the process they limit the kinds of changes that are available.  These limits have huge consequences in the world around us, and you see those consequences playing out in all of your other studies of science.  This is especially true in the field of environmental science!  The reason that we need to have debates about the tradeoffs of different types of energy sources in the "big" world is that energy is conserved in the micro world.  It is not possible to make a car that gets 500 mpg, for example.  By "not possible," I don't mean that the engineering is too difficult.   I mean that it would break the laws of physics.  Gasoline does not contain enough energy to overcome the air resistance, rolling friction, and other losses.  As you will learn later, it is not possible for a system to be 100% efficient.  If you really want to make a difference in climate change, you need to have a thorough understanding of energy, conservation of energy, and (coming in a few weeks) thermodynamics.