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# 3: Work and Energy

• 3.1: The Work - Energy Theorem
For a large number of applications in mechanics, we are not interested in how a force causes the direction of motion of an object to change – we only care about how that force changes the speed of the object.
• 3.2: Conservative and Non-Conservative Forces
Now that we are talking about work done by individual forces, we should make some mention of Newton’s 3rd law.  Every individual force is an interaction with two equal-and-opposite forces involved, so how do we know which one of these to use when computing the work done?
• 3.3: Mechanical Advantage and Power
We will now take a closer look at simple machines from the perspective of work-energy, and discuss the rate at which work is performed.
• 3.4: Energy Conservation Models
At last we will get a glimpse of the advantages of defining work and energy. As with everything else in physics, these advantages are best realized by being flexible with the choice of models used.
• 3.5: Thermal Energy
In the previous section we introduced the term thermal energy. We used this phrase as a catch-all to describe the form that energy takes when non-conservative forces internal to the system do work. It was not clear at that time why we had to introduce this element to our model, so let's examine it closer here.
• 3.6: Force and Potential Energy
We have outlined a way to generate a potential energy function for any conservative force – perform the work integral (which includes that force) between two points in space, and set the result equal to the negative of the change in potential energy. Now we look at doing this process in reverse – getting the force from the potential energy function.
• 3.7: Energy Diagrams
An energy diagram provides us a means to assess features of physical systems at a glance. We will examine a couple of simple examples, and then show how it can be used for more advanced cases in physics and chemistry. It's important to understand that there is no new physics in here – what we have learned so far now is simply represented diagrammatically, making it easier in some cases to see the "big picture" of a physical system.

Image used with permission (CC BY-SA-NC; anonymous).