2.8: Looking Back and Ahead

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We now have quite a few different forms of energy–translational kinetic as well as rotational, thermal, bond, gravitational potential and spring (or elastic) potential energy. A given physical situation could involve any number of these. Energy can be transferred among physical systems either as heat (when there is a temperature difference between two objects) or as work (when one object exerts a force (on another object) that acts through a distance). There are yet other forms of energy that involve electrical interactions, magnetic interactions, and a more general form of the gravitational energy.

By treating thermal interactions and mechanical interactions on an equal footing, we can approach realistic situations without having to automatically assume friction or air resistance is negligible. By now you should be very comfortable with the energy-interaction model. When we encounter new “kinds” of energy, it won’t be a “big deal.” We simply add them to our repertoire of energy-systems that might change in any particular interaction.

Now we are in a position to delve into particle models of matter. Our goal is to be able to understand, in a general or universal way, as many of the properties of matter as we can. As we do this, we will also make a much more direct connection to thermodynamic concepts you have worked with in chemistry courses.

Even as we extend and perfect our energy-interaction model, we recognize that many questions are beyond its reach. For example, our before-and-after approach can’t tell us, “How long did it take an object to fall?” Questions like this involve the dynamics (the details) of interactions. We will spend more time in Part 2 of the course and accompanying course understanding the dynamics of rigid objects. This is fundamentally the relation of force to motion known as Newton’s 2nd law. Using Newton’s laws and kinematics to describe the details of interactions, we can answer questions that are unanswerable using the before-and-after approach. But for right now, we stick to an energy approach and avoid, as much as possible, the details of interactions.

Contributors

Authors of Phys7A (UC Davis Physics Department)