In this chapter, we introduce the important concept of potential energy. This will enable us to formulate the law of conservation of mechanical energy and to apply it to simple systems, making solving problems easier. In the final section on sources of energy, we will consider energy transfers and the general law of conservation of energy. Throughout this textmap, the law of conservation of energy will be applied in increasingly more detail, as you encounter more complex and varied systems, and other forms of energy.
- 8.1: Prelude to Potential Energy and Conservation of Energy
- In George Rhoads’ rolling ball sculpture, the principle of conservation of energy governs the changes in the ball’s kinetic energy and relates them to changes and transfers for other types of energy associated with the ball’s interactions.
- 8.2: Potential Energy of a System
- In Work, we saw that the work done on an object by the constant gravitational force, near the surface of Earth, over any displacement is a function only of the difference in the positions of the end-points of the displacement. This property allows us to define a different kind of energy for the system than its kinetic energy, which is called potential energy. We consider various properties and types of potential energy in the following subsections.
- 8.3: Conservative and Non-Conservative Forces
- A conservative force is one for which the work done is independent of path. Equivalently, a force is conservative if the work done over any closed path is zero. A non-conservative force is one for which the work done depends on the path. The component of a conservative force, in a particular direction, equals the negative of the derivative of the potential energy for that force, with respect to a displacement in that direction.
- 8.4: Conservation of Energy
- A conserved quantity is a physical property that stays constant regardless of the path taken. If non-conservative forces do no work and there are no external forces, the mechanical energy of a particle stays constant. For one-dimensional particle motion, in which the mechanical energy is constant and the potential energy is known, the particle’s position, as a function of time, can be found by evaluating an integral that is derived from the conservation of mechanical energy.
- 8.5: Potential Energy Diagrams and Stability
- Interpreting a one-dimensional potential energy diagram allows you to obtain qualitative, and some quantitative, information about the motion of a particle. For example, the negative of the slope of the potential energy curve, for a particle, equals the one-dimensional component of the conservative force on the particle. Also, at a turning point, the potential energy equals the mechanical energy and the kinetic energy is zero, indicating that the direction of the velocity reverses there.
- 8.6: Sources of Energy
- Energy can be transferred from one system to another and transformed or converted from one type into another. Some of the basic types of energy are kinetic, potential, thermal, and electromagnetic. Renewable energy sources are those that are replenished by ongoing natural processes, over human time scales. Non-renewable energy sources are those that are depleted by consumption, over human time scales.
Thumbnail: Roller coaster "Blue Fire" at Europa Park. (CC SA 3.0; Coaster J).