# The Standards

### Crosscutting Concepts

#### 7. Stability and Change

Below is the progression of the Crosscutting Concept of Stability and Change, followed by Performance Expectations that make use of this Crosscutting Concept.

##### 7. Stability and Change

For both designed and natural systems, conditions that affect stability and factors that control rates of change are critical elements to consider and understand.

##### Primary School (K-2)

Things may change slowly or rapidly.

Some things stay the same while other things change.

##### Elementary School (3-5)

Change is measured in terms of differences over time and may occur at different rates.

Some systems appear stable, but over long periods of time will eventually change.

##### Middle School (6-8)

Stability might be disturbed either by sudden events or gradual changes that accumulate over time.

Explanations of stability and change in natural or designed systems can be constructed by examining the changes over time and processes at different scales, including the atomic scale.

Small changes in one part of a system might cause large changes in another part.

Systems in dynamic equilibrium are stable due to a balance of feedback mechanisms.

##### High School (9-12)

Much of science deals with constructing explanations of how things change and how they remain stable.

Systems can be designed for greater or lesser stability.

Feedback (negative or positive) can stabilize or destabilize a system.

Change and rates of change can be quantified and modeled over very short or very long periods of time. Some system changes are irreversible.

Stability and Change

Stability and Change are the primary concerns of many, if not most scientific and engineering endeavors. “Stability denotes a condition in which some aspects of a system are unchanging, at least at the scale of observation. Stability means that a small disturbance will fade away—that is, the system will stay in, or return to, the stable condition. Such stability can take different forms, with the simplest being a static equilibrium, such as a ladder leaning on a wall. By contrast, a system with steady inflows and outflows (i.e., constant conditions) is said to be in dynamic equilibrium. For example, a dam may be at a constant level with steady quantities of water coming in and out. . . . A repeating pattern of cyclic change—such as the moon orbiting Earth—can also be seen as a stable situation, even though it is clearly not static.

“An understanding of dynamic equilibrium is crucial to understanding the major issues in any complex system—for example, population dynamics in an ecosystem or the relationship between the level of atmospheric carbon dioxide and Earth’s average temperature. Dynamic equilibrium is an equally important concept for understanding the physical forces in matter. Stable matter is a system of atoms in dynamic equilibrium.

“In designing systems for stable operation, the mechanisms of external controls and internal ‘feedback’ loops are important design elements; feedback is important to understanding natural systems as well. A feedback loop is any mechanism in which a condition triggers some action that causes a change in that same condition, such as the temperature of a room triggering the thermostatic control that turns the room’s heater on or off.

“A system can be stable on a small time scale, but on a larger time scale it may be seen to be changing. For example, when looking at a living organism over the course of an hour or a day, it may maintain stability; over longer periods, the organism grows, ages, and eventually dies. For the development of larger systems, such as the variety of living species inhabiting Earth or the formation of a galaxy, the relevant time scales may be very long indeed; such processes occur over millions or even billions of years.” (p. 99-100)

Other Standards That Use This Crosscutting Concept