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A dynamic system is a system that changes over time. It can also be called a “dynamical system.” While some maintain that there is a slight difference

engine, 4-stroke
An engine, an example of a dynamic system. [Animation source: By Zephyris – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/  w/index.php?curid=10896588]
between the two, there seems to be no consensus that the two phrases differ in meaning. An example of a dynamic system is an engine burning fuel. Over time, the fuel is consumed and gases are produced.

framing of a house, static system
The supporting beams of a house form a static system. [Image source: https://en.wikipedia.org/wiki/Framing_(construction)#/media/File:Wood-framed_house.jpg]
A dynamic system contrasts with a static system. For purpose of analysis, a static system does not change in time.  An example of a static system is the structure of a building, where the foundation forms a level platform for the major supports, and the major supports are tied together with cross pieces. In reality, the structure of the building might change in time as it bends in the wind or is shaken by earthquakes.

What is a system?

In science, a “system” means any group of components that can interact with each other and form a complex whole. For example, a car with all its parts is a

ecology of a pond is a system
Frog in a pond. The ecology of the pond can be considered a system. [Image source: By Thompsma – Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=16392401]
proton electron in hydrogen atom
An atom can be considered a system. 
[Image source: Mets501 (Own work); https://en.wikipedia.org/wiki/Proton; CC-BY-SA-3; retrieved Feb. 16, 2018.]
system. An electrical circuit is a system. A bowl of water is a system—there’s the bowl, including all its molecules, and the water, including all its molecules. The ecology of a pond is a system. An atom (lots of interacting parts) is a system. Even an electron can, loosely speaking, be called a “system.” Electrons don’t have components; but they do have a number of properties and they can absorb photons, which are then incorporated into the electron.

In science, the term “system” takes the place of the common English “thing” or “entity.” The advantage of the word “system” is that it doesn’t specify whether the thing is matter, energy, or both. It doesn’t specify whether the thing is animate or inanimate. It specifies only that the parts are inter-related and should be considered to form one thing.F

Feedback Loops

In many dynamic systems, the outputs of some components become the inputs of other components. When an output becomes an input, a feedback loop is formed.

The human body is a system with an almost endless series of feedback loops. One example is the blood glucose/insulin system. Blood glucose is a sugar molecule that the blood carries around the body to nourish cells. When we eat sugar, it is an ingredient (input) to digestion so that it can create blood glucose. Blood glucose, an output of digestion, becomes an input to the pancreas. While blood glucose is a necessary nutrient, at high levels, it becomes toxic. In healthy bodies, an elevated blood glucose level tells the pancreas to produce more insulin to bring the levels down. For people born with diabetes, the pancreas is unable to use the blood glucose input because it was born unable to produce insulin. Such people must use their blood glucose level as an input to a testing device. The person must use the output of the tester as an input to the decision as to whether to give themselves an injection of insulin. The dynamic system continues on virtually endlessly, with the higher levels of insulin being an input to cells, telling them to store blood glucose in the cells, bringing the blood glucose levels down.

Weather systems also include feedback loops. For example, winds and the moisture from the ocean are inputs to the formation of coastal fog. Winds flowing over the ocean towards land pick up individual molecules of water. Individual molecules of water are not visible to our eyes but cause humidity in the air. The winds bring humid air to coastal lands. Humid air is the first output. The humid air becomes an input when it flows over land. If the land is cooler than the ocean, the winds cool. Cool air holds less water as individual molecules than warm air. The water molecules fall out of the air and condense onto dust particles. When a number of water molecules condense onto dust particles, clouds form. If the clouds are near the ground, we call this fog. If they’re higher off the ground, we say that the sky is overcast.

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