A lotta…

… “-ics” actually, Phil.

Physics

Mechanics

Kinematics

Statics

Dynamics (and all things that end in -dynamics, like thermo-, and hydro-, and…)

OK, you get it. That’s a whole lotta “ics” out there and it’s sometimes a little confusing as to what, precisely, is what. And the reality is that it only gets worse once we start getting into simulation approaches to solve physical problems mathematically.

For example, one could be forgiven if surprised to learn that a static finite element analysis (for example, using *STATIC in Abaqus or MECA_STATIQUE in code_aster) can involve bodies that are in motion. As in moving. As in, precisely not static.

Well, we’ll talk a great deal more about FEA and the assumptions that are made there, but as we’re still just getting acquainted let’s keep focused on the basic terms we’re going to be tossing-about casually here. Thus, let’s start with the great-granddaddy of all of them…

Physics

If we’re talking about something that is “in nature”, particularly as regards the physical world, then the word that literally translates from the original Greek as “knowledge of nature” would seem to apply. (This, of course, compares to Metaphysics and Gnosis, both of which I suspect we’ll get into at some point but is well out of scope for now.) Physics has been around since the dawn of civilization and is considered a branch of natural philosophy, which is a term I actually prefer a great deal more than “Science”.

As it is the study of matter and motion, energy and forces, all under the influence of space and time, Physics encompasses all the other “ics” we’re going to talk about today. The goal of Physics is nothing less than the understanding of the physical universe – to understand how everything we can see, hear, touch, etc. behaves.

It is, therefore and by definition, impractical. As in, the goal of Physics has nothing at all to do with practical matters, such as “how to get the most out of this or that”, or “how can we make this better/faster/cheaper”, etc. It is a study about understanding, and to my mind, fits very neatly into the same category as the Quadrivium, the “natural philosophical” arm of the liberal arts (opposed to the practical arts, such as engineering or medicine).

Mechanics

If it’s a physical body in motion under the influence of external forces, then it falls under the broad category of Mechanics. In particular, we’re principally (and almost exclusively) likely to focus herein on what is known as Classical Mechanics or Newtonian Mechanics (vice Quantum Mechanics), defined as a mathematical representation of the laws governing physical, macroscopic, observable (for the most part) with the unaided natural senses, bodies.

Though studied throughout the centuries leading up to the late middle-ages, the foundation of what is typically considered to be modern Classical Mechanics is most often attributed to the work of Newton.

Kinematics

Well, if that’s the case, then what fresh hell is this?

Let me see if I can simplify this. If we care only about the motion of bodies, without any consideration of the forces/energies/whatever causing said motion, then we are dealing with the field of Kinematics.

So, when we’re thinking about the mathematical relationships between displacement, velocity, and acceleration for example, we are then solidly in the realm of Kinematics. Generally speaking, Kinematics is a purely mathematical practice. It is deeply rooted in geometry and absolutely essential to understanding the range of motion of a mechanical system or mechanism.

In summary, Mechanics relies on Kinematics for a mathematical description of the bounds on the paths that may be taken by bodies irrespective of any external forces/energies/whatever.

You may have noticed that we are presently talking quite a bit less about natural philosophy (the understanding of the physical universe) and much more about practice, or the use of that understanding to achieve a certain goal. In other words, we’re gently moving from philosophy and into engineering. So let’s just get there…

Statics and Dynamics

Statics and Dynamics are both branches of Mechanics (and thus, rely on Kinematics), but differ quite significantly in their aims. Statics deals with the analyses of the loads/energies applied to physical systems in what is known as static equilibrium – the point at which the system is no longer in motion under the influence of these loads.

Dynamics, on the other hand, treats of these very same systems while in motion.

But, but, but… (I hear you saying)

Yes, I know. I said only a couple of paragraphs ago that we can perform a “static” analysis on a body that’s in motion! You’re a lying liar who lies, Doctor Boom!

There are a great many ways I can address this, but before I can I need to introduce the concept of Inertia, which is going to be somewhat difficult as we’ve not yet fully introduced the Laws of Motion (though I did talk about the key one, if you were paying attention). So let me appeal to an exercise in physical logic…

  • Say that you see an apple on a table, sitting there, doing nothing, not moving
  • You want to eat said apple
  • So you go over to the table and pick-up the apple
  • And you notice, in picking it up, that the apple is not weightless – that it doesn’t simply float in your hand

Inertia is precisely the resistance of the apple against being moved by your hand. And no, it’s not some sort of “act of will” by the apple to not being eaten (try to get that image out of your head, I dare you). It is, simply, the effect of the mass of the apple under the influence of the forces that otherwise want to keep it right where it is (for example, gravity and the force of the table keeping the apple where it was). The inertial force of the apple resisted your physical forces required to change the state of the apple from at rest to in motion.

So after that very long-winded aside, let me put the issue of static versus dynamic simulation to bed.

  1. If we care about inertial forces, OR are concerned about what is happening to the system while it is in motion, we are dealing with Dynamics
  2. If we don’t care about inertial forces, OR only want to know what is happening to the system while it is at rest, we are dealing with Statics

I hope that clarifies matters and you no longer think of me as this guy.

I think that’s enough to chew-on for now.

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