Talk:Kinematics

Definition

I learned: kinematics is all that regards kinetic energy, which is why I added a brief clause.

I didn't know what a truss was. I checked it, and read that it is a stationary (non-moving) object. Are you sure that one can talk about the kinematics of a truss? (When a truss starts moving under influence of external forces, storms or earthquakes, I can see it, but that would be an exceptional situation). To me the science of a truss belongs to statics a different subfield of mechanics than kinematics. --Paul Wormer 04:43, 3 May 2008 (CDT)

Hello Paul,

I have several arguments. No offense intended!

pw> I learned: kinematics is all that regards kinetic energy, which is why I added a brief clause.

Where is that from? Is there a publication which mentions kinetic energy in reference to kinematics?

According to the online OED, http://www.askoxford.com/, kinematics is "the branch of mechanics concerned with the motion of objects without reference to the forces which cause the motion." No mention of kinetic energy. Some sources mention geometry; more about that later.

There is a critical problem in attempting to describe a trajectory by kinetic energy. Motion is inherently vectorial while energy is a scalar quantity. If the Universe were one dimensional, a motion of a point could be specified by a starting location and the kinetic energy as a function of time. In our universe that fails.

In a more general sense, kinematics provides a means to describe a problem in mechanics, without presuming the solution. For example, we can say that at 3:00 pm the railcar is released from a siding at Yale and begins to roll down grade to Vancouver. Given the profile of the track, the frictions on the car and the assumption of no collision, what time does it reach Vancouver? The track profile and the concepts of position and speed of the car constitute the kinematics. No assumption about the actual speed as a function of position or time is in the kinematics. The full mechanical solution is the working out of the effect of gravity and frictions to produce a speed at each point and ulimately the calculation of elapsed time.

pw> ... statics a different subfield of mechanics than kinematics.

Statics/dynamics and kinematics/mechanics are orthogonal concepts. There is no parallelism between statics and kinematics.

Statics is the particular case of dynamics where the subject is nearly stationary in the chosen frame of reference. "Nearly" because we know that even a static structure deforms as it is loaded. Of course deformations should be small in structures such as bridges and milling machines.

Kinematics on the other hand, is not a special case of mechanics but is a fundamental part of it.

pw> Are you sure that one can talk about the kinematics of a truss?

Certainly! The kinematic description of a truss entails the configuration, dimensions and locations of loads. If a deformation analysis is called for, then deformations are also a part of the kinematics. Any static problem has a kinematic description just as well as a dynamic problem does. According to the Principle of Relativity, there is no reason to discriminate against a static problem.

More about geometry.
Thermal mechanics involves temperture, which is not a geometrical concept. Electrical circuit theory involves circuit topology and currents but not geometry. By referring to the trajectory of the system, rather than its geometry, any theory of mechanics can have a kinematics.

I can not agree with the sentence you have added. If you want to keep it there, I am happy to discuss further. An expert in formal axiomatic theories of mechanics might be able to add something more definite and convincing.

I'm not really happy with my examples from thermal mechanics. They seem too indefinite. Suggestions for improvements are welcome, although I want to consider changes very carefully.

Best Regards, Peter Lyall Easthope 11:18, 7 May 2008 (CDT)

• Peter, thank you for your elaborate answer. I see that our views diverge, so I won't interfere in your work any further and I deleted my addition to your article. Let me just mention a few of my views on the topic of kinematics. (i) A decisive property of an object in motion is its kinetic energy (as a function of time and place). (ii) When you talk of a "load", then you talk about dynamics (namely, you invoke the gravitational force). (iii) I would refer to the flow of gases, temperature and pressure dependence, speed through porous media and so on as gas kinetics, not as kinematics. --Paul Wormer 02:49, 8 May 2008 (CDT)