Talk:Kinematics

message: Removed the section on motion under gravity; it was poorly written, did not fit in with the rest of the article, and wrong.
It seemed to say that Gallileo said that objects of different masses fall at different rates, which is clearly incorrect.
129.78.64.101 (talk) 14:33, 30 April 2008 (UTC)
thread_title: "motion under gravity"

message: This article contains lots of nonstandard notation.   This is potentially confusing.  More standard notation can be found in numerous physics texts including:
Halliday&Resnick, 
Johnson&Cutnell, 
Serway
This page contains several statements at the end about angular "position" and angular velocity that are non-general; that is, they only apply to a two dimensional or planar problem.  MarcusMaximus 09:20, 25 August 2006 (UTC)
replies: text: I notice that the change in the algebraic equations for linear kinematics are not notated consistently with the angular portion.  I prefer describing linear kinematics using vi and vf instead of v and u because there is no obvious relationship between v and u in the equations, while the relationship between the velocities is vital in understanding the equations.  Additionally, if initial and final angular displacements are used in describing angular kinematics, it seems reasonable to be consistent and use initial and final linear displacement to describe linear kinematics instead of using Δ.  I believe it may be sufficient to simply notate the linear equations in both ways, which is what I have done.Tgilro01 20:59, 22 March 2007 (UTC)
thread_title: Sundry comments

message: Can anyone direct me to an article on relativistic kinematics (if one exists)?  If there's no such article yet, it should probably be created either separately or as a subsection of kinematics. HEL 03:09, 5 November 2006 (UTC)
thread_title:  Relativistic kinematics? 

message: Why is u used to represent initial velocity? Is this standard notation? Jhausauer 02:14, 1 June 2007 (UTC)
thread_title:  notation 

message: This article seems a collage of statements taken from different chapters of a physics book, and assembled whithout a logical order, and without specifying clearly where they belong in. There are various levels of kinematics, and either they should be treated separately, starting from the simplest, or the simpler levels should be defined as special cases or the most complex level.
Level 1: Point (or point mass, or particle) motion
In this context it is not correct, in my opinion, to use the expression "translational kinematics"; the word "translational" is used only for rigid body motion.
:Sublevels:
*Linear kinematics in 1-D
*Linear kinematics in 2-D and 3-D 
*Angular kinematics in 2-D and 3-D (for instance, in 3-D, the angular position of a particle rotating about a point cannot be described by a simple angle; you need a unit vector or an angular position vector...)
Level 2: Rigid body motion
Here, and only here, you define the concepts of translation and rotation (see rigid body).
:Sublevels:
*Linear kinematics in 2-D and 3-D, which describes translation (it is the same as above, but first you need to select a point fixed on the body)
*Angular kinematics in 2-D and 3-D, which describes the rotation (and this is a complex part, with several different conventions, explained in rotation (geometry))
Paolo.dL 22:06, 28 June 2007 (UTC)
thread_title:  The levels are not clearly defined and the terminology is questionable 

message: Please do not use the symbol s for a position vector but only the symbol r. s is used in mathmatics and physics for 's = the distance along some curve C in space as measured from some fixed point' (cited from Standard Mathematical Tables). And 'ds = the vector element of a curve (=t ds)'(same source)
It is extremely confusing to use s for a position vector. In it's whole this is a confusing text, mixing different elements together with not so clear relationships with each other.78.21.233.253 (talk) 16:54, 29 November 2007 (UTC)
thread_title:  Position vector: r or s? 

message: It appears that links 1.3.1 to 1.3.4 are broken, that is, they don't move you to their corresponding subsections when clicked...  —Preceding unsigned comment added by Jwedes (talk • contribs) 18:34, 25 August 2008 (UTC)
thread_title:  Broken Links 

message: Most of the symbols here are quite ccomplex. In my first year at university studying Mech. Eng. this was even more complicated as you had to learn what each symbol stood for. Instead of writing down the equations can we have a table to the side telling us what each symbol means and how it is pronounced? NarSakSasLee (talk) 22:14, 23 August 2009 (UTC)
thread_title: Symbols and their meanings

message: cinematics: "describes the motion of objects without consideration of the causes leading to the motion."
dynamics: "customarily refers to the time evolution of physical processes."
in the first one the authors leave out that the motion of objects appears over time. also a problem is that it´s arbitrary what a cause is, because one can see the "cause" as a part of the system and this system has been caused by some other causes. one could also zoom in and say that the moving object can be divided into a caus and an effect. fact is that one has to blend out the affected object and blends out the cause while watching the affected object. it´s just a game of zooming in and out and arbitrarily either blending out the surroundings or the details as if they would disappear.
91.51.204.80 (talk) 14:25, 2 September 2009 (UTC)
thread_title:  dynamics and cinematics 

message: Have changed the initial sections today. The exposition wasn't smooth. There was no mention of the idea of rest and motion stemming out from the fundamental frame dependence of position vectors. Come to think of it, there was no mention of reference frames at all. Have added that. Will get to the sections after "Integral relations" soon. The rest of it needs much standardization. "Linear Motion" shouldn't be the primary heading. It should be "Particle Kinematics" and "Linear Motion" Should appear as a sub-category in Particle Kinematics. Further, the average quantities were introduced after the instantaneous ones. Scanty remarks were made for the limiting definition of velocity or acceleration.
Soumengoswami10 (talk) 19:27, 18 November 2009 (UTC)
thread_title: "Kinematics"

message: * can't begin an article with a quote. no can do. where should it be moved? – Peacock.Lane 10:46, 28 February 2011 (UTC)
thread_title:  can't begin an article with a quote 

message: This article focusses on particle kinematics which is essentially the differential geometry of a curves.  I would like to stream-line this presentation of particle kinematics somewhat, and then expand it to the kinematics of rigid bodies.  Things get more interesting when we consider the kinematics of articulated systems such as in robot kinematics.  I will try to make that connection. Prof McCarthy (talk) 13:11, 25 December 2011 (UTC)
replies: text: After revising the lead and adding section on rigid transformations, I am now examining the section on particle kinematics.  This section seems to be repeating basic results of vector calculus.  I am not sure I can salvage much of it.  Prof McCarthy (talk) 07:19, 30 December 2011 (UTC)
thread_title:  Revisions to Kinematics 

message: I think the section on particle kinematics appears to be general discussion of elementary physics.  It discusses reference frames, the definition of the derivative, integration of ballistic trajectories, relative position vectors, and more.  I will leave it alone, because I just cannot quite figure it out.  Instead, I propose to work on the section on kinematic constraints, which is important and neglected.  Prof McCarthy (talk) 07:33, 31 December 2011 (UTC)
replies: text: Let me just add that the first sentence "Particle kinematics is the study of the kinematics of a single particle." captures the redundancy that is present in much of this section.  Having said this, it is not correct that there is nothing that is useful.  It is just difficult to sort it out.  Prof McCarthy (talk) 07:39, 31 December 2011 (UTC)
thread_title:  Particle kinematics needs work 

message: I think the section Kinematics#Coordinate systems can be revised so the formulas for a moving particle are easier to understand.  I will work on this on my user page and drop it in for evaluation. Prof McCarthy (talk) 21:46, 31 December 2011 (UTC)
thread_title:  Coordinate systems 

message: This section seems to be a duplicate of the position, velocity and acceleration in cylindrical coordinates.  I will add it as a special case.  Prof McCarthy (talk) 22:50, 31 December 2011 (UTC)
thread_title:  Point object in circular motion 

message: I have commented out a few sections because they were non-standard, duplicative or irrelevant.  I think this helps the remaining material flow much better. Prof McCarthy (talk) 23:28, 31 December 2011 (UTC)
thread_title:  Commented out sections 

message: This figure has an arrow and comes from a book called Kinematics of Machinery.  Prof McCarthy (talk) 00:28, 6 January 2012 (UTC)
replies: text: Yet another figure showing a machine. Nothing in it helps the reader to understand what kinematic is (there is a vector Q which is not described, whose meaning is quite difficult to understand even for me; it may possibly be meant to describe an angular kinematic quantity such as angular velocity or angular acceleration, but its direction is not consistent with the right hand rule, and its position in the picture is quite unusual for a such a vector). Please do not insert new figures in the introduction of the article. Please patiently wait for the opinion of other editors in this talk page. Paolo.dL (talk) 10:21, 6 January 2012 (UTC)
thread_title:  Wheel 

message: As, I wrote above, kinematics was first used to describe the motion of projectiles, sliding objects, balls rolling along inclined planes, penduli, and celestial bodies. A description of Galileo's experiments with sliding blocks or rolling balls, for instance, is perfect to introduce the concept of kinematics. For instance, see this intuitively appealing interactive animation, where space, time, and time-variant values of velocity and acceleration are shown. (Remember to press the start button; if you can't see the animation, probably you need to install Java runtime environment). By the way, a sliding plane is considered to be a simple machine, but the focus here is on the kinematics of the block (which is not a machine). Paolo.dL (talk) 21:11, 6 January 2012 (UTC)
thread_title:  Ball rolling or block sliding along inclined plane 

message: I believe it is inappropriate to reformat talk pages, particular entries by other editors.  This removes the historical nature of an evolving conversation.  By taking this action Paolo.dL has reached beyond his assumption of final authority regarding my choice of figure, to domination of my responses to his actions.  This behavior should be banned on Wikipedia.  The fact that nothing will happen should put to rest the myth that there is another editor who will regulate his actions. Prof McCarthy (talk) 22:54, 6 January 2012 (UTC)
replies: text: Refactoring talk pages is allowed in Wikipedia. Especially if it is mild and respectfully performed as in this case. I only changed two section titles, which referred directly and inappropriately to me rather than to the real topic of the discussion, and added two other subsection titles. All the new titles ("Picture for the introduction", "Watt's steam engine", "Wheel", and "Four-bar linkage") showed no point of view and objectively described the contents of the following text, respecting the "historical nature" of the discussion. See also my note above, where I explained everything. This kind of mild refactoring can significantly improve the readability of the page, and this, according to WP:Refactor, is appropriate and in some cases desirable. See also the last comment by Johnuniq on the talk page of WP:Refactor, about "Changing section titles". Johnuniq wrote: "I did one such change recently because the article talk page section heading was addressed to a specific editor, and it is usually best to avoid unnecessary confrontation". Paolo.dL (talk) 00:30, 7 January 2012 (UTC)
thread_title:  Reformatting Talk Pages 

message: *Read through
*Assess
*Add notes for any further work needed
Chaosdruid (talk) 11:27, 28 February 2012 (UTC)
thread_title:  Robotics attention needed 

message: I am sorry for the long chain of revisions.  I just wanted to adjust the order of topics so that it proceeded from particle kinematics to planar rigid body kinematics and then to spatial rigid body kinematics, but the notation and concepts needed sorting out much more than I expected. Prof McCarthy (talk) 10:03, 22 June 2012 (UTC)
thread_title:  Revisions 

message: In the section "Velocity and speed" the symbol 's' (distance travelled) is used a few times, but it has never been introduced/defined.  — Preceding unsigned comment added by Leen Droogendijk (talk • contribs) 13:08, 26 June 2012 (UTC)
replies: text: I will make this correction.  Thank you Prof McCarthy (talk) 04:58, 27 June 2012 (UTC)
thread_title:  Symbol s not introduced 

message: I am sorry that I have made a long list of revisions without explanation.  I thought I could do this easily and quickly, but it did not go as smoothly as I had hoped.  I have provided the matrix calculations that define angular velocity and angular acceleration for a rotating body.  Prof McCarthy (talk) 16:38, 13 July 2012 (UTC)
thread_title: Revisions to section on the rotation of a rigid body

message: Substituted at 21:14, 29 April 2016 (UTC)
thread_title: Assessment comment

message: The substitution should take into account  \mathbf{A}t  instead of only  t :
 \mathbf{A}t = \mathbf{V}-\mathbf{V}_0 
\mathbf{P} - \mathbf{P}_0 = \left(\frac{\mathbf{V} + \mathbf{V}_0}{2}\right) t
 \left(\mathbf{P} - \mathbf{P}_0\right) \circ \mathbf{A} = \left(\frac{\mathbf{V} + \mathbf{V}_0}{2}\right) \circ \mathbf{A} t 
 (\mathbf{P} - \mathbf{P}_0) \circ  \mathbf{A}  = \left( \mathbf{V} - \mathbf{V}_0 \right) \circ \left(\frac{\mathbf{V} + \mathbf{V}_0}{2}\right)  \ , 
— Preceding unsigned comment added by Nooser (talk • contribs) 13:52, 24 September 2016 (UTC)
thread_title:  Invalid division by a vector in "Particle trajectories under constant acceleration" 

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thread_title:  External links modified 

message: I am copying this section over to a course on robotic mechanics on Wikiversity.
https://en.wikiversity.org/wiki/Robotic_Mechanics_and_Modeling/Kinematics
Admazzeo (talk) 20:13, 20 January 2020 (UTC)
thread_title:  Copying section on "Kinematics of a particle trajectory in a non-rotating frame of reference" to Wikiversity 

message: This section is unfocussed, lacks formal vector algebra to match earlier sections, and poorly explained equation derivation for constant acceleration (can be shown from time dependent acceleration in few steps).
V(t) integration assumes constant acceleration without mention.
'translation' not 'displacement'
At 1st citation needed:  Strike [c. n.].  Replace phrase with "By definition of the dot product" before equation.  There is no need to cite a fundamental vector algebra definition.
Strike "In the case of acceleration..."  Replace with "For the case of linear motion, the difference of positions vector, velocity vectors, and acceleration vectors will be colinear.  All angles between vectors will be 0° or 180° giving 1 or -1 for the cosine of these angles respectively.  The vectors can be written as their scalar magnitudes:
v^2 = v0^2 + 2•a•(x - x0) = v0^2 + 2•a•∆x
Strike following sentence.  Define scalar variables after next equation, "where v = |V|, a = |A|, and ∆x = |P| - |P0| = x - x0."
'Curvelinear', not 'curvaceous.'
Remainder of section from "We also know that..." repeats the earlier derivation, is circuitous, and poorly annotated.  If there is no WP page that diagrams the time differentials of position, velocity, and acceleration and explains their relations anywhere else, it could be useful here.  The velocity integration graphical representation and derivation is awkward and unnecessarily weedy.  The graph is lacking significant annotations and those that are there are unhelpful.  (If I had to, I would have used the area of a trapezoid formula instead of calculating areas of a rectangle and triangle and then adding them together.)  The text here uses a lot of unintelligible and incorrect terminology. The graph caption is a good example. Figure 2 is out of place here having undefined terms and parameters involving non-inertial reference frames yet to be addressed. EvolutionOfTruth (talk) 02:15, 31 August 2020 (UTC)
thread_title:  Relative Acceleration problems 

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thread_title: Wiki Education assignment: PHY 381 History of Modern Physics

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