Talk:Coefficient of restitution

message: I added some text on this subject based on experiments performed at Florida State University in the summer of 1955.  What these experiments showed was that, for a variety of metals, e appeared to approach 1.0 at impact speeds approaching 0. As the impact speed increased the value of e dropped, then recovered to a high level, then dropped again to a steady state measure. 
The paper is at www.burgy.50megs.com and also at 
User John Burgeson  —Preceding unsigned comment added by Burgeson (talk • contribs) 19:41, 15 January 2009 (UTC)
thread_title:  e (COR) is not a constant 

message: There is something very important in this article, and in everyone's understanding that needs to be cleared up. The coefficient of restitution is not a property of an object; it is not defined for any one object. Rather, it pertains only to interactions (collisions) between two objects, just as the coefficient of friction is only defined for two surfaces that interface. Hence the statement "A golf ball has a COR of 0.78" is incorrect and nonsensical. 141.213.247.14 06:45, 24 October 2007 (UTC)
replies: text: The article does say this.  Quote: "An important point: the COR is a property of a collision, not necessarily an object. For example, if you had 5 different types of objects colliding, you would have {5 \choose 2} = 10 different CORs (ignoring the possible ways and orientations in which the objects collide), one for each possible collision between any two object types."  --Numsgil 16:04, 24 October 2007 (UTC)
thread_title:  Not a property of objects 

message: The article states that a cannonball has a low Coefficient of Restitution. I'm not so sure about that. Certainly a ball bearing has a very high Coefficient of Restitution as demonstrated when one bounces a ball bearing on a piece of concrete. If one dropped a cannonball on a suitably non-deforming surface, might it bounce quite high? --Dumbo1 16:28, 13 January 2006 (UTC)
replies: text: I suppose it depends on the surface. That confuses things a lot and I can't be positive that the article is right.  Run!  18:04, 13 January 2006 (UTC)

text: If the surface deforms, then the height of the bounce will depend to some degree on the coefficient of restitution of the surface, as well as that of the object bouncing. I think the article is wrong in this respect, specifically that of the example of the cannonball. At a guess I think that the coefficient of a cannonball is greater not less than that of a rubberball. --Dumbo1 00:59, 2 February 2006 (UTC)
replies: text:  the coefficient of restituion all depends on the to materials interacting. For example, a ball bearing bouncing on solid steel has a high coefficient of resitution, but that same ball bearing boucing on, say, vinyl flooring will have a much lower coefficient of restituion. (Cabin Tom 03:29, 20 April 2006 (UTC)

text: What "failed to parse" messages?  --Numsgil 06:22, 20 April 2006 (UTC)
replies: text:  It seems to have been fixed...(Cabin Tom 23:43, 22 April 2006 (UTC)
thread_title:  Older Discussions 

message: Seems to be disagreement both on the page and on the web about what the signs of the equations need to be.
As I understand it COR is, for any realistic collision, positive and between 0 and 1.  Maybe an sbsolute value would take care of this?  --Numsgil 02:18, 21 April 2006 (UTC)
thread_title:  Equation Sign 

message: Hi, this is a question bearing on the discussion.  I am actually doing a computer simulation of collisions.  If two balls with COR's c1 and c2 collide, what COR should be used? It would depend on the relative masses I guess.  Also, if one has COR 0, seems that the final COR should be 0, even if the other has COR=1.  A steel ball will stick to a putty ball, no matter what their relative masses.
- Alberto
Yes information about an equation that deals with two cor's would be greatly helpful
: I've written a small caveat on the page: CORs are for collisions, not objects.  That said, in any sort of simulation it's not going to be practical to pick CORs for every object.  I have no idea if this is correct or not, but a good half way solution would be to use the minimum of the two CORs of the colliding objects as the COR for the collision.  That should take care of the case Alberto pointed out where putty and a steel ball collide.  Again, I'm not sure if this is physically accurate or not.  --Numsgil 10:41, 29 October 2006 (UTC)
replies: text: The COR is defined as a velocity relative to what the thing hit.  However, in this case, it's actually a 3 body system (floor, basketball, tennisball).  You're right, that it's like a Newton's cradle, but I don't think you'd ever see a COR > 1.  --Numsgil (talk) 04:16, 4 April 2008 (UTC)

text: No, the CORs are still less than one.  Remember that you are dropping both balls.  Meaning that you cannot treat the basketball as part of the table (the two never (well, that's an oversimplification.  The two never have the same instantaneous velocity unless the basketball is at its peak compression, and thus has stored potential energy waiting to be released)  have the same instantaneous velocity for one thing).  The basketball hits the table first, and the table causes it to reverse direction.  Then the basketball/superball collision occurs, and a great deal of energy from the basketball gets transferred to the superball.  Well, that's a simplification too, the collisions actually occur at the same time, so again it's very similar to newton's balls.  But the point here is that a COR must be less than one unless you are generating energy.  Dropping balls does not generate energy.  Dropping landmines on the other hand...  Hmm, I think that would be a more interesting classroom demonstration. :)  --Numsgil (talk) 03:02, 19 May 2008 (UTC)
thread_title:  Combining two COR's 

message: How in the world are the equations for the final velocities derived?
: You mean in the "use" section?  I had to derive them myself, though they're very similar to equations given by Andre Lamonthe in one of his intro to game programming books.  It's been a while, I don't have my original notes for how they're derived, but I could reconstruct the method of derivation if you're interested/skeptical --Numsgil 02:50, 29 October 2006 (UTC)
replies: text: edit: Here's what I said at the top of the page when I rewrote the page: "I've basically entirely rewritten the page, expanding it from a stub. I couldn't find a reference to the equations I listed in "Usage" for the general collision case. They are modified slightly from equations listed in Tricks of the Game Programming Gurus 2nd Edition by Andre Lamothe and the equations listed on the elastic collision page. --Numsgil 02:23, 11 February 2006 (UTC)

text: Posted the way to find the derivation, I've left the solving of the system of equations as an excercise for the reader.  Should be managable for anyone with any sort of algebra background.  --Numsgil 10:42, 29 October 2006 (UTC)

text: Thanks.  The first equation is the conservation of momentum that holds true for all collisions.  The second is the definition of the coefficient of restitution.  Both equations are then manipulated to put it into a form that linear algebra is good at, specifically the "standard form" aX + bY = c.  For the purposes of the derivation, you can treat c, v1 initial and v2 initial as constanst and v1final and v2final as your unknowns, since you know c, v1 initial and v2 initial before the collision occurs.  Tell me if I've been obtuse at any point in my reasoning, I'm trying to bridge the gap between pure theoretical and the practical level of algebra most computer scientists (say, game programmers) are comfortable with. --Numsgil 22:23, 2 November 2006 (UTC)
thread_title:  What the hell? 

message: Currently, "COR" redirects to Heart.  Perhaps a redirect here or disambiguation page should be considered.  --Half Cat 18:10, 2 April 2007 (UTC)
thread_title:  "COR" Redirect 

message: I have seen that McGraw Hill's Sci-Tech Dictionary claims that e can be used for the restitution coefficient.  Is this correct? I apologize for linking to answers.com, but it seemed most convenient at the time.  —Preceding unsigned comment added by 69.124.113.50 (talk) 01:58, 10 January 2008 (UTC)
replies: text: fwiw, this notation is employed in STEP. Mr Dactyl (talk) 15:05, 3 December 2008 (UTC)
thread_title:  Could the COR be abbreviated as "e"? 

message: Shouldn't things like "Newton's Experimental Law" and "Newton's Law of Impact" redirect to this page? PT (talk) 21:58, 6 December 2008 (UTC)
thread_title:  "NEL": redirect? 

message: Velocities  —Preceding unsigned comment added by 173.2.20.96 (talk) 01:09, 15 December 2008 (UTC)
replies: text: Velocities are in fact scalar along the direction of collision.  If two objects are colliding, there's a normal vector for the collision.  Take the velocities of either body at the point of collision and dot product it with this collision normal.  That's the scalar velocity term to use in the equations.  --Numsgil (talk) 23:47, 17 May 2009 (UTC)
replies: text:  What does "direction of collision" even mean? In the center-of-mass frame, there is a still a vector relative velocity for each particle, and it can change direction before and after the collision. Yes, you can always define a scalar from a vector, but that's not always relevant. What's wrong with "relative speed"?  — Preceding unsigned comment added by 2.249.139.10 (talk) 20:35, 14 April 2018 (UTC)
thread_title:  Velocity shouldnt be scalar 

message: A major defect with this article, and one which, for me, makes it unintelligible is that the terms 'velocity' and 'speed' are used interchangebly, COR being defined in terms of speed and then in terms of velocity in different parts.  There is no consistency here.  In English, 'velocity' is always a vector, and 'speed' is the corresponding scalar quantity; 'scalar velocity' is an oxymoron, which might reduce to 'speed' or 'the magnitude of a velocity'.  If this is what is intended the term 'speed' should be used.  'Velocities are in fact scalar along the direction of collision' is absurd for several reasons; not only are velocities vectors, and therefore not scalar, but what possible meaning can be given to '...along the direction of collision' ? I feel that the entire article should be revised by someone who is capable of using appropriate language.  I have abandoned the attempt to make sense of it, and have reached for a textbook written by a competent author. Wikki fails again. Dr Andrew Smith.  — Preceding unsigned comment added by 82.32.54.67 (talk) 07:35, 26 October 2012 (UTC)
replies: text: Don't like it?  Fix it.  --38.110.159.138 (talk) 17:51, 28 February 2013 (UTC)
replies: text:  I'm with the good doctor. Sometimes you find yourself thinking: Life's too short to mess with this . . .  86.183.83.105 (talk) 04:39, 4 October 2013 (UTC)
thread_title:  Velocity & Speed 

message: I feel an expert is needed here to clean this article up. I found contradictions, highlighted by the Talk Section "Not a property of objects", and in the introduction "The coefficient of restitution (COR) of two colliding objects is a positive real number between 0.0 and 1.0" followed by "A COR greater than one is theoretically possible."
I've corrected those, but if I can find two within the first read of the article, there's probably a heap of them waiting to be found. It probably needs a rewrite. 
Richard n 14:14, 12 June 2014 (UTC)
thread_title:  Expert Needed 

message: I've now given this a clean-up as required and removed the expert-needed flag. I added the Newton's Experimental Law redirect, changed the symbol to "e" (which is common), and added some more introductory context, details of the values of e, and a simple example of how it can be used. 
For reference, the derivation of "speed of separation = e x speed of approach" from conservation of momentum and energy is given here: http://www.solitaryroad.com/c1013.html
RichardNeill (talk) 19:28, 6 July 2015 (UTC)
thread_title:  done 

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thread_title:  External links modified 

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