PDA

View Full Version : Re: CoP v's CoF



sdalne74
06-09-2009, 11:57 PM
Hello Dave,

As mentioned by some other guys, Force is vector quantity and is supposed to
act on a predefined point. Thus, there can't be really a center for it. When
you consider force distributed over an area you are considering pressure.
Thus, as pressure is distributed quantity it can have center.
But, there is a point of discussion here. If you have several forces applied
at different points.Then, there could be a point where you can apply
resultant force which could create similar effect as that of the individual
forces. You can call this point as COF.
Correct me if am wrong some where.
Thanks,
Sarang Dalne
ANSYS, Inc.

On Wed, Jun 10, 2009 at 4:59 AM, Dave Smith
wrote:

> Dave Smith -
>
> I'de like to make this a discussion if possible, so Ive posted the replies
> so far and added my own reply to them.
>
> Original Post
>
> What opinions do you have regarding the use and interchangeability of the
> terms Centre of Force (CoF) and Centre of Pressure (CoP) especially when
> applied to analysing data from a pressure mat system or pressure insole
> system where one might be interested in pressure or force plantar to the
> human foot during ambulatory activities.
>
> Replies so far
>
> I'd assume that they mean the same thing, but I see COP used more often
> and that's the one I'd go with. Now if only you guys would spell
> "center" correctly! ;)
>
> Brian Schulz, Ph.D.
>
> I'm sure I'm not shining any new light on the subject, but force is a
> vector quantity and by definition a force distributed over a surface
> becomes a pressure. To me, this means that "center of force" is
> somewhat incorrect as there is no "center", just the point origin of
> the force. Since pressure is distributed on a surface it can have a
> center. That is just my opinion, so take it for what it's worth...
>
> Andrew
>
> I would have two quick concerns with using COF instead of COP.
>
>
>
> (1) force is a vector and already classified with a single point of
> application/origin, so there really isn't a centre to it. Pressure (a scalar
> quantity) is force over an area (the plantar surface of the foot in this
> case), so there can be a centre to it.
>
>
>
> (2) COF is commonly used to abbreviate "coefficient of friction" in similar
> research. So it may be confusing to use it to abbreviate a different term.
>
>
>
> Robert D. Catena, Ph.D.
>
>
>
> Centre of pressure is the traditional name (e.g., Elftman, H., A
> cinematic study of the distribution of pressure in the human foot. The
> Anatomical Record, 59:481-491, 1934) and especially allies to pressure
> mapping systems but has also been used for force platform systems.
> Centre of force is not a common name and might be better replaced by
> "point of force application". In any case a force does not have a
> centre.
>
>
>
> Gordon Robertson
>
>
>
> What you have to consider is how pressure mats/insoles actually work. They
> measure the normal force over and area, from which you can derive pressure.
> So to consider these two terms as representative of the outcome measure is
> unfair really as both force plates and pressure systems essentially measure
> force. The term centre of force really isn't used that frequently simply
> because it refers to a point of force application, which whilst it may be
> theoretical is a point. By the very nature of this point we should discuss
> this as centre of pressure.
>
> Hope this helps
>
> Dominic
>
> Dr Dominic Thewlis
> Lecturer In Biomechanics
>
> My Reply
>
> So the general consensus appears to be 3 things -
>
> 1) That CoP is preferable to CoF since when considering force applied over
> an area it really becomes pressure by definition.
>
> 2) CoF cannot be the same as CoP since force is a vector and pressure is
> scalar.
>
> 3) Force is a point application and so cannot have a centre (or center for
> Brian)
>
>
>
> 1) I can see the logic of that but as Dominic pointed out "What you
> have to consider is how pressure mats/insoles actually work."
>
>
>
> They characterise normal force or rather discreet force locations
> perpendicular to a reference plane e.g. the pressure mat on the floor. The
> area that each force point is spread over is relative to the size of the
> sensor cell and nothing about the centre of pressure can be known until we
> have data from more than one cell.
>
>
>
> 2) Isn't the pressure mat scenario a special case? Doesn't pressure
> become a vector when defined by a reference plane? i.e. it is given
> direction. Similar to speed (mph) becomes velocity when a direction is
> specified.
>
> 3) Can one not consider the many force points defined by the multiple
> cells of a pressure mat as having a central location that is the CoF?
>
> So each load cell of the pressure mat only characterises normal force. If
> it were possible to have load cells that also characterised horizontal or
> shear forces then the resultant force vector for each cell would have a 3D
> nature and the summation of all these vectors would result in a single point
> 3D force vector that would be similar to the 3D force vector characterised
> by a force plate software output. So isn't it fair to say that, in terms of
> a pressure mat or insole system, the summation of all forces, normal and
> shear, can be described as the Centre of Force and the summation of all
> normal forces the Centre of Pressure? Therefore since we do not have
> systems that characterise discreet force locations in 3D, only those that
> characterise normal force, then the only sensible term to use would be CoP.
>
>
>
> Dave Smith Podiatrist, MSc App Biomechanics
>
> ---------------------------------------------------------------
> Information about BIOMCH-L: http://www.Biomch-L.org
> Archives: http://listserv.surfnet.nl/archives/Biomch-L.html
> ---------------------------------------------------------------
>



--
Sarang Dalne,
ANSYS, Inc.
Canonsburg, PA, USA.