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*To*: Multiple recipients of list <dinosaur@lepomis.psych.upenn.edu>*Subject*: Re: Stego/Ankylo limbs (long)*From*: Dinogeorge@aol.com*Date*: Sun, 4 Feb 1996 14:48:07 -0500*Reply-to*: Dinogeorge@aol.com*Sender*: dinosaur@lepomis.psych.upenn.edu

In a message dated 96-02-04 12:50:55 EST, Robert.J.Meyerson@uwrf.edu (Rob Meyerson) writes: >Sorry, your wrong on this one. Set this one up as a physics problem >regarding >the justification of stresses (I can't think of the real term, but this is >close >enough). With the feet under the body, the stress diagram looks like this: > > 1 > 1 w > 1 > V > >In this case, the force on the feet (w) equals the mass times the >acceleration due to gravity (simple Newtonian stuff). > >With the forelimb held off to the side, the diagram looks like this: > > --------->1 > \ A 1 > \ 1 > \ 1 > \ 1 x > H \ 1 > \ 1 > \ 1 > \ 1 > >V > >Where: > H = Hypotenus (with a value equal to w). > > A = The angle between hypotenuse and horizontal. > > X = Total load applied to the feet. > >Trigonometry says that X = H.cos A. Since the cosine of any angle (provided >A < 90) is always less than one, then X < H. Therefore, the total load >applied >to the feet is less with the feet held out to the side. > > Oops. This ain't right at all. The _load on the feet_ is the same regardless of where they are placed on the ground, and it depends mainly on the weight of the body and how it is distributed therein. What the hypotenuse of the above triangle measures, in how much longer it is than the vertical vector, is the _extra_ force that needs to be distributed along the leg muscles in order to hold the leg out at the specified angle from the sides. The vertical leg minimizes the difference between the length of the hypotenuse and the vertical (effectively to zero), which is why it's the "preferred" stance of graviportal animals.

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