[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Stego/Ankylo limbs (long)



On Sun, 4 Feb 1996, Rob Meyerson wrote:

> >> hind legs.  The front limbs wouldn't have to do much more than steer.  
> >> So they wouldn't have to be efficent in locomotion.
> >
> >Wrong.  Unless the front legs have some sort of wheels or runners on 
> >them, they need to be just as efficient in locomotion as the hind legs, 
> >or at least be able to keep up.
> 
> However, since the main thrust is being provided by the hindlegs, the forelegs
> can afford to lose a little efficency for the sake of stability.  A decent
> trade-off in my book.

But if you produce a lot of thrust with the hindlimb and the forelimb 
can't keep up, the animal is going to fall on its face!


> >> >        Finally, the reinforcements you mention in the shoulder are
> >> >consistent with a large charging animal that would have to change course
> >> >quickly or lunge sideways.
> >> 
> >> However, the reinforcements I mentioned only match up (and therefore, only
> >> apply) if the humerus is held horizontal to the shoulder articulation.
> >
> >I fail to see how that is true.
> 
> The feature I point out is the reinforcement process located on the upper
> humerus, which lines up with a similar process on the scapulacoricoid.  These
> two processes match up only when the humerus is held horizontally out from the
> shoulder attachment.

Then the "reinforcement procees" must actually be something else!  Sorry, 
but given the trackway data, there is simply no way ceratopians could be 
holding their humeri horizontally.


> 
> >> >        By sprawling, not only are you moving the Triceratops closer to 
> >> > the
> >> >ground, but you are placing more weight on the forelimbs making them sink
> >> >even deeper into the sand.  You are not suggesting "dune surfing" are you?
> >> 
> >> Actually, by holding the feet out to the side, it spreads the total weight 
> >> of
> >> the animal over a wide area.  This would reduce the total load on the 
> >> forefeet.
> >
> >No, it does not.  The weight of the animal is distributed over the area 
> >of the parts of its feet in contact with the ground.  No more; no less.  If 
> >the animal slopes down sharply towards the forelimbs, these must bear a 
> >large 
> >portion of the animal's total weight, and, as mentioned above, the animal 
> >sinks into the sand.
> 
> 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:
> 
>          |
>          | w
>          |
>          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:
> 
>         --------->|
>          \ A      |
>           \       |
>            \      |
>             \     | x
>           H  \    |
>               \   |
>                \  |
>                 \ |
>                  >V
> 
> Where:
>         H = Hypotenuse (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.
> 

Fascinating.  How do you get gravity to act obliquely like that?

Check your diagrams again.  Weight is weight.  It always acts downwards.

What you have done is *increased* the total force on the legs 
and also introduced a horizontal component to the force, which, incidentally, 
will serve to shove the legs out from under the body, causing them to slip 
sideways, particularly on a slippery surface like sand!

This is why we build columns vertically, not slanted.

> Rob
> 
> ***

Nick Pharris
Pacific Lutheran University
Tacoma, WA 98447
(206)535-8204
PharriNJ@PLU.edu

"If you can't convince them, confuse them." -- Harry S. Truman