Re: T. rex mechanix

[ngear@gvtc.com]

bettyc@flyinggoat.com wrote:
>you are basing your assumptions on the ratio of 20 feet ladder x 6 ft
>height of human---try the ratio of 20 foot ladder and 15 foot tall
>person.  Suddenly pivots do not look so odd.

Doubtless someone will already have pointed out that a rex 15 feet tall
would have been closer to 40 than 20 feet long, but the real problem here is
that the height of the person (or rex) is irrelevant.  It would take just as
much time and energy for a 15 foot person to swing a 20 foot ladder as it
would a 6 or even 3 foot tall person.  What matters is the proportion of
your plant (the longest dimension of your contact with the ground) to your
moment (inertia) about the vertical axis.

You can demonstrate this for yourself by doing the Maytag (not for people
with back problems; use low heel shoes only).  First plant your feet far
apart and twist left and right with your arms down at your side and note how
fast and easily you can do this.  Next try it again placing your feet right
against each other.  You will notice your pivot speed drops a bit and your
feet will try to slip more.  Keeping your feet together, extend your arms
out sideways and see how fast you can twist.  Slower still, right?  Next try
it holding weights out at arms length (gallon milk or water bottles work
pretty well).  By this point you will probably be down to a fraction of your
original twist speed and ground friction is becoming a serious
consideration.  (And you are still a long way from having a beam that
reached nearly 40 feet in length weighing something like six tons or more.)

While it is true the rex foot was bigger than ours it was still a small
fraction of the rex's effective body length, and I think this is the
proportion we need to focus on.


Nicholas Wren