For what it's worth. It might be helpful to remember that when our body
accelerates(*) "upward" due to a single force applied to the bottom of our
feet, it goes into compression with the greatest stresses in the vicinity
of our feet and relatively small stresses in the vicinity of our head.
When our body accelerates "upward" due to a single force applied to the
top of our head (ouch!), it goes into tension with the greatest stresses
in the vicinity of our head and relatively small stresses in the vicinity
of our feet. Such differential stresses always accompany acceleration.
On the other hand, if we are not accelerating there are no such
differential stresses. Thus, differential stresses are a clear signature
of acceleration.
(*) In the above, "acceleration" is, of course, meant in the preferred
sense of "wrt a local inertial (i.e., free fall) frame."
John
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A. John Mallinckrodt web: www.intranet.csupomona.edu/~ajm
Professor of Physics email: mallinckrodt@csupomona.edu
Physics Department voice: 909-869-4054
Cal Poly Pomona fax: 909-869-5090
Pomona, CA 91768 office: Building 8, Room 223