  The title page of Chapter 9 in Fundamentals of Physics (Halliday, Resnick,
and Walker, 8th Edition, p. 201) shows a dramatic photograph of two Big Horn
sheep butting heads and promises to explain how sheep survive such violent
clashes without serious injury. However, the answer presented in sample problem
9-4 (p. 213) errs in presuming an interaction time of 0.27 s which results in
an unrealistically long stopping distance of 0.62 m. Furthermore, the assertion
that the horns provide necessary cushioning of the blow is inconsistent with
the absence of concussions in domestic breeds of hornless sheep. Results from
traumatic brain injury (TBI) research allow acceleration tolerance of sheep to
be estimated as 450 g facilitating an analysis of sheep collisions that is more
consistent with available observations (stopping distance less than 1 cm,
impact time of roughly 2 ms).
