body like the spearthrower: the mass, the length, and the
concentration of mass (see chapter 5).
Before leaving this section it is warthwhile trying to
approximate the magnitude of the loading about the wrist for a
spearthrower of a given design. Initially a reasonable figure for
the angular acceleration of the spearthrower had to be established. This was done by calculating the angular acceleration f r o m . the
velocity/time curves in section 4.4. In the time period just before
2
release the acceleration were as follows: M. Edwards, 823 rads/sec
; the writer, 873 and 1004 rads/sec2 . The differences between
Edwards' and m y own accelerations is probably due to his being taken
over a longer time span (.04 compared with .02 sec.). If a
spearthrower of cylindrical construction is used with a length of one
metre and a mass of .2kgs the moment of inertia can be easily
calcutated (table 10). And if the angular acceleration is set at 800
rads/sec the inertial torque is in the order of 60 Newton /Metres.
This figure is very close to that for heavy tennis racket during a
tennis serve, which Plagenhoef (1971:158) calculated at 53
Newton/metres. Remembering that the wrist velocity is also
comparable between the two actions the similarity is perhaps not that
surprising. The dimensions of the spearthrower used in this
calculation were similar to those used by the writer in the film
4 . 6 : C o n clu sio n
The above a p p ro x im a tio n l i k e a l l th e m echanics in t h i s s e c t i o n i s based on th e model o f th e sp e a rth ro w e r r o t a t i n g by i t s e l f . A lthough a d e q u a te a s a f i r s t a p p ro x im a tio n , i t c l e a r l y d o e s n o t
r e p r e s e n t th e t o t a l p i c t u r e . The s p e a r /s p e a rth r o w e r a r t i c u l a t i o n , w hich adds a v\hole ran g e o f te c h n o lo g ic a l f a c t o r s which a f f e c t th e
CHAPTER FIVE
SPEAR AND SPEARTHRCWER ARTICULATION
5 . 1 : I n t r o d u c t i o n
I h e a r t i c u l a t i o n o f t h e s p e a r and s p e a r th r o w e r i s p r o b a b ly th e m o st m e c h a n ic a lly com plex a r e a o f s p e a r and s p e a r th r o w e r o p e r a t i o n . I t w i l l be r e c o g n is e d t h a t s p e a r p e rfo rm a n c e i s b a s e d on e s s e n t i a l l y l i n e a r c h a r a c t e r i s t i c s o f m o tio n , i e . i t s v e l o c i t y in th e d i r e c t i o n o f a g iv e n t a r g e t . The s p e a r th r o w e r on th e o t h e r hand i s e s s e n t i a l l y a r o t a t i o n a l s y s te m . The b a s i c pro b lem i s t h e r e f o r e to a r t i c u l a t e a r o t a t i o n a l sy stem su ch t h a t i t t r a n s f e r s e n e r g y i n t o a l i n e a r sy stem s u c e s s f u l l y . I t w i l l be shown t h a t th e a n a l y s i s o f t h i s a r t i c u l a t i o n a llo w s t h e i d e n t i f i c a t i o n o f s e v e r a l im p o r ta n t m e c h a n ic a l r e l a t i o n s h i p s w hich h av e h e a v i l y in f l u e n c e d th e s t r u c t u r e o f b o th s p e a r s and s p e a r th r o w e r s in A u s t r a l i a . P r e v io u s a p p ro a c h e s t o s p e a r /s p e a r t h r o w e r a r t i c u l a t i o n h ave depended on an e s s e n t i a l l y s t a t i c m odel (s e e s e c t i o n 4 : 5 a ) . T h is m odel w i l l be d i s c u s s e d f i r s t b e c a u s e i t a p p p e a r s a p p l i c a b l e f o r th e s t a g e o f t h e th ro w b e f o r e th e s p e a r th r o w e r b e g in s to r o t a t e . I t s p r e d i c t i o n s a b o u t s p e a r and s p e a r th r o w e r m o rp h o lo g y w i l l be com pared w ith th o s e r e s u l t i n g from an a n a l y s i s o f s p e a r and s p e a r th r o w e r a r t i c u l a t i o n u s in g a m ore dynam ic a p p ro a c h .
5 . 2 : S t a t i c Models The s p e a r th r o w e r , jo in e d to th e b a se o f th e sp e a r by a peg o r s o c k e t i s p r i o r to th ro w in g , h e ld in a p o s i t i o n such t h a t i t s long a x is i s p a r a l l e l to t h a t o f th e s p e a r . As see n in s e c t i o n 4 :5 a th e sp e a rth ro w e r in t h i s p o s i t i o n e x e r t s a to rq u e ab o u t th e w r i s t . By th e same p r i n c i p l e o f moments th e s p e a r w i l l a ls o e x e r t a to rq u e ab o u t th e w r i s t , ( i f th e s p e a rth ro w e r d o es n o t p iv o t on th e s p e a r 's COG) eq u al to i t s mass and th e d i s t a n c e (r) from th e s p e a r 's COG to th e p iv o t o f th e w r i s t . I f th e s p e a r i s o f s u f f i c i e n t le n g th and mass th e to rq u e produced by th e s p e a r w i l l be o p p o s ite to t h a t produced by th e s p e a rth ro w e r and o f a g r e a t e r m a g n itu d e . The w r is t o f th e th ro w er i s t h e r e f o r e b ein g tu rn e d downward away from th e th ro w e r. T h is im balance o f moments h a s le a d s e v e r a l w r i t e r s to argue t h a t s p e c i f i c r e l a t i o n s h i p s m ust o ccu r betw een th e s p e a r and th e s p e a r th r o w e r .
The sec o n d , and p o s s ib ly th e m ost commonly used model i s what may be term ed th e w eight co m p en satio n p r i n c i p l e . T h is i s e s s e n t i a l l y a v e ry underd ev elo p ed b io m e c h a n ic a lly based argum ent which assum es t h a t humans have an optimum load a g a in s t which th e y may throw o r h o ld . As see n in c h a p te r fo u r t h i s may be th e c a se in term s o f e n erg y p r o d u c tio n , b u t i t s u n q u a lif ie d use w i l l be shown to have produced some u n s a t i s f a c t o r y p r e d i c t i o n s ab o u t s p e a r /s p e a r th r o w e r a r t i c u l a t i o n .
D avidson (1936:451) argued t h a t heavy s p e a r s co u ld be thrown b e s t w ith a s h o r t s p e a rth ro w e r and t h a t l i g h t w eight s p e a r s co u ld be thrown w ith e i t h e r long o r s h o r t s p e a rth r o w e rs . The re a so n fo r
Page 85 D a v id s o n 's p r e d i c t i o n i s t h a t he was i m p l i c i t l y u sin g a l im it e d w eig h t arg u m en t. Because a l l w eight h e ld in one hand r e q u i r e s an e f f o r t from th e arm to h o ld i t , heavy s p e a r s h e ld in th e same hand as a long heavy s p e a rth ro w e r would be to o heavy in combined mass to throw . W ith l i g h t s p e a r s i t d o es n o t m a tte r g r e a t l y a b o u t th e added mass so b o th long and s h o r t s p e a rth r o w e rs may be u sed .
D a v id so n 's p r e d i c t i o n ab o u t th e r e l a t i o n s h i p betw een heavy s p e a r s and s h o r t sp e a rth ro w e r w i l l be shown to be b a s i c l y c o r r e c t , b u t n o t f o r th e re a s o n s p ro p o se d . He was how ever, i n c o r r e c t in assum ing t h a t s h o r t s p e a rth r o w e rs a r e n e c e s s a r i l y l i g h t e r .
P e e ts (1960) in fo rw ard in g a s o l u t i o n to th e American "b a n n e rsto n e " problem argued t h a t w eig h t c o u ld be added to a s p e a rth ro w e r to c o u n te r th e to rq u e o f th e s p e a r . Thus, long heavy s p e a r s r e q u ir e d h e a v ie r and p o s s ib ly lo n g e r sp e a r th ro w e rs . P e e ts ' argum ent h a s a ls o been used by Cosgrove (1978) to d e f in e th e f u n c tio n o f th e s h e l l a tta c h m e n t to th e p roxim al end o f Q ueensland s p e a rth r o w e rs .
P a l t e r (1 9 7 6 ), on th e o th e r hand, claim ed t h a t t h e r e a r e no such d e v ic e s f o r adding mass to th e s p e a rth ro w e r in use in A u s t r a l ia d e s p i t e th e le n g th s o f th e s p e a r s u se d . He con clu d ed t h a t " th e le n g th o f th e s e weapons seems to v a ry in d e p e n d e n tly w ith r e s p e c t to th e le n g th and w eight o f th e sp e a rth ro w e r p r o j e c t i l e s " (1 9 7 6 :5 0 3 ). 1. "B an n e rsto n es" a r e p r e h i s t o r i c a r t i f a c t s o f v a r io u s form s which have been found in a s s o c i a t i o n w ith s p e a rth r o w e rs in e a r l y American s i t e s (see s e c t i o n 5 .7 )
T h is c o n c lu s io n i s n o t based on any t h e o r e t i c a l p r i n c i p l e s , b u t on a few b r i e f r e f e r e n c e s to th e e th n o g ra p h y , w hich h as v e ry l i t t l e