12 l/4 由角动量定理 e-=--- dl d(o dJ dt morcos=@ ml+ mr=mra dt 12 考虑到O=c 7g120 cos at cOs dt 20 2407 7ll 0 7 12 v = 由角动量定理 t J t J t L M d d d d( ) d d = = = t r m l m r m r t mgr d d ) 2 12 1 ( d d cos 2 2 = + = 即 考虑到 =t ) 7 12 cos( 24 7 cos d 2 d 0 0 t l t g t r v v lg = =