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~- (Ro+rb) x 8F, = dd ~-- Ri x 8m Vi (4.313)
Ro x ~: 8F: + ~- rl x 8Fr = ddt L- (Ro + -o,) x 8m Vt (4.314)
d
= ddt E Ro x 8m V: + ddt ~- }:b X 8m Vi.
(4.315)
-
= Ro x z~8m V: + Ro x E8mv:
+ddt y-) b X 8m V: (4.316)
We -note that
Ro x ~_ 8m Vi = Ro x X) 8m (Ro + 77lb) (4.317)
= Ro x >] 8mRo + R,
N Rox~8mrb (4.318)
-. O
N (4.3 19)
The first term on the right-hand side ofEq. (4.318) vanishes because Ro x Ro = 0
and the second term vanishes because the body is assumed to be rigid. Furthermore,
using Eq. (4.294), we obtain Ro x 8F1 - Ro x 8m V/.
With these simplifications, Eq. (4.316) reduces to
E7bx8F,=/t~rbx8m.V: (4.320) ..
Thus, according to Eq. (4.320), the statement Gr.=dllt/dt of Eq. (4.311) is
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