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WalkingInMud
04-22-2008, 03:15 PM
Let r(t) be a v.v.f -with the first and second derivatives r' and r''. Determine formula for
d/dt [r.(r' x r'')] -in terms of r:

How do we approach this one?

maybe:
d/dt [r.(r' x r'')] = d/dt [r.r' x r.r''] = ((r'.r' + r.r'') x r'.r'') + (r.r' x (r'.r'' + r.r''')) ??
...and then what?

(since it looks messier that the initial expression I guess my differentiation is wrong or I should diff at this early stage)

I'm thinking that there must be some simple vector calculus identities that make this easier or something.

Is anyone able to give me a starting point -- or starting direction? -- thanks heaps

WalkingInMud
04-22-2008, 05:30 PM
;)

Firstly, we gotta assume that the 3rd derivative exists,

Then, d/dt[r.(r' x r'')] = r'.(r' x r'') + r.[(r'' x r'') + (r' x r''')].

now, r'' x r'' = 0 is a given since they are both parallel (well actually they're the same), also,

r'(r' x r'') = 0 as well -- since (r' x r'') is orthogonal to r' (and r'')

so, the final answer is r.(r' x r''')

:cool: