Vector Calculus

  • Thread starter rad0786
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  • #1
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Hey... i was hopeing somebody can help me with a homework question... its about vector calc.... taking the deriviative.

d/dx[r(t) dot r(t)] = r'(t) dot r(t) + r(t) dot r'(t) = 2r'(t) dot r(t)

I know it sounds sillly, but i was just wondering how on earth they got
2r'(t) dot r(t) ????
 

Answers and Replies

  • #2
Hurkyl
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Well, go through the list of properties of the dot product -- any of them look useful?
 
  • #3
robphy
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Another approach... what must be true for that last equality
r'(t) dot r(t) + r(t) dot r'(t) = 2r'(t) dot r(t)
to happen (assuming that each side is correct)?
This should lead to the property hinted by Hurkyl.
 
  • #4
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Okay i see it now.

It comes from the property....

a dot (b + c) = a dot b + a dot c

:)

Thanks for poinitng that out...
 
  • #5
Hurkyl
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Ah, right -- that one's so fundamental I forgot it even needed to be invoked! (I had only noticed you needed a.b = b.a)
 

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