I know that D ⃗ v f = ∇ f ⋅ ⃗ v Dv→f=∇f⋅v→ is the directional derivative. My question is why must the vector ⃗ v v→ be a unit vector? I am sure there is an obvious answer, but my book doesn't really explain it.

Reference https://www.physicsforums.com/threads/directional-derivative.906813/


closed as off-topic by Marcus Müller, MBaz, Peter K. Mar 31 '17 at 14:34

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  • "This question does not appear to be about signal processing within the scope defined in the help center." – Marcus Müller, MBaz, Peter K.
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locked by jojek Apr 4 '17 at 7:34

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  • $\begingroup$ This is a math question without any relation to signal processing... $\endgroup$ – Marcus Müller Mar 31 '17 at 13:00

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