Vector Calculus, Unit normal to surface help

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Homework Help Overview

The discussion revolves around finding the gradient of a scalar function \(\phi(x,y,z) = x^3 + 2xy + yz^3\) at a specific point and determining the direction of the unit normal to the surface defined by \(\phi(x,y,z) = 11\) at that point. The problem is situated within the context of vector calculus.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • Participants discuss the calculation of the gradient \(\nabla\phi\) and its magnitude, questioning how the specific value of 11 for the surface affects the unit normal direction. There is also a clarification regarding a numerical error in the magnitude calculation.

Discussion Status

Participants are actively engaging with the problem, providing feedback on calculations and clarifying misunderstandings. There is an acknowledgment of the relationship between the surface value and the normal direction, though no consensus has been reached on the implications of that relationship.

Contextual Notes

There are indications of potential confusion regarding the calculations and the significance of the surface equation, as well as a mention of a typo in the numerical values discussed.

tarwe
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Homework Statement


If \phi(x,y,z) = x3 + 2xy +yz3 find \nabla\phi at the point P=(1,1,2) and direction of the unit normal to the surface \phi(x,y,z) = 11 at P.

Homework Equations





The Attempt at a Solution


Worked out \nabla\phi to be 5i + 10j + 12k
Got |\nabla\phi|= √256

so the unit normal to surface is surely \frac{5+10+12}{√269}
but how does the =11 bit make a difference?

Thanksss
 
Last edited:
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welcome to pf!

hi tarwe! welcome to pf! :smile:
tarwe said:
but how does the =11 bit make a difference?

it doesn't :wink:

(btw, it's not 256)
 


tiny-tim said:
hi tarwe! welcome to pf! :smile:


it doesn't :wink:

(btw, it's not 256)


thank you!
that was a typo, meant 269, was thinking in binary :/
 
tarwe said:

Homework Statement


If \phi(x,y,z) = x3 + 2xy +yz3 find \nabla\phi at the point P=(1,1,2) and direction of the unit normal to the surface \phi(x,y,z) = 11 at P.

Homework Equations


The Attempt at a Solution


Worked out \nabla\phi to be 5i + 10j + 12k
Got |\nabla\phi|= √256

so the unit n0ormal to surface is surely \frac{5+10+12}{√269}
You mean \frac{5i+10j+12k}{√269}

but how does the =11 bit make a difference?
It would change the exact position but not the normal to the surface.

Thanksss

Homework Statement


Homework Equations


The Attempt at a Solution

 

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