Gradient, unit normal in vector calculus

  1. Aug 26, 2011 #1
    1. The problem statement, all variables and given/known data

    there is a surface xy3z2=4. What is the unit normal to this surface at a pt in the surface (-1,-1,2)??

    2. Relevant equations
    what is a unit normal to a scalar region? how can it be calculated?


    3. The attempt at a solution
    i calculated the gradient (del operator) of this surface at the given point to be 3i-12j+4k . this is in the direction of the unit normal along which max. rate of increase occurs. So far so good. But how dowe get the unit normal from this ? i dont know how to go further.
     
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  3. Aug 26, 2011 #2
    here, all points in the surface has a constant value =4, irrespective of the location of the points in the surface. does this have something to do with the calculation of the unit normal? it does sound absurd..but i'm equally blank..so just throwing all that's on my mind.
     
  4. Aug 26, 2011 #3
    by the way, i'm writing down the gradient of the surface as i calculated it, so that any mistakes i've made can be corrected too.
    [itex]\nablaS[/itex] = y3z2i+3xy2z2j+2xy3zk.
    now,[itex]\nablaS[/itex] at the point (-1,-1,2) = 3i-12j+4k

    the unit normal as i got is 3i-12j+4k
    -----------------------------
    13

    but my textbook says it is (-1/rt 11, -3/rt 11, 1/rt 11)
    i've no idea how to arrive at this one .. any help is appreciated
     
  5. Aug 26, 2011 #4

    vela

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    There's a theorem that says whenever you have a surface defined by f(x,y,z) = c, where c is a constant, the normal to the surface is given by grad f. This is exactly the case you have here, so taking the gradient will indeed give you the normal.

    The reason you're not getting the right answer is that you simply calculated the numbers wrong. The x-component, for example, should be (-1)322=-4, not 3. (I didn't check the rest.)
     
  6. Aug 26, 2011 #5

    I like Serena

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    Hi msslowlearner! :smile:

    When I calculate your gradient I get a different result.
    Your gradient formula is correct, but filling in (-1, -1, 2) should give a different result.

    Furthermore, your problem statement asks for a unit normal.
    This means your vector should have length 1.
     
  7. Aug 26, 2011 #6
    i dont think it should have length 1. its the vector/mod vector, which is not 1. its the smallest multipliable unit of the vector, when multipled by the length of the vector gives the actual vector .
     
  8. Aug 26, 2011 #7
    yes, the x-component was wrong. now, i get the gradient as -4i -12j +4k
    whose magnitude is sqrt(176). if the unit normal vector is supposed to be (gradient/magnitude of gradient), i still am not arrivng at the answer !!
     
  9. Aug 26, 2011 #8
    hang on hang on !! i got it .. simple math.. sqrt(176) = 4*sqrt(11). sorry guys.. my mistake.. thanks for the help :)
     
  10. Aug 26, 2011 #9

    I like Serena

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    Huh? :confused:
    I don't understand you. Could you elaborate?

    Note that your textbook says: (-1/rt 11, -3/rt 11, 1/rt 11)
    This should be read as:
    [tex](\frac {-1} {\sqrt {11}}, \frac {-3} {\sqrt {11}}, \frac {1} {\sqrt {11}})[/tex]
     
  11. Aug 26, 2011 #10
    the magnitude of my vector was 4*rt(11). when i multiply my unit vector with the magnitude, i get my vector again = -4i -12j +4k :)
    the terms i used to explain may have been obscure,but what i intended to say was tat the unit vector here has a length which is not equal to 1, but it is still a unit vector for a given vector.
     
  12. Aug 26, 2011 #11

    I like Serena

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    I'm sorry to say, that what you say is still obscure.
    A unit vector is defined to be a vector of length 1.
    To obtain a unit vector from a vector it needs to be divided by its length.
     
  13. Aug 26, 2011 #12
    somehow, i dont get it .. i still think it need not be necessarily 1. but since ur a PF contributor, what you say should be right ..i'm just a beginner .. i'll look my concepts again.. thanks for the help :)
     
  14. Aug 26, 2011 #13

    vela

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    You mean the normal vector you first calculated, (-4, -12, 4), isn't length 1. By dividing it by its magnitude, you get the unit normal, (-1/√11, -3/√11, 1/√11), which is length 1. As ILS noted earlier, that's why it's called the unit normal.
     
  15. Aug 27, 2011 #14
    i'm sorry people .. i see it now .. unit normal vector has a length 1 ... personal apologies to "i like serena" .. i kinda questioned ur intelligence.. sorry ... sometimes i miss out the so obvious tiny little things tat matter the most .. sorry guys
     
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