Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Parabola Three Normals

  1. Sep 16, 2012 #1


    User Avatar
    Gold Member

    1. The problem statement, all variables and given/known data
    Three normals are drawn from the point (14,7) to the curve [itex]\large y^{2} -16x-8y=0[/itex]. Find the co-ordinates of the feet of the normals.

    2. Relevant equations
    Converting the equation of parabola in the form of a perfect square I get
    [itex]\large (y-4)^{2}=16(x+1)[/itex]

    3. The attempt at a solution
    I know that for the parabola [itex]\large y^{2}=4ax[/itex] the equation of normal is [itex]\large y=-tx+2at+at^{3}[/itex]. But what will be the equation of normal for the parabola in this question? If I get to know the equation of normal in terms of t then I can get a cubic equation in t and by solving that equation I can obtain the coordinates of feet of normals. But I'm falling short of the equation of normal. How do I get it?
  2. jcsd
  3. Sep 16, 2012 #2


    User Avatar
    Homework Helper

    Remember how if we have some parabola y=x2 and we then substitute x'-1 for x, to get y=(x'-1)2, this is now a parabola that has moved 1 unit to the right. Also, if we substitute y'-1 for y and get y'-1=(x'-1)2 this is now a parabola that now moved up 1 unit as well, so its apex is now at (1,1).

    This also applies to all functions. So if we have a tangent to the parabola y=x2 as y= mx+c, then if we move the parabola across a units and up b units (a and b could be negative too so we can move then down and left) to get the parabola y-b=(x-a)2 then if we do the same transformation to y=mx+c to get y-b=m(x-a)+c this will now still be a tangent to the parabola at the same point.

    So where am I going with this?

    Well, you've already told me that

    So then what is the normal for the parabola [itex]\large (y-4)^{2}=16(x+1)[/itex] now?
  4. Sep 16, 2012 #3


    User Avatar
    Gold Member

    Yep. That worked! Thank You once again for helping me. Can you please solve my another question which I have posted in this forum titled "Locus of circumcentre". I really need some hints to get started with.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook