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

Hi there,To find the gradient of a curve, we draw a chord on the

  1. Jan 24, 2012 #1
    Hi there,

    To find the gradient of a curve, we draw a chord on the curve and then makes the 2nd coordinates ( B ) tends to A ( 1st coordinates ).

    To find the gradient of the chord, i.e, ΔY/ΔX, we replace the two coordinates into the equation of the curve. But my question is why do we replace them ?
  2. jcsd
  3. Jan 24, 2012 #2
    Re: Differentiation

    The gradient field evaluated along a curve gives you vectors normal to the curve. If you draw a chord (hence, a straight line), its gradient will be lines (or, rather vectors along these lines) of the form y = ax +b
  4. Jan 24, 2012 #3
    Re: Differentiation

    Sorry, but I didn't quite understood there. Can you explain more simpler please. Sorry again.
  5. Jan 24, 2012 #4


    User Avatar
    Science Advisor

    Re: Differentiation

    I think meldraft is misunderstanding your use of the word "gradient". In the United States, we typically just use the word "gradient" to mean [itex]\nabla f= \partial f/\partial x\vec{i}+ \partial f/\partial y\vec{j}+ \partial f/\partial z\vec{k}[/itex], which will give a vector normal to the curve f(x,y,z)= constant.

    But in the United Kindom, and other places, "gradient" is used to mean just "derivative".
    And that is defined as the limit of the "difference quotient":
    [tex]\frac{f(x)- f(x_0)}{x- x_0}= \frac{y(x)- y(x_0)}{x- x_0}[/tex]

    Geometrically that "difference quotient" is the slope of the line from [itex](x_0, y_0)[/itex] to (x, y) so the limit gives the slope of the tangent line.
  6. Jan 25, 2012 #5
    Re: Differentiation

    Ahhh I didn't know that, thanks HallsofIvy!

    So, if I understand correctly, your question is why we evaluate the formula that HallsofIvy wrote, with points A and B.

    As HallsofIvy explained, the limit gives you the tangent line.

    In other words, if you don't evaluate the formula, you won't get a number for the slope (derivative) of the curve at this point.
  7. Jan 26, 2012 #6
    Re: Differentiation

    Yeah by gradient, I meant the slope of a line. My question is why do we replace the 2 coordinates of the line into the equation of the curve to find the gradient of the line ?

    And ultimately move the line and make it become a tangent to the curve so as to find the gradient of the curve.

    Just to say that, I'm still at a low level and have just started learning Differentiation and I don't know anything about difference quotient.

    Searched for difference quotient in Wikipedia and I got to know that its related to function and difference of 2 points. The rest I didn't understood.
  8. Jan 26, 2012 #7
    Re: Differentiation

    The concept is that you split the curve into infinitesimally small lines, each one after the other, and each with a different slope. The equation that gives you the slope of a line is:


    where points 1,2 are the beginning and the end of the line, respectively (or, in principle, any 2 points along the line)

    Now, if you do this for the entire curve, you get its derivative:

    [tex]f'(x)=lim_{Δx->0} \frac{f(x+Δx)-f(x)}{Δx}[/tex]
  9. Jan 28, 2012 #8
    Re: Differentiation

    You mean finding the gradient of every lines until ΔX becomes 0 ?

    Here's an image : http://i1162.photobucket.com/albums/q531/Jadaav/Image0198.jpg

    The image is not that clear.
  10. Jan 28, 2012 #9
    Re: Differentiation

    Is this from a numerical analysis/computational mechanics course?

    What that example is probably demonstrating, is that by calculating f(x) and f(Δx) and replacing in the derivative formula, you get the general formula for the actual derivative. This happens because Δx can be neglected from the result, since it's infinitely small (therefore its length is practically 0). Hence, you have proven that the derivative of x^2 is 2x :)

    Edit: as for your question, no, if you do it by hand you do not actually calculate every single segment (although, theoretically that is the concept). By calculating that limit you get a new function that basically describes how the slope changes, as x grows larger/smaller (that function is the derivative of the curve)
  11. Jan 29, 2012 #10
    Re: Differentiation

    What derivative formula ? From the example, f(x) and f(Δx) were replaced in the equation of the curve which in this case is y=x^2.

    My original question was why do we replace f(x) and f(Δx) in the equation. What does it do ?

    For me its a kind of a simultaneous equation and from what I know is that we do simultaneous equation to know at which coordinate(s) two lines meet.
  12. Jan 29, 2012 #11


    User Avatar
    Science Advisor

    Re: Differentiation

    First, we don't "replace [itex]f(\Delta x)[/itex]", we replace [itex]f(x+ \Delta x)[/itex]. Now, apparently, I don't understand your qestion. We replace f(x) with [itex]x^2[/itex] because we are asked to find the derivative of the function [itex]f(x)= x^2[/itex]. And, we replace [itex]f(x+\Delta x)[/itex] with [itex](x+\Delta x)^2[/itex] because that is what [itex]f(x+\Delta x)[/itex] means!

    Is the difficulty that you do not understand "functional notation"? If we were given, instead, that [itex]f(x)= x^3[/itex], then we would know that [itex]f(2)= 2^3= 8[/itex], [itex]f(a)= a^3[/itex], [itex]f(x+1)= (x+ 1)^3= x^3+ 3x^2+ 3x+ 1[/itex], and that [itex]f(x+ \Delta x)= (x+ \Delta x)^3= x^3+ 3x^2\Delta x+ 3x(\Delta x)^2+ (\Delta x)^3[/itex]. Then, of course, [itex]f(x+\Delta x)- f(x)= x^3+ 3x^2\Delta x+ 3x(\Delta x)^2+ (\Delta x)^3- x^3= \Delta x(3x^2+ 3x\Delta x+ (\Delta x)^2)[/itex]
    That means
    [tex]\frac{f(x+\Delta x)- f(x)}{\Delta x}= \frac{\Delta x(3x^2+ 3x\Delta x+ (\Delta x)^2)}{\Delta x}= 3x^2+ 3x\Delta x+ (\Delta x)^2[/tex]
    Now, what is the limit of that as [itex]\Delta x[/itex] goes to 0? What is the derivative of [itex]f(x)= x^3[/itex]?
  13. Jan 30, 2012 #12
    Re: Differentiation

    Sorry, typo :smile:
  14. Jan 30, 2012 #13
    Re: Differentiation

    The answer is 3X2

    Edit : I do know how to work it out. But I don't know what each step does and I'm trying to know why. Its like I learned it by heart what to do from my school teacher and I know that's wrong.
    Last edited: Jan 30, 2012
  15. Jan 30, 2012 #14


    User Avatar
    Science Advisor

    Re: Differentiation

    The basic idea is that you are looking at the slope of the line from the point [itex](x_0, f(x_0))[/itex] to the "neighboring" point [itex](x_0, f(x_0+ \Delta x))[/itex] (a "secant line" because a "secant line" in a circle is two a line that passes through distinct points on the circle while a "tangent line" only touches a single point on the circle). The reason we replace [itex]x_0[/itex] with [itex]x_0+ \Delta x[/itex] is that it moves us over a little bit so that we do have two points and so can calculate a slope. Then take the limit as [itex]\Delta x[/itex] goes to 0. (Not "until ΔX becomes 0"- take the limit. If you don't have a pretty good grasp of limits it will be hard to understand the derivative.) This can be done because, geometrically, the limit of those "secant lines" is the "tangent line" to the curve at that point.

    This has, by the way, nothing to do with "differential equations" so I am moving it to "Calculus and Analysis".
    Last edited by a moderator: Jan 30, 2012
  16. Jan 31, 2012 #15
    Re: Differentiation

    You mean to calculate the slope of the tangent ?

    So let me say what I've understood :

    At first we were at the point [itex]x_0[/itex], f([itex]x_0[/itex]) and by replacing the neighbouring coordinate, we moved a little bit and now we see the initial coordinate( where I was ) and the neighbouring coordinate. So we can calculate the slope of the secant line ( gradient ).

    Am I right here ?

    Edit : I've checked Wikipedia and it gives the definition of limit used in Mathematics. But do you know somewhere that I can have deeper understanding for it ?
    Last edited: Jan 31, 2012
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook