Drawing Tangents

[Patts]

Hello, I tried using the polynomial line which goes
through the points of degree 2. But the equation given is
y = -2E-06x2 + 0.0023x + 0.236 which I dont understand.
If I use the method of linest, I am left with y=mx + b, I
can diff but I will not be able to get velocity at each
point as the x would be eliminated once diff.

To be more precise, I have a set of x values which is the
height from 100cm to 190cm. The y values are the time it
takes for the object to fall, in sec.

any other suggestions

regards

 

[Jerry W. Lewis]

To fit a quadratic with LINEST: Select 3 columns instead of 2, and use
=LINEST(yData,xData^{1,2})

If there is a discrepancy between LINEST and the chart trendline, you
should trust the chart trendline (assuming that you have an "XY
(Scatter)" chart instead of "Line" chart). LINEST in versions prior to
Excel 2003 use an algorithm that is mathematically exact, but
numerically unstable.

Can you clarify your statement "which I don't understand"? The equation
of the fitted line is a straightforward quadratic equation. If you want
to calcualte with the formula, you should format the equation to dispay
numbers in scientific notation with 14 decimal places. The default
format rounds the coefficients too severly to be of any use in calculations.

Jerry

 

[Bernard V Liengme]

All this sounds like a physics homework problem.
Seems to me you have your x's and y's reversed. If y = time and x= distance
then dx/dy is velocity.
As for y=-2E-06x2+ 0.0023x + 0.236 what do you not understand? For -2E-06
read 2 x 10^-6.
Bernard

 

[Tushar Mehta]

See the replies to the other discussion on the same subject in the same
newsgroup.

--
Regards,

Tushar Mehta, MS MVP -- Excel
www.tushar-mehta.com
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