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Poko Mac
12-01-1998, 08:30 PM
Just a quick (hopefully) question to the group,

I am demonstrating how to calculate angular momentum of several segments
from filtered co-ordinate data using Microsoft Excel. To do this the
angular velocity of the segment, in free body state, must be obtained.

My problem arises when the segment moves out of the 0 - 90 degree
quadrant. If you use the simple TAN relationship (difY/difX) to
calculate the angle of the segment from the raw co-ordinates then as the
segment approaches 180 degrees the angle with the horizontal approaches
zero, which mathematically makes sense. In the second quadrant an
anti-clockwise displacement would lead to a negative angular velocity
(if you utilise the same angle as used in the first quadrant) which goes
against the sign convention.

In practice you could take this angle from 180 or add it to 90 degrees
to get the full angle of the segment, assuming your right horizontal is
zero degrees. The problem is the spreadshhet, without more complex
calculations, does not know what quadrant the segment is in! Is there a
simple way to calculate the 'true' angle for the segment, i.e. the true
angular displacement from the original?

Maybe i am overlooking something fundamental or it could be more
involved, has anyone out there solved the problem using spreadsheets or
any other information would be gratefully accepted.

I know i could solve the problem using C++ or Matlab but the
demonstration was as much about using MS Excel as calculating the
angular momentum.

As usual i will post a summary of replies.

Thanks, Dave

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