In
Part 4 of this series on How to Draw a Circle in Scratch the G-poly (center, radius) script was completed. It will take just the
addition of a single block to convert the G-poly (center, radius) script to the G-poly
(center, radius, arc) script. A comparison of the two scripts is shown below.

Where
did the added block come from? In Part 4 we decided to fix the number of sides
of the regular polygon at 60. The [turn (right 360/(number of sides)] block
then reduced to [turn (right 360/60)] that reduces to [turn right (6)].

We
can set up the proportion for 60 sides to 360º as shown below and solve for x, the length of the side that corresponds to 1º.

60:360 = x:1

x = 0.1666…

A
new [set] block named *(arc length)º*and a new variable named

*arc*are created. The variable

*arc*is then multiplied by 0.01666… and stored in

*(arc length)º.*

Now the script will draw a full
circle if arc is set to 360 or the
arc for any angle from 0º to 360º.

For
all practical purposes, we have created a geometry compass in Scratch! I made a
new background for the G-poly (circle,
center, radius, arc) script and renamed the script Geometry Compass. In the following graphic the center was set at
(15, -25), the radius to 125, and the arc to 281º.

The Geometry Compass project can be
viewed and downloaded at this link.

https://scratch.mit.edu/projects/70542210/

Click on the Play button to watch a short video of the Geometry Compass in action.

Parts 1-5 of this series of Circle posts (in separate PDF files) can be obtained at no cost by sending an email request to grandadscience@gmail.com.

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