Sunday, July 26, 2015

How to Draw a Circle in Scratch - Part 5

   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.
   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

No comments:

Post a Comment