Find bouncing angle of ball when you know the ball's heading and bumper's heading; 1) by rotating the system; 2) using algebraic linear combination.
Great exposition. I'm loving this approach, particularly the graphs that pop up to show things, and the way you showed the step by step by clicking buttons.
Only improvements I can see is to perhaps make it clearer what you're trying to learn. I was a bit confused when we switched to "how does Etoys do it on the walls" and then "traditional geometry." In other words, just a simple transition sentence or two to make it flow better.
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