...
c = (4, 5)
sqrt(4^2 + 5^2))
The PVector class in processing has a method that returns this value: mag();
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Modify example 2 to include a gravity force. The ball circles should fall towards the bottom of the screen, where it will bounce up again.
(Harder) Further modify the example so that each mouse press adds a new circle, and releasing the mouse sets the new circles velocity.
(Very Hard) Further modify the example so that each ball bounces off any ball it collides with.