I have posted a number of CoCo BASIC articles that used the example of bouncing a ball around the screen. I have a simple routine I use for this, which involves an X and Y location variable, and movement MX and MY variables that will be positive or negative for which direction the ball is moving.
To my surprise, I found nearly this exact code in the old VIC-20 manual as a bouncing ball example!
This code called it DX and DY (delta), but it’s basically the same code I’ve been using all these years. I had no idea I learned this from the VIC-20 manual back in 1982!