One of the best ways to learn is having a good problem to solve. Well, I know it has been a long time since I have worked on any coding, but I had a problem that needed solving. I've been teaching my students to combine vectors into resultants and it is a little difficult when those vectors are at different angles. I thought this would make a good visualization and would be pretty straight forward, meaning I'd have the ability to make the code work. I know there are other vector simulations and visualizations online, but it's always nice to make exactly what you need to fit your needs and there is the satisfaction of having done it yourself.
I made a visualization for combination of forces (both with "helper" vectors and without) and a visualization for splitting a vector into its x and y components. Click and drag to move the vectors around.