2D Physics Tutorial

I've been writing the content of this tutorial for a couple of weeks now, and I thought I'd submit a relatively brief tutorial on the main components of what I've made so far. I hope you enjoy it, and I hope you can follow all of it. Please leave any critisisms you may have, it's all appreciated. Thanks
Oh and I'm also currently working on more content to put into a second tutorial as a sequel to this one. Leave any ideas you have or anything you want to be put in and I'll work on putting it all in!