Here is the Crash Course vid on sliding friction. Good one. I like the graphic showing how mu-Fn is the MAX force of friction. The actual force may not be at the maximum value.
Here’s that neat little Inclined Plane with Pulley Problem widget:
Even better if you go to their website (bigger and easier to read)
Why not check out this “worksheet” too? It’s got two inclined planes! Hardcore, and cool!
And here is the website for the above widget.