This video introduces the main formula you'll need for F=ma problems involving force (F=μN), then shows you how to use it to solve a few basic examples. The key to friction is that the force of friction is perpendicular to the normal force pressing two surfaces together, so friction problems always require both X and Y equations. :(
Static vs Kinetic Friction (μs vs μk)
This video covers the important differences between static and kinetic friction. It also explains why μs is ALWAYS bigger than μk, a fact which is likely to make it onto your next exam in the form of a short answer or multiple choice question!
Force Required to Overcome Static Friction
This video explains how to solve the type of problem where they ask you how hard an object must be pushed to "break free" of static friction and start moving (sometimes referred to as "breakaway").
Force Required to Prevent Painting From Sliding Down Wall
This video covers a common example that's covered by most books, though the object isn't always a painting. The basic idea is they'll ask you the normal force (or sometimes μs) required such that the friction will be high enough to prevent an object from sliding down the wall.
Static Friction of Crate Pressed by Angled Force
This video works an example where they ask you to calculate the static friction force required to resist an angled force.
Hockey Puck Getting Shot By Angled Stick (with Friction)
This video covers a fun example from the world of sports: a hockey player slamming a slap shot! Because the stick acts on the puck at an angle, pre-loading flex into the stick, the stick is pressing the puck into the ice, increasing the friction that acts upon it, which in turn affects the acceleration. Complicated!
If you do not have an account, you should get one, because it is awesome! You can save a playlist for each test or each chapter, and save your "greatest hits" into a "watch right before the final" list (not that we recommend cramming, but when in Rome...)