Khan Academy
Khan Academy: What Is Normal Force?
If you have ever run into a solid wall, you have experienced the pain of a normal force. This webpage reviews over the force exerted when two objects touch.
Khan Academy
Khan Academy: What Is Friction?
Learn about the force of friction on this reference page which defines why friction is important and how it can change physic problems.
Khan Academy
Khan Academy: What Are Inclines?
A webpage with equations and examples of how to solve inclined planes problems. Learn how to deal with slopes when surfaces usually aren't perfectly horizontal.