Science Struck
Science Struck: Guide to Free Body Diagrams in Physics
A detailed explanation of the different types of forces that can be applied to an object and how to represent them in a free body diagram. Includes examples and some practice problems to try, with solutions provided.
Physics Classroom
The Physics Classroom: Newton's First Law of Motion
A clear explanation of Newton's First Law of Motion, the law of inertia, with diagrams and real-world examples. Useful for both students and teachers. Written by a high school physics teacher.