Last week I attended a workshop about a website called Defined Stem. It was created by the people who developed United Streaming. If you have not explored this site, you are missing a treasure trove of resources. While you typically think of STEM resources as being solely for your science, technology, engineering, and math teachers, you will find resources well beyond. As many of the real-life videos chronicle, the STEM professions call on skill sets well beyond strictly science, technology, engineering, and math.
The projects that are already developed in this website follow the Understanding By Design format and are truly project-based learning units. If you strictly think of doing projects as problem-based learning, you are incorrect in your assumption. The following Prezi delineates the difference.
If you still are having problems understanding the difference, I especially like the Edutopia website and their detail of this methodology. They also have several videos of examples of project-based learning that find real-world problems at the the core of the content. If you are looking for ways to challenge your students with the standards they must know, PBL is the avenue to take. Explore the Defined Stem website. You can request a 14 free trial. I know you will be excited with the content there. This is a great way to integrate language arts and history into STEM curriculum.