Why Gliders Fly
This video does a good job explaining how gliders can fly without an engine.
Jim Burch, Study Soaring
Freely available on the Internet, this study guide helps prepare anyone for the knowledge test and practical exam to become a US glider pilot. Jim is a highly experienced flight instructor and FAA flight examiner. His online guide is succinct, well illustrated, and avoids some of the errors contained in the official “Glider Pilot’s Handbook”, which is typically used as primary reference for flight instruction.