Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: Dry friction is a force that opposes the relative lateral motion of two solid surfaces in contact. Professor Lewin got his PhD in Nuclear Physics at the Technical University in Delft, the Netherlands in 1965. He joined the Physics faculty at MIT in 1966 and became a pioneer in the new field of X-ray Astronomy. His 105 online lectures are world renowned and watched by about 2 million people annually. Lewin has received five teaching awards. He is the only MIT Professor featured in "The Best 300 Professors" of The Princeton Review. Professor Lewin co-authored with Warren Goldstein the book "For the Love of Physics" (Free Press, Simon & Schuster), which in 2012 has been translated in 9 languages and will be translated in a total of 11 languages. About this book Bill Gates wrote:"For the Love of Physics captures Walter Lewin's extraordinary intellect, passion for physics, and brilliance as a teacher. Hopefully this book will bring more people into the orbit of this extraordinary educator and scientist". Professor Lewin's honors and awards include the NASA Award for Exceptional Scientific Achievement (1978), twice recipient of the Alexander von Humboldt Award (1984 and 1991), a Guggenheim Fellowship (1984), MIT's Science Council Prize for Excellence in Undergraduate Teaching (1984) and the W. Buechner Teaching Prize of the MIT Department of Physics (1988). In 1997, he was the recipient of a NASA Group Achievement Award for the Discovery of the Bursting Pulsar. He is a member of the Royal Dutch Academy of Sciences (elected 1993), Fellow of the American Physical Society.