Professor Moriarty requested that we re-visit one aspect of physics in the game Portal 2 - humour him! Previous videos at https://www.youtube.com/watch?v=pw6JIB3pM2o and https://www.youtube.com/watch?v=UKAQ1PtcgVA Visit our website at http://www.sixtysymbols.com/ We're on Facebook at http://www.facebook.com/sixtysymbols And Twitter at http://twitter.com/#!/periodicvideos This project features scientists from The University of Nottingham http://www.nottingham.ac.uk/physics/index.aspx Sixty Symbols videos by Brady Haran A run-down of Brady's channels: http://periodicvideos.blogspot.co.uk/2012/06/here-are-my-channels.html