What is Energy?

Energy is the most powerful and useful concept in all of physics, but what exactly is it? You can start your free trial of The Great Courses Plus at Get your own Space Time t­shirt at Tweet at us! @pbsspacetime Email us! pbsspacetime [at] gmail [dot] com Comment on Reddit: Help translate our videos! Previous Episode: Kronos: Devourer of Worlds Written and Hosted by Matt O'Dowd Produced by Rusty Ward Graphics by Matthew Ranker Assistant Editing and Sound Design by Mike Petrow and Meah Denee Barrington Made by Kornhaber Brown (www.kornhaberbrown.com) In physics energy is not a substance, nor is it mystical. Energy is a number. A quantity. And the quantity itself isn’t even particularly fundamental. Instead, it’s a mathematical relationship between other, more fundamental quantities. It was 17th century polymath Gottfried Leibnitz who first figured out the mathematical form of what we call kinetic energy – the energy of motion. He realized that the sum of mass times velocity squared for a system of particles bouncing around on a flat surface is always conserved, assuming no friction and perfect bounciness. Leibnitz called this early incarnation of energy vis viva – the living force. Big Bang CoolAsCats David Nicklas Faisal Saud Anton Lifshits Joey Redner Quasar Tambe Barsbay Mayank M. Mehrota Mars Yentur Mark Rosenthal Dean Fuqua Justin Lloyd Roman Pinchuk Hypernova Shaun Williams Edmund Fokschaner Max Levine Matthew O’Connor Eugene Lawson Martha Hunt Joseph Salomone Chuck Zegar Jordan Young Ratfeast John Hofmann Barry Hatfield Craig Peterson Jane Myers Brandon Cook James Flowers Greg Allen Denys Ivanov Nick Virtue Alexey Eromenko Nicholas Rose Scott Gossett Mark Dykstra Mark Vasile Patrick Murray Sultan Alkhulaifi Alex Seto Michal-Peanut Karmi Erik Stein Daniel Lyons Kevin Warne JJ Bagnell J Rejc Avi Goldfinger John Pettit Florian Stiglmayr Benoit Pagé-Guitard Nathan Leniz Jessica Fraley Loro Lukic Brandon Labonte David Crane Greg Weiss "The Great Courses Plus is currently available to watch through a web browser to almost anyone in the world and optimized for the US market. The Great Courses Plus is currently working to both optimize the product globally and accept credit card payments globally."