Energy -- need a proper definition please Please could someone give me a proper definition of energy. My teacher said in class today that "there is no such thing as different types of energy". He said that although we learn this in introductory physics, a physicist would not agree there are different types and would define it in a much different way. He said something about it being "the ability to do something" which therefore means the notion of having different types of energy isn't quite right. Is he right? From researching the internet I can see that energy is very abstract but could someone give me a proper physics definition of what it is. I found a video clip from professor Brian Cox who says that he defines energy as: "the length of a space-time four vector in the time direction" What does that mean? Thanks for any help!