You can get a great workout by walking up the stairs in a tall building. It’s a unit of energy, defined as the amount of energy needed to raise the temperature of 1 liter of water by 1 degree Celsius. (This is your common “food calorie”: chemists confusingly use a calorie measure that’s 1,000 times smaller.) We call the energy input (or output) “work.” Work is done on a system by applying a force over some distance. This depends on the mass of the book, the gravitational field (g), and the height of the book above the floor (y).