(Today is a Wednesday, but that doesn’t have as much of a ring to it.) Here’s a question: if everyone shuffled their feet everywhere they went, how much heat would that generate per year? Here’s an answer, with absolutely no sources to back it up (see, it’s supposed to just sort of be a fun little estimation, okay?)
There are about 7 billion people in the world, each person has two feet, and each foot supports half the person’s weight. Each foot travels the same distance that its person walks (or limps, in this case), which is … let’s see, about a hour per day if you add everything up, at about a meter per second, for a couple hundred days a year. That gives us precisely one-thousand-point-zero-one kilometers per year.
The work done by friction is the distance traveled, times the coefficient of friction, times the weight that the ground supports. Two feet times the coefficient of friction is about one foot. A person contains about 80 kilograms, I guess, which becomes 800 newtons when you consider that whole gravity thing.
Put it all together and it’s about 6 quadrillion joules per year. To give you an idea of how much that is, it’s enough energy to do 6 quadrillion joules of work. (Don’t like my analogy? Do some math yourself for once.)