This is one of those things I thought I understood clearly, but it appears I didn’t quite get it. I was working on the book, and needed to describe the difference between power and energy, as well as tie it in to torque and acceleration. With the help of John Fiorenza once again, I think I’ve got it. Here’s the bit from the book:
“Power,” a word in general English use since the 1500s, is loosely described as the “ability to act or produce an effect”. It’s a time-dependent concept – for example, horsepower is the result of torque (or Force) and RPM (revolutions per minute: one revolution of a shaft achieved over a unit of time – a minute). “Energy,” for the purpose of our discussions of battery packs and range and all that, is different from power. Energy is, equally loosely, the capacity for power. This all gets to Newton’s First Law: Force = Mass x Acceleration. You take a motorcycle, you accelerate it, that’s Force. You accelerate it over a period of time, and that’s power. In order to apply power, you need energy. Force, Acceleration and Mass are physical properties, thanks to Newton. Power and Energy are concepts we’ve created to describe what you can do with them.
I think what gets confusing is that a lot of the charts and curves you see plot torque along with horsepower, like the one above.
Here’s kind of a white paper on the whole thing, from MIT: Understanding Motor Characteristics , if you’re feeling the need to geek out.