I just want to clear this up a little bit for people who aren't familiar with electronics and find this thread later. This is not the case for all electric conventions, but more of a happenstance that works in the case of motors since they operate regardless of which direction current is flowing. For example, it is a convention that the longer leg on an LED is positive, and the shorter negative. Sure, you could ignore this and shorten the long-leg to be shorter than the short leg (for whatever reason), but you can't change which side is positive and negative like you can on a motor. If you plug that thing in backwards, you'll likely be buying a new LED unless you're quick to remove it. A lot of conventions are there for a reason, and sometimes just to make your life easier. Unfortunately, not everyone follows the same conventions, so sometimes you run into the case of something like motors where plugging it it one way may give one result, while doing the same thing on another product gives the opposite.