You Know What They Say When You Assume

Posted by Alfred Ban Humboldt.

I was in class today, and we were studying conservation of energy formulas. We started with something pretty broad, but went on to narrow it down to a single equation. When we started doing examples, I noticed how many assumptions we were making. I found a distinct moment during one of the examples where I would have given up because we weren't given enough information. But then he said something interesting, "There is a velocity here, but among friends we can just call it zero."

He went on, "I know it should be nonzero, but since the area we're using is so big, and since we're all friends here, the velocity is so small compared to the second velocity that we're going to say it's zero."

Among friends? Among friends, why not just make up an answer? It would save us a lot of work. It didn't end (or begin) there. In the next example he said, "We're not given a value for the head loss of the system, so we'll call it zero."

No, we're not given the head loss, so we can't complete the problem. There's an old joke among physicists: you're given a problem starting with, "Assuming a horse is a perfect sphere..."