It is a natural human tendency to name things; we all do it when we come across something new to us. One of the most important aspects of science is the art of giving names to newly discovered objects or concepts. As well as fulfilling our need to mark intellectual territory, naming a complicated concept lets us talk about it in an efficient manner. Can you imagine taking a calculus course which didn't use the words "integral" or "derivative"? In the interest of saving time, the first lessons in any subject must impart this kind of basic vocabulary.

In most scientific subjects, new students understand this instinctively because they are learning about things which are truly new to them. In introductory biology, you learn the meaning the of the words "cell", "mitochondria", and "nucleus" without batting an eye, while chemistry students observe the novel processes of "sublimation" and "condensation" and absorb the meanings of the new words with relative ease. Physics students start at a disadvantage in that the vocabulary of physics is made up of everyday words which must take on new meanings. The boundary between common language and scientific jargon becomes harder to discern.

Most common words have a large number of possible meanings. Take the word "force", for example; how many meanings can you think of for this word? How many meanings are there in the dictionary? Most (but not all) of them share the sense of causing something to happen against resistance, and this strong connotation colors every use of the word. Unless you are very careful, you will interpret the word with its connotation rather than its scientific definition. Other words, such as "work", have scientific definitions completely different from their colloquial ones.

You may be scoffing at the idea that anyone could confuse the scientific meaning of "force" with the various common definitions, but I can tell you with confidence that this is the most common mistake that beginning students make. When you hear the word "force" in physics lecture for the first time, the natural tendency of most people is to substitute a familiar meaning for the word, not to pay close attention and learn a new definition. This is especially easy to do because the scientific meaning and the colloquial meaning are very close; whatever the lecturer is saying still seems to make sense. The same thing is true of any scientific term which enjoys common usage in other contexts.

If you practice a science long enough, you eventually learn to be very cautious of this kind of confusion. When communicating in a scientific context, I have learned to take each word as having only one possible meaning, regardless of how it is being used. As a beginning physics student, you need to realize that every piece of vocabulary has a unique meaning, one which is precisely defined in the context of the subject. The fuzzy concepts of force which surround the word's English usage must be set aside, to be replaced by the mathematical formulation of Newton. You must see work as the transfer of energy from one system to another rather than an activity which tires you out. When you give something a charge, you might be putting more particles on it, or you might be taking them away.

What I've asked you to do is made all the more difficult by the fact that the physical intuition built up over your lifetime is mostly wrong. People have to train themselves to observe the world carefully; they do not do it naturally. For centuries the world thought that thrown objects moved in circular paths, even though anyone could have easily tested this. You are starting out with many faulty assumptions about how the world works, and these ideas are intimately linked to the language that you use to describe them. This is why it's so important to disassociate the common meanings of words from their scientific ones.

Here's a concrete, if simple, example of the problem. Let's consider the hypothesis that the acceleration of a tennis ball bouncing up into the air is the same as when it is falling. There are two ways to read this sentence. The colloquial definition of acceleration is a speeding up, as opposed to deceleration, a slowing down. If I interpret our hypothesis with this understanding, it is clearly nonsense; the ball is slowing down on its way up and speeding up on its way down. This is obvious to anyone, and many students reject the hypothesis on this basis even after doing the experiment. I have seen this misconception appear countless times on lab reports.

If we use the scientific definition of acceleration, on the other hand, the hypothesis becomes quite plausible. Recall that the scientific definition of acceleration is just the second derivative of position, or the first derivative of velocity. Using this mathematical definition, it is clear that the slope of a negative (downward) velocity of increasing magnitude could be the same as the slope of a positive (upward) velocity of decreasing magnitude. Your measurements of the changing speed of the ball will tell whether the accelerations are actually the same, within the experimental uncertainties.

Most of my students have fallen prey to this kind of error in reasoning at one time or another. It is very easy to do in physics, more so than in other scientific fields, because the words we use are so firmly ingrained in their colloquial meanings. You really need to create a separate space in your mind to contain the scientific definitions, and learn to draw meaning from that space alone when working in a scientific context. If you learn to use the scientific vocabulary with precision, however, I guarantee that the whole subject will become much clearer and more sensible.

Whether you believe it's useful or not, of course, is up to you. But at least you will understand what your teachers are telling you.

This essay can still be found on the web at the University of Michigan Introductory Physics Labs website.