Because people have asked me lots of questions about stuff in the past, I figured I’d post some of the relevant ones on this page. If your question has not been answered, please ask a grownup for help.
Question: What are units?
Answer: Units of measurement are the terms that we use to describe the size of something.
You probably know that units include things like seconds, kilograms, meters, and so forth. What’s important about units (and why we care about making sure we get them right) is that they give us some idea of the magnitude of the thing we’re looking at. For example, if I told you that I weighed 120, you’d either think that I was very skinny (if you thought I meant pounds) or very fat (if you thought I meant kilograms, which I do). Without units, you simply don’t know which is right.
Scientists usually try to keep units straight for this very reason. It may seem that we’re being very annoying with our units and our significant figures and such, but these are the things that ultimately make it easier to figure out what we’re talking about.
Question: Where did units originally come from?
Answer: Units were devised to satisfy a powerful need for standardization.
After all, if everybody measured things using different scales, nobody would ever figure out what was going on. Additionally, the earliest units were based on simple phenomena so they could be easily reproduced, which is why zero degrees Celsius is the freezing point of water and 100 degrees is its boiling point.
Question: Why are standard units of measurement important to scientists?
Answer: Without standard units of measurement, scientists would have a huge problem understanding what other scientists were saying.
If I were to refer to a meter as “the length of my leg” and another scientists were to refer to it as “the length of a Saint Bernard dog”, we’d have a lot of trouble when it came to do actual science. To avoid this problem, the SI system of units gives us a convenient and, more importantly, standard set of units that we can agree on.
Question: How does a conversion factor differ from a measurement?
Answer: A measurement is a bit of information that you collect, whereas a conversion factor is used to then change the units of this measurement.
For example, if you were to measure my height, you’d find that it is 180 centimeters. If you want to then change the units from centimeters to something else, you’d need a conversion factor to perform this calculation.
Question: Why should we use SI units?
Answer: Because that’s what everybody in the scientific community uses.
If you don’t use SI units, you simply won’t be able to conveniently be able to discuss your work with anybody else.
Question: What are the units of density?
Answer: Anything mass divided by anything volume (such as g/mL and kg/m3)