Measuring the World

Before a scientist can explain anything, they have to answer a much simpler question first: how much?

How long is it? How heavy? How hot? How much water is in the glass?

That is what measuring is — and it is the first skill in science, because everything else you will ever learn sits on top of it. A wrong measurement will make a perfectly correct formula give you a wrong answer.

By the end of this lesson you will be able to:

• say what measuring actually means, in your own words
• name the five things we measure most often, and the instrument for each
• read a scale without making the mistake almost everyone makes
• explain why a number on its own is never a finished answer

Take your time with this one. There is almost nothing to memorise here — only things to understand.

What measuring actually is

Suppose I ask how long your pencil is, and you have no ruler anywhere. You could still answer me. You might hold it against your hand and say, "about four fingers."

You have just measured something. You compared the pencil against a thing whose size you already knew — your finger — and you counted how many times it fitted.

That is all measuring ever is:

Measuring means comparing a thing against a known amount, and counting how many times that amount fits into it.

Now, the trouble with fingers. Yours are not the same size as mine. So your "four fingers" and my "four fingers" are two different lengths, and neither of us can check the other's work. A measurement nobody else can check is not much use in science.

So the world sat down and agreed on a set of fixed amounts that never change and belong to nobody. One centimetre is the same centimetre in Dhaka, in Chattogram, and in Tokyo. Those agreed fixed amounts are called units.