Physics can be difficult to learn, but this blog aims to help you get into physics by connecting your GCSE physics lessons with things you see in the world around you.

## Wednesday, 9 November 2011

### To Be Precise

Measuring things is something we come into contact with every day – that bag of apples in the supermarket says it weighs 1.2kg, we give our height in centimetres if we’re young, or feet and inches if we’re less young, and we’ll describe things in terms of London bus units (or at least the news people do). Previously, we’ve looked at our basic units of measure and how they are set in stone so that everybody around the world knows that their 1kg = 1kg somewhere else and so on.

Whenever something is measured, the results are known as data (a plural word, which can get a bit confusing). And these data falls into two categories. If something has a very definite number attached to it, like shoe size, or the number of people on a bus, then this is called discrete data. It means that you can be very sure on the accuracy of your data, because you can’t really have half a person on the bus. Ok, I hear you say, but you can have half shoe sizes. But you can’t have a shoe size of 6.59 or something like that. These are distinct pieces of data.

The other type of data is called continuous data, and this is the more common form of data. Time, angles, distance, weight and more all fall into this category. And the instruments you use to measure your data, and how accurately you can measure with them will have an effect on your data. Let’s take a simple example. We want to measure our hand span – and there is a choice of three items. One, is a ruler that measures in inches. The second is a ruler that measures in centimetres, and the third is a ruler that measures in millimetres.

The inch rule has the largest gaps between the markings, so perhaps we can measure to within half an inch of the true size. That means we think the real measurement is, for example, 7.5ins ± 0.25ins. Similarly, with our centimetre ruler, we can see to the nearest 0.5cm, so we know that the real hand span is 20 ± 0.25cm. This is slightly more accurate as a centimetre is smaller than an inch. And using our millimetre ruler we can see to the closest millimetre, so we know that the distance we’re measuring in 205 ± 0.5mm. That’s a lot more accurate still.

There are other, better ways to measure even more accurately with a whole range of specially designed tools which you will learn about during your experiments. But always bear in mind that you are limited by your equipment!