1/ How to Change Units It is very important that you are aware that different systems of units exist. Furthermore, you must be able to convert between units. Being able to change between units (for example, converting from millimetres to metres) is a useful skill in Science. The following conversion diagrams will help you change from one unit to another. If you want to change millimetre to metre, you divide by 1000 (follow the arrow from mm to m); or if you want to change kilometre to millimetre, you multiply by 1000×1000. The same method can be used to change millilitre to litre or kilolitre. Use figure 2.2 to change volumes: Worked Example 1: Conversion 1 Question: Express 3 800 mm in metres. Answer Step 1 : Find the two units on the conversion diagram. Use Figure 2.1 . Millimetre is on the left and metre in the middle. Step 2 : Decide whether you are moving to the left or to the right. You need to go from mm to m, so you are moving from left to right. Step 3 : Read from the diagram what you must do and find the answer. 3 800 mm ÷ 1000 = 3,8 m Worked Example 2: Conversion 2 Question: Convert 4,56 kg to g. Answer Step 1 : Find the two units on the conversion diagram. Use Figure 2.1. Kilogram is the same as kilometre and gram the same as metre. Step 2 : Decide whether you are moving to the left or to the right. You need to go from kg to g, so it is from right to left. Step 3 : Read from the diagram what you must do and find the answer. 4,56 kg × 1000 = 4560 g 2/ Two other useful conversions Very often in Science you need to convert speed and temperature. The following two rules will help you do this: Converting speed When converting km·h$^{−1}$ to m·s$^{−1}$ you divide by 3,6. For example 72 km·h$^{−1}$÷ 3,6 = 20 m·s$^{−1}$. When converting m·s$^{−1}$ to km·h$^{−1}$, you multiply by 3,6. For example 30 m·s$^{−1}$×3,6 = 108km·h$^{−1}$. Converting temperature Converting between the kelvin and celsius temperature scales is easy. To convert from celsius to kelvin add 273. To convert from kelvin to celsius subtract 273. Representing the kelvin temperature by T$_{K}$ and the celsius temperature by To$_{C}$, T$_{K}$ = To$_{C}$+ 2733/ A sanity test A sanity test is a method of checking whether an answer makes sense. All we have to do is to take a careful look at our answer and ask the question Does the answer make sense? Imagine you were calculating the number of people in a classroom. If the answer you got was 1 000 000 people you would know it was wrong — it is not possible to have that many people in a classroom. That is all a sanity test is — is your answer insane or not? It is useful to have an idea of some numbers before we start. For example, let us consider masses. An average person has a mass around 70 kg, while the heaviest person in medical history had a mass of 635 kg. If you ever have to calculate a person’s mass and you get 7 000 kg, this should fail your sanity check — your answer is insane and you must have made a mistake somewhere. In the same way an answer of 0.01 kg should fail your sanity test. The only problem with a sanity check is that you must know what typical values for things are. For example, finding the number of learners in a classroom you need to know that there are usually 20–50 people in a classroom. If you get and answer of 2500, you should realise that it is wrong. Xem thêm: Grade 10 - Physics - Units High School Students Studying the Sciences Physics