Length (III): Metre

1Summary 重點摘要

1 m = 100 cm. Desk is about 1 m.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Step 1: When to use metresWhen measuring longer objects, centimetres give very big numbers, which is not handy. Measuring a door in centimetres means writing 200 centimetres; a blackboard comes to more than 300 centimetres, awkward to read and write. Use the larger unit, the metre, with symbol m: a door is about 2 m tall and a blackboard about 3 m long, far simpler numbers. A common mistake is to use centimetres for everything, producing hundreds of centimetres for long objects. Before measuring, estimate the length first; for anything over 1 metre, the metre is the more suitable unit, and the answer is easier for others to understand.
2. Step 2: The conversionRemember the link between metres and centimetres firmly: 1 m = 100 cm, so 2 m is two lots of 100 cm, which is 200 cm, and 5 m is 500 cm. With this conversion in mind, we can change between metres and centimetres. The other way, 200 cm holds two lots of 100, so it is 2 m. A common mistake is to remember it as 1 m = 10 cm, mixing it with another unit, which makes every answer ten times off. One way to remember: the metre is the big brother of the centimetre, one metre equalling one hundred centimetres. After converting, divide back by 100 once to check the direction is right.
3. Step 3: Estimate around youEstimate first, then measure; it is a good habit: a door is about 2 m tall, a desk about 1 m long, a book about 30 cm tall, and one step about half a metre. Keep estimating and checking, and your eyes grow sharper. The smaller the gap between your estimate and the real measure, the more you have improved. A common mistake is to estimate everything in whole metres, ignoring the part below 1 metre. Use a known length nearby as a reference: the desk top is about 1 m, so ask whether the object is a few times that or a fraction of it, and the estimate comes close to the truth.
4. Step 4: Measure with a tapeUse a tape measure for long objects like a blackboard: line the start of the tape with one end of the board, pull it straight, then read the number at the other end. If the tape runs from 0 to 3 m, the board is 3 m long. Keep the tape straight while measuring, never bent or sagging, or the result will come out shorter than the truth. A common mistake is not starting from the 0 mark but from the tip of the case or some other number, or reading the scale at a slant. Find where 0 is before starting, read with the eye directly over the mark, and align both ends, and the result will be reliable.
5. Step 5: Metres to centimetresTo change metres into centimetres, multiply: 3 m = 3 × 100 = 300 cm. Every metre holds 100 cm, so count how many lots of 100 there are; thinking this way avoids errors. The other way round, to change centimetres into metres, divide: 450 cm = 450 ÷ 100 = 4 m 50 cm. A common mistake is to reverse the direction, working out 3 m as 3 ÷ 100, or to be stuck when the division is not exact; 450 cm is really 4 m with 50 cm left over. After converting, check with common sense: metres to centimetres should make the number bigger, centimetres to metres should make it smaller.
6. Step 6: Pick the right unitBefore measuring, think how long the object is, then choose the unit: a pencil is short, so use centimetres, 18 cm; a playground is long, so use metres, 80 m; a textbook is about 26 cm, a classroom door about 2 m. With the right unit the numbers stay short and easy to read, and others understand easily. A common mistake is picking a unit too large or too small, writing a pencil as 0.18 m or a playground as 8000 cm, both awkward; another is leaving out the unit and writing a bare number, so no one can tell 18 centimetres from 18 metres. Always write the unit together with the answer.

4Diagrams Diagrams

5Worked example Example

6Common mistakes Mistakes

7Quick check Quick check

8Recap Recap

• 1 m = 100 cm.
• Metres for long things, centimetres for short.
• A door is about 2 m; a desk about 1 m.
• Convert before comparing.
• m to cm: multiply by 100.