Length (IV): Kilometre

1Summary 重點摘要

1 km = 1000 m. Roads use km.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Part 1: Use km for long distancesUse kilometres for long distances; this is the basic rule for choosing units: the longer the distance, the bigger the unit. Example: the trip from home to school may be 2 km, and two stations are about 1 to 2 km apart. Written in metres, such distances give very big numbers, since 2 km is 2000 m, awkward to write and to read, so people use kilometres. Take care to find a familiar reference, such as the length of a playground, before estimating, instead of guessing from nothing.
2. Part 2: 1 km = 1000 mKilometres and metres are linked: 1 km = 1000 m, the biggest rate among the common length conversions. Why remember a track? A standard running track lap is 400 m: two laps are 800 m, and two and a half laps are 1000 m, exactly 1 km, so the body's experience anchors the abstract number. Remember this link and converting becomes easy. Take care that 1 km = 1000 m goes by thousands, unlike the hundreds between m and cm; keep the two rates apart in your memory.
3. Part 3: km to m, multiply by 1000To change km to m, multiply by 1000, because 1 km holds 1000 metres. For example, 3 km = 3 × 1000 = 3000 m, and 5 km = 5 × 1000 = 5000 m. Going from a bigger unit to a smaller unit makes the number bigger; the smaller the unit, the bigger the number, a rule shared by every conversion. The usual error is multiplying by 100, writing 3 km as 300 m, which shrinks the number instead; between km and m stand three zeros, and the count of zeros must match the rate, so km to m always multiplies by 1000.
4. Part 4: m to km, divide by 1000To change m to km, divide by 1000, the opposite direction from the last step. For example, 4000 m = 4000 ÷ 1000 = 4 km. If it does not divide exactly, as with 2500 m, write 2 km with 500 m left over, that is 2 km 500 m, both units together. Take care that a smaller unit turning into a bigger one should shrink the number; if the number grows after converting, multiply and divide have been swapped, so check at once. Also, the 500 m in 2 km 500 m is not a decimal, and 2.500 km must never be written.
5. Part 5: Match the units before addingBefore adding or subtracting lengths, make the units match, the fixed routine of length working. Why match first? Numbers with different units cannot be combined directly, just as 2 tens and 3 ones cannot be called 23 tens. One stretch is 2 km and another is 500 m; change 2 km into 2000 m first, then 2000 + 500 = 2500 m, which is 2 km 500 m. Take care to change the bigger unit into the smaller one when matching, which avoids decimals and keeps the working steady.
6. Part 6: Everyday useKilometres appear everywhere in daily life, on road signs, maps and traffic news. A road sign saying the library is 3 km away means 3000 m more to go. If a bus stop comes every 500 m, the whole trip passes 3000 ÷ 500 = 6 stop gaps, which shows how kilometres serve daily life. Take care to think of 1000 m on seeing km; when estimating travel time, convert the distance into a familiar unit first, for example 3 km is about seven and a half laps of the playground track, and then the estimate has something to stand on.

4Diagrams Diagrams

5Worked example Example

6Common mistakes Mistakes

7Quick check Quick check

8Recap Recap

• 1 km = 1000 m
• Long distances (roads, between places) are measured in km
• km to m: multiply by 1000, e.g. 3 km = 3000 m
• m to km: divide by 1000, e.g. 4000 m = 4 km
• Always match the units before adding or subtracting