Length (V): Millimetre

1Summary 重點摘要

1 cm = 10 mm. Pencil tip uses mm.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Part 1: What mm measuresMillimetres measure tiny, thin things; they are the smallest of the common length units. Example: a 10-dollar coin is about 24 mm across, a notebook about 5 mm thick, and an ant about 6 mm long. Centimetres are not accurate enough for these, but millimetres are just right. Why use a small unit for small things? The smaller the unit, the finer the result of measuring. Take care to choose the unit before estimating; an ant written as 0.6 cm is less clear than 6 mm.
2. Part 2: 1 cm = 10 mmLook at a ruler: between the marks 1 and 2 there are 10 small steps, and each small step is 1 mm, so 1 cm = 10 mm. Why exactly ten steps? Centimetres and millimetres are linked by exactly ten times, a rate different from the hundreds between m and cm. So 2 cm = 20 mm and 3 cm = 30 mm, and so on. Take care to tell the small steps from the big ones on the ruler; small steps are mm, big ones are cm, and the small steps must be counted one by one without skipping any.
3. Part 3: cm to mm, multiply by 10To change cm to mm, multiply by 10, because 1 cm holds 10 millimetres. Example: 5 cm = 5 × 10 = 50 mm. For the mixed form 3 cm 4 mm: first change 3 cm into 30 mm, then add the 4 mm, giving 30 + 4 = 34 mm. A bigger unit turning into a smaller one makes the number bigger. The usual error is forgetting the ones part of a mixed form, writing 3 cm 4 mm as 30 mm and losing the 4; only after joining both parts is the conversion complete.
4. Part 4: mm to cm, divide by 10To change mm to cm, divide by 10, the opposite direction from the last step. Example: 70 mm = 70 ÷ 10 = 7 cm. If it does not divide exactly, as with 46 mm, that is 4 cm with 6 mm left, written 4 cm 6 mm; both forms are correct. Take care that a smaller unit turning into a bigger one should shrink the number; if the number grows instead, multiply and divide have been swapped. Another usual error is dividing by 100, which is the rate between cm and m and has nothing to do with mm, so never borrow the wrong rate.
5. Part 5: Measuring mm with a rulerWhen measuring small objects, line one end up with the 0 mark; this is the first step of using a ruler. Example: to measure an eraser, if the other end passes 3 cm and 2 more small steps, read it as 3 cm 2 mm, which is 32 mm. Count the small steps carefully one by one, since each is 1 mm and being one step off means 1 mm wrong. Take care that the object lies flat against the ruler; slanted or held in the air it cannot be read truly, and the start must sit on 0, for starting elsewhere means reading the gap between the two marks instead.
6. Part 6: Adding mixed unitsBefore adding or subtracting mixed units, match them in mm first; this is the fixed routine for mixed units. Example: a pencil is 7 cm 5 mm long and sharpening takes off 1 cm 2 mm: change them to 75 mm and 12 mm, then 75 - 12 = 63 mm, which is 6 cm 3 mm. Why match first? Units that differ cannot be added or subtracted directly. Take care that forgetting to change back into mixed units at the end is also a common slip; before answering, read clearly which form the question asks for.

4Diagrams Diagrams

5Worked example Example

6Common mistakes Mistakes

7Quick check Quick check

8Recap Recap

• 1 cm = 10 mm
• mm measures tiny, thin things: coins, notebook thickness
• cm to mm × 10; mm to cm ÷ 10
• Mixed units: 3 cm 4 mm = 34 mm
• Match all units to mm before calculating