Length (2): Centimetre
BasicUse centimetre to measure shorter objects, learn to use ruler.
After this lesson, you can:
- Centimetre is unit for short objects
- Use ruler align with 0 mark
- Ruler must be straight along object
Before this lesson you should know: Length (1): Comparing Length
1Summary 重點摘要
Use centimetre to measure shorter objects, learn to use ruler.
2Vocabulary cards Vocabulary
C
Centimetre厘米
Unit for shorter objects, symbol cm.
R
Ruler間尺
Tool for measuring length.
M
Measure量度
Use ruler to measure length.
P
Paper clip萬字夾
Small clip to hold paper.
E
Eraser膠擦
Thing to erase pencil writing.
P
Pencil鉛筆
Pen for writing.
1
1 cm1 厘米
One mark on ruler.
E
Estimate估計
Guess length roughly without ruler.
3Step by step Steps
- Step 1: Know centimetreThe centimetre is the unit for measuring shorter objects, written as cm. Small things like paper clips, erasers and pencils suit centimetres well. One centimetre is about as wide as a fingernail; feel it with a finger first to help remember how long it is.
- Step 2: Use rulerWhen measuring, line one end of the object up with the 0 mark of the ruler, then see which number the other end of the object reaches. Lining up with 0 is the key; with the wrong starting point the length read off will be inaccurate. Keep your eyes level with the scale, not looking sideways.
- Step 3: Read answerWhichever mark the far end of the object reaches is its length in centimetres. For example, if the end reaches 8, the object is 8 centimetres long, written 8 cm. Read the scale carefully, mark by mark. One mark misread makes the answer wrong by 1 cm.
- Step 4: EstimateEstimate before measuring: a paper clip is about 3 cm, an eraser about 5 cm, a pencil about 18 cm. With an estimate ready, a measured result of 30 cm would warn you that something went wrong, so the problem is found in time.
- Step 5: ComparePut the measured lengths together to compare: paper clip 3 cm, eraser 5 cm, pencil 18 cm. Since 3 < 5 < 18, the paper clip is shortest, the eraser a little longer, and the pencil longest. Comparing makes the results of measuring more useful.
- Step 6: NotesWhile measuring, keep the ruler tight against the object, placed square and straight, never slanted; put both the object and the ruler on a flat desk. Measure a second time after the first; if the two results match, you can be confident. These good habits keep measurements accurate and reliable.
4Diagrams Diagrams
5Worked example Example
Example: measure a pencil with a ruler. How many centimetres long is it? Then compare it with an eraser.
- Lay the pencil flat and line the back end with the ruler's 0 mark; the starting point must be exact.
- Look at the mark at the pencil tip: it points exactly at 18, read as 18 centimetres.
- So the pencil is 18 cm long; remember to write the unit.
- Then measure the eraser: lined up with 0, its end reaches 5, so 5 cm; 18 > 5, the pencil is 13 cm longer than the eraser.
- Answer: The pencil is 18 cm long. With the back end on the 0 mark and the tip reaching 18, the length is 18 cm; the eraser is 5 cm, and 18 - 5 = 13, so the pencil is 13 cm longer than the eraser.
6Common mistakes Mistakes
✗ ✗ Not align with 0 mark
✓ ✓ Must align with 0 mark to be accurate!
✗ ✗ Measure diagonally
✓ ✓ Ruler must be straight along object!
✗ ✗ Counting marks instead of reading the number
✓ ✓ Read the number where the end reaches!
✗ ✗ Starting at 1
✓ ✓ Align with the 0 mark, not 1!
7Quick check Quick check
Q1What unit for short objects?
Answer: Centimetre
Did you get it right?
Q2Align with what on ruler?
Answer: 0 mark
Did you get it right?
Q3Paper clip about how long?
Answer: 3 cm
Did you get it right?
Q4An eraser is about how many cm?
Answer: 5 cm
Did you get it right?
Q5How to place the ruler for an accurate measure?
Answer: Straight along the object, aligned with 0
Did you get it right?
8Recap Recap
- Centimetre is unit for short objects
- Use ruler align with 0 mark
- Ruler must be straight along object
- Paper clip about 3 cm, eraser about 5 cm
- Can estimate then measure