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Grade 3 (age 8-9)

grade 3 Length & Time

To measure a length, choose a unit that fits the size of the thing you are measuring. Short things like a paper clip or an eraser are measured in centimeters and millimeters, long things like a classroom in meters, and long distances in kilometers; 1 cm is 10 mm and 1 m is 100 cm. On a clock, the short hand points to the hour and the long hand to the minutes; 1 hour is 60 minutes and 1 minute is 60 seconds. Change the length on the ruler to see the cm and mm marks move together, and change the hour and minutes on the clock to read the time the hands show.

Units of Length

📏 Units of Length
①Split 1 cm into 10 equal parts and each part is 1 mm, so 1 cm = 10 mm
②Things you can hold, like a paper clip, an eraser or a pencil, are measured in cm and mm
③Long things like a classroom or a hallway are measured in meters: 1 m = 100 cm
④Long distances, like from school to home, are measured in kilometers: 1 km = 1000 m

Measuring with a Ruler

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💡 How to Use a Ruler
①Line up one end of the object with the 0 mark on the ruler
②Read the mark where the other end lands; in the picture, it is where the shaded strip stops
③The numbered marks are 1 cm apart, and each small space between them is 1 mm
④The slightly longer mark in the middle is the 5 mm point, a handy landmark when you count the small marks

Reading a Clock

3
30
⏰ How to Read a Clock
①Read the hour from the number the short hand points to; if it is between two numbers, the hour is the number it has just passed
②Each small mark the long hand passes is 1 minute, and each number it passes adds 5 minutes
③When the long hand points to 12 it is exactly on the hour, and when it points to 6 it is 30 minutes past
④One full turn of the long hand is 60 minutes, or 1 hour

Summary

Metric Length Units
1 m = 100 cm = 1000 mm
To change cm to mm, multiply by 10; to change m to cm, multiply by 100
Time Units
1 hour = 60 minutes, 1 minute = 60 seconds
Time units group by 60 to make the next bigger unit
One Day
1 day = 24 hours
The short hand goes around twice a day: 12 hours of a.m. and 12 hours of p.m.
🎯 Summary
①Length units get bigger in the order mm, cm, m, km
②1 cm = 10 mm, 1 m = 100 cm, 1 km = 1000 m
③To measure, line one end up with 0 and read the mark at the other end
④The short hand shows the hour and the long hand shows the minutes
⑤1 hour = 60 minutes, 1 minute = 60 seconds, 1 day = 24 hours

Examples and Unit Test

Example 1
How many millimeters are in 3 cm?
1
1 cm = 10 mm.
2
3 cm is 3 groups of 10 mm, so 3 × 10 = 30 mm.
▸ 30 mm
To change cm to mm, multiply by 10.
Example 2
How many meters and centimeters is 250 cm?
1
100 cm make 1 m, so see how many groups of 100 cm fit in 250 cm.
2
250 cm = 200 cm + 50 cm, and 200 cm is 2 m, so it is 2 m 50 cm.
▸ 2 m 50 cm
The number of 100 cm groups gives the meters, and what is left over gives the centimeters.
Unit Test
How many centimeters are in 1 m?
①10 cm
②100 cm
③1000 cm
④60 cm
▸ ② 100 cm
1
Cut 1 m into 1 cm pieces and you get 100 pieces.
2
So 1 m = 100 cm, and the answer is ②.
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