Learning Objectives
- Measure lengths using standard units (metre, centimetre, kilometre)
- Convert between metres, centimetres, and kilometres
- Estimate and measure lengths of objects
- Solve word problems involving length
Key Concepts
Units of Length
We use different units to measure different lengths. Centimetre (cm) is used for small things like a pencil or notebook. Metre (m) is used for medium things like a room or a rope. Kilometre (km) is used for long distances like the distance between two cities. Always choose the right unit for what you are measuring!
Converting Between Units
1 metre = 100 centimetres. So 3 metres = 300 centimetres. To convert metres to centimetres, multiply by 100. To convert centimetres to metres, divide by 100. For example, 250 cm = 2 m 50 cm. Also, 1 kilometre = 1000 metres. So 5 km = 5000 metres.
Measuring with a Ruler
Place the zero mark of the ruler exactly at one end of the object. Read the number at the other end. That is the length. If the end falls between two marks, estimate to the nearest half centimetre. For example, if it falls between 7 cm and 8 cm, closer to 7, the length is about 7 cm.
Word Problems on Length
If a ribbon is 2 m 50 cm long and you cut off 75 cm, what is left? First convert everything to cm: 2 m 50 cm = 250 cm. Then subtract: 250 - 75 = 175 cm = 1 m 75 cm. Always make sure both measurements are in the same unit before adding or subtracting.
Important Terms
- Centimetre (cm): A small unit of length; about the width of your little finger
- Metre (m): A medium unit of length; 1 m = 100 cm
- Kilometre (km): A large unit of length; 1 km = 1000 m
- Conversion: Changing from one unit to another
- Estimate: Making a smart guess about the length before measuring
Quick Revision
- 1 km = 1000 m
- 1 m = 100 cm
- Use cm for small things, m for medium, km for long distances
- Convert to the same unit before adding or subtracting
- Always start measuring from the zero mark of the ruler
- Estimate first, then measure to check