Q1
The SI unit of length is the:
Ametre
Bkilogram
Csecond
Dhandspan
CBSEClass 6 (CBSE)ScienceCh 5 · Topic 1
Need for Measurement and Standard Units
Understanding why standard units are necessary for accurate measurement.
Core concept
If two people measure the same table using their own handspans, they will likely get different results, since everyone's hand size is different — this shows why non-standard units are unreliable. The metre was originally defined based on the Earth's size, but is now defined very precisely using the speed of light, making it consistent everywhere on Earth (and beyond) regardless of who measures it or where. Other standard units include the kilogram for mass and the second for time.
Exam highlight
Key diagram
- Show two people measuring the same table with handspans getting different results, versus a standard scale.
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Quick notes
• Standard units allow consistent, reliable measurement understood by everyone.
• Non-standard units like handspans vary between people and are unreliable.
• The metre (m) is the SI unit of length.
• Other SI base units include the kilogram (mass) and the second (time).