World Without Measurements
8 Minutes
Measurements for kids: learn what physical quantities are, fundamental versus derived units, and why SI units exist. A science lesson in English.
- How standard units compare known and unknown properties
- Fundamental quantities and derived quantities like density
- Oliver Smoot's bridge measurement and standard SI units
What this lesson teaches
Measurement is the process of comparing an unknown property of an object or an action with a known property and expressing it in a meaningful way.
Fundamental and derived quantities
- Anything that can be measured about an object or an action is called a physical quantity.
- Fundamental quantities are basic measurements like length in meters, time in seconds, and mass in kilograms.
- Derived quantities are calculated using more than one fundamental quantity, such as area, volume, and density.
- Density is found by dividing mass by volume, while speed and acceleration are derived using length and time.
The Smoot measurement
In America, a person named Oliver Smoot used his own height as a base unit called one smoot. He laid down repeatedly across a bridge to find its length was 364.4 smoots. He also measured his house to be 25 smoots long and 18 smoots wide, calculating an area of 450 square smoots. While someone can name a unit after themselves, having everyone use different units creates confusion.
Why standard units are needed
When people use different everyday measures, such as a forearm length (muzam) at a flower shop, kitchen spoons, or personal body lengths, results are not standard. To compare quantities accurately, people need an agreed reference point. For mass, scientists made a standard cylinder of platinum and iridium in France as one kilogram, which was copied so markets worldwide have uniform weights.
The International System of Units
In 1960, scientists from around the world gathered to create the International System of Units, known as SI units. They established seven fundamental units: the meter for length, the kilogram for mass, the second for time, the Kelvin for temperature, the candela for luminous intensity, the mole for amount of substance, and a unit for electric current. Units like the meter and second are designed using the travel of light, ensuring that everyone across the globe uses the exact same measurements.