01Why this matters
A number without a unit is incomplete as a measurement. Shared units allow experiments and calculations to be compared.
02See it
03Understand it
The International System of Units, or SI, provides a shared language for physical measurements.
The SI base units include metre for length, kilogram for mass, second for time and ampere for electric current. Derived units combine base units: speed uses metres per second, and force uses newtons, where 1 N = 1 kg·m/s². Prefixes change scale but not the physical quantity.
04Guided example
A cart travels 12 m in 3 s. Express its average speed in SI units.
- Identify the measured quantities: distance in metres and time in seconds.
- Divide distance by time: 12/3 = 4.
- Carry the units through the division: average speed = 4 m/s.
05Try it
06Practise it
07Use it in the real world
A sensor reading of 250 milliseconds means 0.250 seconds. Convert it before combining it with a speed measured in metres per second.
Go a little deeper
Dimensional checks can expose mistakes. Distance divided by time has speed dimensions; distance multiplied by time does not. A dimensionally correct formula still needs physical justification.
08Quick check
09Summary
The International System of Units, or SI, provides a shared language for physical measurements. Carry the units through the division: average speed = 4 m/s.
Before moving on, explain the worked example in your own words. Use the hint on any question you want to revisit.
10Key idea
1 N = 1 kg·m/s²
N: newton; kg: kilogram; m: metre; s: second.
1 m/s = 3.6 km/h
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