SI units
The standard units scientists share worldwide: metre (m) for length, kilogram (kg) for mass and second (s) for time.
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Analogy
Like everyone agreeing that a cup of tea means the same amount, so recipes work anywhere.
Example
2.5 km = 2.5 x 1,000 = 2,500 m450 g = 450 / 1,000 = 0.45 kg90 min = 90 x 60 = 5,400 s
In practice
Always convert to SI units before using a formula, or the answer comes out wrong.
Practice
Flashcards
- What are the SI units for length, mass and time?Metre (m), kilogram (kg) and second (s).
- Convert 36 km/h to m/s.36 / 3.6 = 10 m/s.
Questions people ask
Why do we use SI units in physics?
Physics formulas only work when every quantity uses matching units. SI units are the agreed international set, so a result in Japan means the same as one in Brazil. Mixing, say, grams with kilograms inside one formula gives an answer that is wrong by a factor of a thousand.
What is the difference between a base unit and a derived unit?
A base unit stands alone, such as the metre, kilogram or second. A derived unit is built by combining base units. Speed is measured in metres per second (m/s), and force in newtons (N), where one newton equals one kilogram metre per second squared.