Units (Astrophysics)

2 marks · Normal

Explain why the unit of speed in astrophysics is usually written as m/s rather than km/h. Give one reason.

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The SI unit for speed is m/s because it is based on the fundamental units of metre (m) for length and second (s) for time. This makes calculations and conversions more consistent in science, including astrophysics.

Also accept: m/s is the SI unit, so it is used universally in science for consistency.

Common mistakes: Saying 'easier to write' without linking to SI units.

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Marking points

B1 · 1 mark · States that m/s is the SI unit (not km/h).

B2 · 1 mark · Links m/s to the fundamental SI units metre (m) and second (s), or mentions scientific consistency/calculations.

Worked solution

Physics and astrophysics use the SI system. The SI unit for speed is the metre per second (m/s), derived from the base units metre and second. Using m/s keeps calculations consistent and makes conversions easier.