Physics: Astrophysics: Units

Astrophysics uses the same seven SI units as forces and motion: kg, m, m/s, m/s2, N, s and N/kg. Nothing new needs memorising here - the specification deliberately reuses the mechanics units so that an orbit calculation and a car acceleration calculation follow exactly the same rules. The difference is scale. Distances are measured in millions of kilometres, masses in multiples of Earth's mass, and orbital periods in days or years rather than seconds. But every calculation still needs SI units before you substitute, and the size of the numbers involved is exactly why standard form becomes essential in this section in a way it rarely is in forces and motion questions closer to everyday scales.