Question 1

Why are some unit conversions exact and others approximate?

Accepted Answer

Conversions defined by international agreement are exact — for instance, 1 inch ≡ 25.4 mm and 1 pound ≡ 0.45359237 kg. Others, like nautical mile to metre (1852 m, exact by definition) appear approximate but are also defined values. Truly approximate conversions arise only when the defining relationship involves a measured physical constant.

Question 2

How do I convert compound units like km/h to m/s?

Accepted Answer

Convert each component separately: 1 km = 1000 m and 1 h = 3600 s, so divide by 3.6. For example, 90 km/h = 90 / 3.6 = 25 m/s. The same principle applies to any compound unit — handle numerator and denominator units independently.