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What is 72,000 km/hour in Système Relativistique (SR) units?

I am getting 1/14490 or 1/14490th of the speed of light.

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1 Answer

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The speed of light is close to $3\times 10^5$ km per second that is to say $3\times 10^5\times3600=1.08 \times 10^9$ km per hour.

So $72000$ km per hour is $\frac{72000}{1.08 \times 10^9}=\frac{1}{15000}$ For sure, if you use $c=299 792 458$ meters per second as given here , then $$\frac{72000}{299 792 458 \times 10^{-3} \times 3600}=\frac{10000}{149896229}=\frac{1}{14989.6229}\approx \frac{1}{14990}$$ but not $\frac{1}{14490}$ which is probably a small typo.

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