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An astronaut on the moon receives a radio signal from Earth that takes 1.28 s to arrive. Using c = 3.00 x 10^8 m/s, what is the distance from Earth to the moon?
A2.34 x 10^5 km
B3.84 x 10^5 km
C3.84 x 10^8 km
D4.27 x 10^8 km
Answer & Solution
Correct answer: B. 3.84 x 10^5 km
1. Distance equals speed multiplied by time, d = v multiplied by t.
2. Substitute v = 3.00 x 10^8 m/s and t = 1.28 s.
3. Multiplying gives d = 3.84 x 10^8 m, which is 3.84 x 10^5 km.
4. 3.84 x 10^8 km shifts the decimal point three places too far, and 2.34 x 10^5 km does not match this multiplication.
5. 4.27 x 10^8 km does not come from this calculation at all.
6. So 3.84 x 10^5 km is correct.
_Source: OpenStax Physics (CC BY 4.0), Ch 10 "Special Relativity", section 10.1 Postulates of Special Relativity_
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