Question

Radio waves of speed $3\times {10}^8$ m/s are reflected off the moon and received back on earth. The time elapsed between the sending of the signal and receiving it back at the earth station is 2.5 s. What is the distance of the moon from the earth(in km)?

• A. $8.75\times {10}^5$
• B. $3.75\times {10}^4$
• C. $3.75\times {10}^5$
• D. $8.75\times {10}^4$

Answer 1

Answer: (C)

$3.75\times {10}^5$

During the trip the distance traveled by the signal is $2\times \text{distance between earth and moon}=2x$

We know $time=2.5s=\frac{distance}{speed}=2x/(3\times {10}^8)$

so $2x=7.5\times {10}^8$ so distance $x=3.75\times {10}^{8}meter=3.75\times {10}^{5}Km$