Chapter 13, Problem 1RQ

To determine

Choose the correct option from the given choices to find the time taken by the radio waves to reach a town 12000 km away.

Answer to Problem 1RQ

The correct option from the given choices is $\left(\text{d}\right)40\text{ms}$.

Explanation of Solution

Calculation:

Write the general expression to calculate the speed.

$\text{speed}=\frac{\text{distance}}{\text{time}}$        (1)

Rearrange the equation (1) as follows,

$\text{time}=\frac{\text{distance}}{\text{speed}}$

Substitute $3\times {10}^8\frac{\text{m}}{\text{s}}$ for speed and 12000 km for distance in above equation.

$\begin{array}{c}\text{time}=\frac{\text{12000}\text{km}}{3\times {10}^8\frac{\text{m}}{\text{s}}}\\ =\frac{\text{12000}\times {10}^3\text{m}}{3\times {10}^8\frac{\text{m}}{\text{s}}}\left\{\because 1\text{k}={10}^3\right\}\\ =0.04\times {10}^3\times {10}^{-3}\text{s}\\ =40\text{ms}\left\{\because 1\text{m}={10}^{-3}\right\}\end{array}$

The time taken by the radio wave is $40\text{ms}$. Therefore, option (d) is correct and option (a), (b), (c), and (e) are incorrect.

Conclusion:

Thus, the correct option from the given choices is $\left(\text{d}\right)40\text{ms}$.