question_answer

An aeroplane flying horizontally 1 km above the ground is observed at an elevation of 60o and after 10 seconds the elevation is observed to be\[{{30}^{o}}\]. The uniform speed of the aeroplane in \[km/h\] is [IIT 1965]

A) 240

B) \[240\sqrt{3}\]

C) \[60\sqrt{3}\]

D) None of these

Correct Answer: B

Solution :

\[d=H\cot {{30}^{o}}-H\cot {{60}^{o}};\] Time taken = 10 second Speed =\[\frac{\cot {{30}^{o}}-\cot {{60}^{o}}}{10}\times 60\times 60=240\sqrt{3}\].