A 1.2 m tall girl spots a balloon moving with the wind in a horizontal line at a height of 88.2 m from the ground. The height of 88.2 m from the ground. The angle of elevation of the balloon from the eyes of the girl at any instant is 60°. After some time, the angle of elevation reduces to 30° (see Fig. 9.13). Find the distance travelled by the balloon during the interval.
Let the initial position of the balloon be A and final position be B.
Height of balloon above the girl height = 88.2 m - 1.2 m = 87 m
Distance travelled by the balloon =
DE = CE - CD
In right ΔBEC,
tan 30° = BE/CE
⇒ 1/√3= 87/CE
⇒ CE = 87√3 m
also,
In right ΔADC,
tan 60° = AD/CD
⇒ √3= 87/CD
⇒ CD = 87/√3 m = 29√3 m
Distance travelled by the balloon = DE = CE - CD = (87√3 - 29√3) m = 29√3(3 - 1) m = 58√3 m.
