Question

The angle of elevation of a jet plane from a point A on the ground is 60o .After a flight of 30 seconds , the angle of elevation changes to 30o .If the jet is flying at a constant height of 3600root3, find the speed of the jet plane.

Answer 1

Let D and E be the initial and final positions of the plane respectively


Consider the ΔABD

tan60=BD/AB

√3=1500√3/AB

AB=1500

Consider the ΔACE

tan30=CE/AC

1√3=1500√3/AC

AC=4500

BC = AC - AB
= 4500 - 1500
= 3000 m
DE = BC = 3000 m

So the plane travels a distance of 3000m in 15 seconds.
Therefore, speed of the plane =3000/15
= 200 m/s