Question

A girl of height 90 cm is walking away from the base of a lamp-post at a speed of 1.2 m/s. If the lamp is 3.6 m above the ground, find the length of her shadow after 4 seconds.

Answer 1

Let AB denote the lamp-post and CD the girl after walking for 4 seconds away from the lamp-post (see Fig. 6.32). From the figure, you can see that DE is the shadow of the girl. Let DE be x metres.

Now, BD = 1.2 m × 4 = 4.8 m.

Note that in ΔABE and ΔCDE,

∠B =∠D (Each is of 90° because lamp-post, as well as the girl, are standing vertical to the ground)

So, the shadow of the girl after walking for 4 seconds is 1.6 m long.