Question

An object is placed in front of a convex mirror at a distance of 60m from its pole. Object starts moving towards the mirror at 20 $\frac{m}{s}$. Find the average velocity of the image for a time interval of 2 sec if the focal length of the mirror is 20m.

• A. 20 m/s
• B. 10 m/s
• C. 30 m/s
• D. None of the above

Answer 1

The correct option is D

None of the above

$\frac{1}{v}$ + $\frac{1}{u}$ = $\frac{1}{f}$

$\frac{1}{v}$ - $\frac{1}{-60}$ = $\frac{-1}{20}$

$\frac{1}{v}$ - $\frac{1}{60}$ = $\frac{1}{20}$

v = 15 m

Now as object moves at 20 $\frac{m}{s}$ , in two seconds it reaches u = -20m

$\frac{1}{v}$ + $\frac{1}{-20}$ = $\frac{1}{20}$

$\frac{1}{u}$ = $\frac{1}{20}$ + $\frac{1}{20}$

u = 10 m
Now average speed of the image is given by = $\frac{15-10}{2}$
= 2.5 $\frac{m}{s}$