Question

Image of an object approaching a convex mirror of radius of curvature $20 \, m \, $ along its optical axis is observed to move from $\frac{25}{3}m$ to $\frac{50}{7}m$ in $30 \, s$ . If the object is moving uniformly, then its speed is

• A. $ \, 3 \, km \, h^{- 1}$
• B. $ \, 4 \, km \, h^{- 1}$
• C. $ \, 5 \, km \, h^{- 1}$
• D. $ \, 6 \, km \, h^{- 1}$

Answer 1

Answer: (A)

Using mirror formula,
Or $\frac{1}{u_{1}}= \, \frac{3}{25} \, - \, \frac{1}{10}$
Or $u_{1} \, = \, 50 \, m$
And
$\therefore \frac{1}{u_{2}} \, = \, \frac{7}{50} \, - \, \frac{1}{10}$
Or $u_{2} \, = \, 25 \, m$
Speed of object $=\frac{u_{1} - \, u_{2}}{t i m e}$
$=\frac{25}{30 \, }m \, s^{- 1}$
$= \, 3 \, km \, h^{- 1}$