Question

What is the speed of the object (in $kmh⁻1$ ) if an 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?

• A. 3
• B. 4
• C. 5
• D. 6

Answer 1

Answer: (A)

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