Image of an object approaching a convex mirror of radius of curvature 20m slong its optical axis is observed to move from `(25)/(3)`m to `(50)/(7)`m in 30 seconds. What is the speed of the object in km per hour?

Answer

3

Explanation

`(1)/(v) + (1)/(u) = (1)/(f) implies f = (R)/(2) = -10m`
`1^(st)` condition `(-3)/(25) + (1)/(u_(1)) = (-1)/(10) implies u_(1) = 50 m`
`2^(nd)` condition `(-7)/(50) + (1)/(u_(2)) = -(1)/(10) implies u_(2) = 25 `m
Velocity = `(triangleu)/(triangle t) = (25)/(30) xx (18)/(5) = 3km//h`