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

`R=20 cm, f=10 m`
For mirror,
`(1)/(v)+(1)/(u)=(1)/(f)`
`(1)/(25//3)+(1)/(u_(1))=(1)/(10)`
`rArr u_(1)=-50 cm`
`& (1)/(50//7)+(1)/(u_(2))=(1)/(10)`
`rArr (1)/(u_(2))=(1)/(25) rArr u_(2)=-25 cm`
So, speed `=|(Deltau)/(Deltat)|=(35)/(30) m//sec.=(5)/(6)m//sec` .
`&` in `km//hr=(5)/(6)xx(18)/(5)=3 km//hr` .