Question

Car $B$ overtakes another car $A$ at a relative speed of $40\, ms ^{-1}$. How fast will the image of car $B$ appear to move in the mirror of focal length $10 \,cm$ fitted in car $A$, when the car $B$ is $1.9 \,m$ away from the car $A$?

• A. $4 \,ms ^{-1}$
• B. $0.2 \,ms ^{-1}$
• C. $40 \,ms ^{-1}$
• D. $0.1 \,ms ^{-1}$

Answer 1

Answer: (D)

Mirror used is convex mirror (rear-view mirror)
$\therefore V _{ I / m }=- m ^{2} V _{ O / m }$
Given,
$ V _{ O / m }=40\, m / s $
$ m =\frac{ f }{ f - u }=\frac{10}{10+190}=\frac{10}{200} $
$\therefore V _{ I / m }=-\frac{1}{400} \times 40=-0.1 m / s$
$\therefore $ Car will appear to move with speed $0.1 \,m / s$.