A car is fitted with a convex side-view mirror of focal length 20 cm . A second car 2.8 m behind the first car is overtaking the first car at relative speed of 15 m / s . The speed of the image of the

Question

A car is fitted with a convex side-view mirror of focal length 20 cm . A second car 2.8 m behind the first car is overtaking the first car at relative speed of 15 m / s . The speed of the image of the second car as seen in the mirror of the first one is :

TestHub · Accepted Answer

v 1 ​ + u 1 ​ = f 1 ​ − v 2 1 ​ d t d v ​ − u 2 1 ​ d t d u ​ = 0 d t d v ​ = − u 2 v 2 ​ ( d t d u ​ ) f = 20 cm u 1 ​ + − 280 1 ​ = 20 1 ​ ⇒ v = 15 280 ​ cm v 1 ​ = − ( 15 × 280 280 ​ ) 2 × 15 = 15 1 ​ m / s

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