Question

the speedometer of a car moving eastward reads 50 km per hr. it passes another car which travels westward at 50 km per hr what is the relative velocity of car a with respect to car b.
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Answer 1

The mirror is moving at speed V2=70 km/h along with the truck.

So, The truck seen the can approaching towards it with velocity,

relative velocity of can wrt to truck =V1/2

=V1−V2=50−70

=−20 km/h

So for the mirroe Object velocity is =−20 km/h and so for a plane mirror, image velocity be =−V=20 km/h.

As the mirroe itself moving with velocity V2=70 km/h

The observer sees the velocity of image of can as : 

V′=V2−V1/2=90 km/h