A car is moving on a straight road at a speed of 50 km/h and a truck is ahead of the car and having a large plane mirror fixed vertically on the back of truck. Truck is running at a speed of 70 km/h. For a stationary observer standing between car and truck. What is the speed of image of car in the mirror?

40 km/h

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70 km/h

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20 km/h

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90 km/h

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Solution

The correct option is D 90 km/h
The mirror is moving at speed $V_{2} = 70 k m / h$ along with the truck.
So, The truck seen the can approaching towards it with velocity,
relative velocity of can wrt to truck $= V_{1 / 2}$
$= V_{1} - V_{2} = 50 - 70$
$= - 20 k m / h$
So for the mirroe Object velocity is $= - 20 k m / h$ and so for a plane mirror, image velocity be $= - V = 20 k m / h$ .
As the mirroe itself moving with velocity $V_{2} = 70 k m / h$
The observer sees the velocity of image of can as :
$V^{'} = V_{2} - V_{1 / 2} = 90 k m / h$

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