Two cars are moving in the same direction with the same speed 30 km/hr. They are separated by a distance of 5 km, the speed of a car moving in the opposite direction if it meets these two cars at an interval of 4 minutes, will be

(1) 40 km/hr

(2) 45 km/hr

(3) 30 km/hr

(4) 15 km/hr

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Relative motion

(2) The two car (say A and B) are moving with same velocity, the relative velocity of one (say B) with respect to the other A,​ vBA=vBvA=vv=0

So the relative separation between them (= 5 km) always remains the same.

Now if the velocity of car (say C) moving in opposite direction to A and B, is vC relative to ground then the velocity of car C relative to A and B will be vrel.=vCv

But as v is opposite to vC

So vrel=vc(30)=(vC+30)km​​​/​​hr.

So, the time taken by it to cross the cars A and B t=dvrel​       460=5vC+30  


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