two particles P and Q start simultaneously from a point A with velocities 15 m/s and 20 m/s.the accelaration of both the bodies are equal in magnitude but opposite in direction.at point B velocity of P is 30m/s.find d velocity of Q at point B?(sir if we apply first equation of motion answer is coming 5 m/s,if we apply 3rd equation answer is imaginary,if we solve by average velocity method(because displacement and time both are equal for btoh the bodies) answer is coming 25m/s......which one should we accept?? )

Since you get an imaginary answer, the given question has to be wrong. It is physically not possible with the given numbers. 

Hi Ayush,

The first thing you must note is that P & Q will not reach point B simultaneously. (As a matter of fact one must verify if Q can reach point B).

At B velocity of P is 30m/s.

You can also check that Q will never reach point B. (When velocity of Q becomes 0, use 3rd law of motion).

You can see that, 0^2 - 20^2 = -2ax

And for P, let displacement is y. So 30^2 - 15^2 = 2ay. See that y>x. Hence Q will never reach the point B.

Regards,

Ashwin (IIT Madras).

f1st one is correct.........u have to rule out the 3rd equation method as the answer is imaginary..which cant be accepted

first one