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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?? )

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?? )

Grade:12

4 Answers

Prajwal kr
49 Points
9 years ago

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

Ashwin Muralidharan IIT Madras
290 Points
9 years ago

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).

vishal ojha
35 Points
9 years ago

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

vivek kumar
57 Points
9 years ago

first one

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