Here is a problem on mechanics.
Problem; Two particles move in a uniform gravitational field with an acceleration g. AT the initial moment the particles were located at one point and moved with velocities v1=3.0 m/s and v2=4.0 m/s horizontally in opposite directions. Find the distance between the particles at the moment when their velocity vectors become mutually perpendicular.
Here is my ans. (Got to fig. it out in a concise way after scribbling through pages unsuccessfully. )