A particle of mass m moves in the plane xy due to the force varying with velocity as F = a (yi = xj), where a is a positive constant, i and j are the unit vectors of the x and y axes. At the initial moment t - 0 the particle was located at the point x = y = 0 and possessed a velocity Vo directed along the unit vector j. Find the law of motion x (t), y (t) of the particle, and also the equation of its trajectory.