A force of constant magnitude F = 10 N acts on a particle moving in a plane such that it is perpendicular to the velocity v→(|v→|=v=5m/s) of the body, and the force is always directed towards a fixed point. Then the angle (in radian) turned by the velocity vector of the particle as it covers a distance S = 10 m is (take : mass of the particle as m = 2 kg)