In the XY- plane , the region `y gt 0` has a uniform magnetic field `B_1 hatk` and the region ` y lt 0 `has another uniform magnetic field `B_2 hatk`. A positively charged particle is projected from the origin along the positive y- axis with speed ` v_0= pi m s^(-1)` at t=0 , as shown in the figure . Neglect gravity in this problem . Let t=T be the time when the particle crosses the X- axis from below for the first time . If `B_2 = 4B_1`, the average speed to the particle , in `ms^(-1)`, along the x- axis in the time interval T is __________ .

Answer

2