In the xy-plane, the region y>0 has a uniform magnetic field B1k^ and the region y<0 has another uniform magnetic field B2(−k^).
A positively charged particle is projected from the origin along the positive y-axis with speed v0=π ms−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 above for the second time. If B2=4B1, the average velocity of the particle, in ms−1, along the x-axis in the time interval T is _____ .