Three particles of masses 2,3 and 5 move under the influence of a force field so that their position vectors relative to a fixed coordinate system are given respectively by r1=2ti-3j+3t2k,r2=t+1i+3tj-4k,r_3=t2i-tj-2t-1k where t is time. Find the total external torque of the system applied at the origin at t=1.