&Sum:∑F+∑/=|∑△mh2la n F lni s fi 0and∑fr1=0 S0, we have∑Fn=∑△mtr2a met s Let tnet=∑ ∑Fn Net torque about given axis, and       +  =  i i i i i t i i i t i F r f r m r 2 & sum: i r n ˆ :  Finri = f inri = 0  = 0 i i i and f r So, we have          =  i i i i it i F r m r 2 =  =  i net i it i Let   F r —— Net torque about given axis, and