Newton's second law for rotation (转动定律)： Divide the body into many small parts,for each part we have: F+f=△m,a, 万×F+万×万=△m,i×a, △m ∑万×F+∑×f=∑△m,7×a The torque∑7×手=0 of inner force: The net t=∑店×F torque: mi z i r  Newton’s second law for rotation（转动定律）: Fi  i f  Divide the body into many small parts, for each part we have: i i i i F f m a       i i i i i i i r F r f m r a             i i i i i i i r F r f m r a               The net torque： z i Fi r       The torque of inner force:    0 i i r f  