The relationship between△fand△s In general △r≠△S △s Can △F=△s Yes. for two cases 1)1D motion without 1ox changing direction 2)When△t→>0 after take limit: dr=dsThe relationship between and : r   s 1 r  P1 2 r  P2 r   x y O z    r s s In general, Can ? r = s  Yes, for two cases: 1) 1D motion without changing direction 2) When after take limit: t → 0, dr = ds 