Discussion: A very small displacement during a small time interval A very small displacement: dr= AB A very small distance: ds B ds= AB When time interval rde dr approaches to 0 dr=ds B Let: 0A=OC Acre CB= dr AB=AC+cB Cautio d≠d dr= ac+cb=rde+drrDiscussion: A very small displacement during a small time interval O y A r  B r  B A x dr  ds ds = AB dr = ds When time interval  approaches to 0: dr = AB  A very small displacement: A very small distance: C dr AC = rd drr ˆ ˆ dr = AC +CB = rd +  Let: OA= OC AB = AC +CB d rd CB = dr Cautio n! dr  dr 