UNIVERSITY PHYSICS I CHAPTER 4 Kinematics ii Motion in two and Three Dimensions 84.1 The position velocity, and acceleration vectors in two dimensions 1. The position vector and displacement Let the plane of the two-dimensional motion be the x-y plane of a Cartesian coordinate system. r(t=x(t)i+y(t) 本s r(t+4-r(t) =|x(t+∠r)-x(t)i +[y(t+4)-y(t)lj1 1. The position vector and displacement Let the plane of the two –dimensional motion be the x-y plane of a Cartesian coordinate system. y t t y t j x t t x t i r t t r t r r r r t x t i y t j f i ˆ [ ( ) ( )] ˆ [ ( ) ( )] ( ) ( ) ˆ ( ) ˆ ( ) ( ) + + − = + − = + − = − = + ∆ ∆ ∆ ∆ r r r r r r x y O ∆s §4.1 The position, velocity, and acceleration vectors in two dimensions