Recall that the trajectory of a particle P is defined by its coordinate vector r measured in a fixed reference frame F. As the particle moves, its coordinate vector r (t) traces its trajectory, which is a curve in space, given by: r ( t ) = x ( t ) x ^ ^ + y ( t ) y ^ ^ + z ( t ) z ^ ^ , where x̂, ŷ, and ẑ are the unit vectors along the x, y and z axes of the reference frame F, respectively.