The vector position of a particle varies in time according to the expression r = 6.20 i - 6.40t j where r is in meters and t is in seconds. (a) Find an expression for the velocity of the particle as a function of time. (Use any variable or symbol stated above as necessary.) V = -12.8 m/s X (b) Determine the acceleration of the particle as a function of time. (Use any variable or symbol stated above as necessary.) a = -12.8 m/s 2 X (c) Calculate the particle's position and velocity at t = 2.00 s. F= 6.2 m X 6.4 m/s X