4- A freight train leaves a train station and travels due north on a straight track. After hours, the train is 5(t) = 18c2 21?3 kilometres north of the train station (for 0 s t s 9) a) Find an expression for the velocity of the train at any time 0 s t s 9 b) Find the velocity of the train after 3 hours, and after 7 hours. c) Find the acceleration of the train after 1 hour. 5- Each of the graphs in the figure below shows the position of a p article moving along the x-axis as a function of time, 0 s t s 5. During this time interval, which particle has a) A Constant velocity b) The greatest initial Velocity c) The greatest average Velocity d) Zero Average Velocity e) a negative acceleration for 0 s t s 5 i)