A railroad engine is being positioned in a train yard over straight track. Its velocity is shown in the graph below in 5-second intervals as well as in a table of values. The graph is linear between t = 0 and ? = 15 and has horizontal tangent lines at t = 25, t= 35, t=40, and t = 50. At t = 0, the engine is in front of a control tower. t (seconds) v(t) ft per second 0 12 6 10 -6 feet per second 20 -1 30 35 45 50 55 time (seconds) 60 a. At what values of t does the engine have no acceleration? b. Write an expression for the speed of the engine from 0