In an amusement-park ride, cars rolling along at high speed suddenly head up a long, straight ramp. They roll up the ramp. reverse direction at the highest point, then roll backward back down the ramp. In each of the following segments of the motion, are the cars accelerating, or is their acceleration zero? If accelerating, which way does their acceleration vector point? 

a. As the cars roll up the ramp. 

b. At the highest point on the ramp. 

c. As the cars roll back down the ramp.