Answer
(a) An object falling down with increasing speed.
(b) A car running with constant speed in a circular track.
(c) An object moving upward with decreasing speed.
Work Step by Step
In the cases (a) and (c), removing the effect of air resistance, the only force acting on the object is the weight, that points downward. So, in (a), the object moves in the same direction as the net force and in (c) in the opposite direction.
In the case (b), the net force is the force that the road apply on the tires of the car. This force is horizontal and points to the center of the circular track.
