we say accelearation is v dv/dx. does it mean a=0, when v is 0? when ball is at the highest point of treajectory??

When an object(in this case ball) is projected vertically upwards,the direction of motion of the object and its velocity are along vertical in upward direction.Now at the highest point of its path,the velocity of the body gets zero,but its acceleration is not zero.As the object moves up,it is always attracted by earth in downward direction,i.e acceleration due to gravity acting vertically downward direction.When object reaches at the highest point of its trajectory it stays for a moment its velocity gets zero but it is still under the action of gravity i.e it is still accelerating downward.