Average acceleration is defined as:

.

In words, this definition indicates that the acceleration is equal to the rate at which the velocity is changing. Since is a positive scalar, the two vectors in the definition, and , have the same direction.

Point D is a turning point, hence the velocity of the ball at point D is zero. Consider a small portion of the motion with an initial point at point D and a final point just after point D. Since the velocity at point D is zero,

=

You indicated that the acceleration at point D is directed into the page. Since and have the same direction, and since , this means that the final velocity of the ball (i.e., the velocity just after point D) is also directed into the page. Is the ball really moving into the page just after point D?