F.4 PHYF.4 PHYF.4 PHY

The main reason is that the direction of acceleration and the motion are opposite.
Take right as positive.Suppose the constant acceleration of the body is -a and the initial velocity is +u.Obviously,the speed would decreases to zero and then the body will go to the left.So the body would have zero velocity at a particular time.
Note1: Even if the acceleration is negative,the body is also said to be accelerating.
Note2:Of course the body would not have zero velocity forever because acceleration implies a change of velocity.The body would only have zero velocity at an instant.

2009-08-22 22:29:27 補充：
I would like to add something to note2: "A body can have zero velocity but also be accelerating." doesn't mean the body have zero velocity forever.

Acceleration and velocity are entirely two different physical quantities. An acceleration will lead to a change (increase or decrease) of velocity.
A simple example is an object ( a car, a ship, or a man) starts to move from rest. At the instant when the object starts moving, the velocity is zero, but there is an acceleration. It is because of such acceleration that brings an increase of velocity from zero.