If velocity is zero is acceleration zero, and can acceleration be negati. and velocity be post. and vice versa

Answer 1

No. For example, at t = 0, I hit a ball. Right at that moment, the ball is not moving (v = 0), but I am imparting force, which imparts acceleration. The ball has 0 velocity, but it has positive acceleration.

Accleration is considered to be negative when it is acting in the opposite direction of the velocity. For example, if I throw a ball up, the upwards velocity is positive, and the gravitational acceleration downwards is negative. As long as the ball is travelling up, the ball has positive velocity, and negative acceleration.

In the same scenario with the ball, when the ball reaches the highest point and begins to come down, the velocity becomes "negative" as it is going the opposite direction of the initial velocity.

Acceleration and velocity are vectors, which cannot have a negative magnitude (i.e, they are always positive). Vectors are a line with an arrow to show direction. Vectors can be split into dimensional vectors, (up/down, forward/backwards, right/left), and if you are working with multiple vectors, they may be added and subtracted. This is when they may be treated as positive or negative.

Answer 2

Acceleration is the change in velocity. If velocity is zero, there is no change occurring at all. So yes, if velocity is zero, acceleration is zero.

Negative acceleration is a downward change in velocity (slowing down). So velocity can be positive and there be negative acceleration.

There is no negative velocity Velocity can never be negative, under any conditions.