Final answer:
The initial velocity of a ball thrown downward from a bridge at time [t=0] and striking the water below is positive, considering that downward motion is taken as the positive direction in this mechanics scenario.
Step-by-step explanation:
When a ball is thrown down from a bridge at time [t=0] and strikes the water below, the initial velocity of the ball is the speed at which it is thrown downward, not taking into account its acceleration due to gravity. In this context, initial velocity refers to the velocity at the exact moment the ball starts its descent. If the ball is thrown downward, as opposed to being dropped, it has a positive initial velocity since it is moving downwards from the bridge to the water, which is the convention for positive direction in this scenario.
From the provided information, such as mentioning a ball released with an initial velocity of 4.9 m/s downward and considering the acceleration due to gravity, we can understand that in physics, particularly in the study of mechanics and projectile motion, the convention is to consider downward motion as positive when it comes to velocity measurements. As such, since the ball is thrown downward, its initial velocity would be positive if we're adhering to this convention.