When a ball is thrown upward at 15 m/s, the magnitude of its initial velocity is 15 m/s. However, as the ball moves upward against the force of gravity, its velocity gradually decreases. At the highest point of its trajectory, the ball momentarily comes to a stop before it starts descending.
The direction of the ball's velocity is determined by its motion. When the ball is thrown upward, its velocity vector points in the opposite direction to the force of gravity. We can consider upward as the positive direction and downward as the negative direction.
Therefore, the magnitude of the ball's velocity remains 15 m/s throughout its upward motion, but its direction changes from initially positive (upward) to negative (downward) as it reaches the highest point and starts descending.