Final answer:
The final velocity of the ball thrown vertically upward with an initial speed of 21 m/s, when it returns to its starting level, is -21 m/s. The negative sign indicates that the direction of the velocity is now downward.
Step-by-step explanation:
When a ball is thrown vertically upward with an initial speed of 21 m/s and then returns to the level at which it started, its velocity will be -21 m/s. The negative sign represents that the direction of the velocity is now opposite to the initial direction. The magnitude will be the same as it was upon the ball's release, but its direction will be the opposite given that the ball is moving downward when it returns.
Understanding this requires knowing the principles of physics governing vertical motion: When an object is projected upwards, it will come down with the same speed and in the opposite direction (if we ignore air resistance). This is because the force of gravity, which is acting downwards, decelerates the ball until it stops at its peak, then accelerates it back downwards along the same path, reversing the direction of its velocity.
Therefore, in this question, if the 'upward' direction was initially taken as positive, the final velocity of the ball, when it returns to its starting level, is -21 m/s (downwards). This means that the ball is moving downwards with a speed of 21 m/s.
Learn more about Vertical Motion