Final answer:
The ball's velocity is not constant because velocity includes direction as well as speed, and the direction changes continuously in circular motion. Even though the ball's speed may be constant, the direction change causes a centripetal acceleration, thus changing the velocity.
Step-by-step explanation:
When a ball attached to a string is whirled in a horizontal circle with a constant speed, the velocity of the ball is not constant due to the continual change of direction. Velocity is a vector quantity, meaning it has both magnitude (speed) and direction. Although the speed (scalar quantity) of the ball remains constant, since its direction is constantly changing as it moves around the circle, the velocity is not constant.
In circular motion, when an object maintains a constant speed, it is experiencing a change in direction at every point of the motion, creating a constant inward acceleration known as centripetal acceleration. This implies that while the speed might be constant, the velocity is not, as velocity is dependent on both speed and direction. In physics, this is a fundamental concept when studying uniform circular motion and dynamics.
When considering similar scenarios such as the behavior of a ball on a pendulum or an object tied to a string being swung in a circle, it's essential to understand that while some components of the motion, such as horizontal speed in a vacuum, might remain unaffected by gravity or other forces, rotation around a circle always involves an acceleration due to the continuous change in the direction of motion.