* Circular Motion: When the ball is whirled on a string, it's moving in a circle. This circular motion requires a constant force towards the center of the circle, called the centripetal force. This force is provided by the tension in the string.

* Inertia: Objects in motion tend to stay in motion in a straight line at a constant speed (Newton's First Law). This tendency to resist changes in motion is called inertia.

* The Break: When the string breaks, the centripetal force (the tension in the string) disappears. There's no longer a force pulling the ball towards the center of the circle.

* Straight Line Motion: Because of inertia, the ball continues to move in the direction it was traveling *at the instant the string broke*. This direction is tangential to the circle, meaning it forms a straight line that's perpendicular to the radius of the circle at the point where the string broke.

In summary: The ball moves off in a straight line because the force that was keeping it in circular motion (the tension in the string) disappears, and inertia causes it to continue moving in the direction it was traveling at that moment.