Final answer:
If the string breaks, no external force causes the can to move in a straight line; it continues in a straight-line path due to its inertia as per Newton's first law of motion. The centripetal force, such as tension, acts towards the center and is no longer present after the break.
Step-by-step explanation:
If a string that holds a whirling can in its circular path breaks, the force that causes it to then move in a straight-line path is actually no force. According to Newton's first law of motion, an object in motion will stay in motion with the same speed and in the same direction unless acted upon by an unbalanced external force. In the context of circular motion, the centripetal force acts towards the center of the circular path to keep the object moving in that path. When the string breaks, this force no longer acts on the can, which means there is no external force causing a change in its state of motion. Consequently, the can moves in a straight line tangent to the point where the string broke, which is in accordance with Newton's first law.
In contrast, centrifugal force is an apparent force experienced in a rotating reference frame and is not responsible for the object moving in a straight line after the string breaks. The actual force that stretched the string when the mass was moving in a circular path was the tension force, which is a type of centripetal force. It originates from the mass itself as it is pulled inwards while trying to move straight due to its inertia, creating tension in the string.