Final answer:
A ball stops rolling on a flat surface primarily due to the force of friction, which acts opposite to the movement of the ball, transforming kinetic energy into heat and gradually slowing the ball until it comes to a stop.
Step-by-step explanation:
The reason a ball stops rolling on a flat surface is primarily due to friction. When you roll a ball along a surface, like a sidewalk, it eventually comes to a stop because of the rubbing force, which is friction, exerted by the sidewalk onto the ball. According to Newton's first law of motion, a body in motion tends to remain in motion at a constant velocity unless acted upon by a net external force. In this case, friction is that net external force, and it acts in the opposite direction of the ball's movement.
Friction is a resistance to motion that transforms some of the ball's kinetic energy into heat, thereby slowing the object down. Without friction, such as in the vacuum of space, objects can continue to move indefinitely. This concept can be observed in an air hockey table scenario where, when the air is turned on, the puck glides smoothly over a nearly frictionless surface for much longer distances than when the air is off. Hence, friction is responsible for the slowing down of the ball, not gravity, conservation of energy, or the absence of friction.
If we were to consider other possibilities: the absence of friction would mean no force to stop the ball; gravity primarily acts perpendicular to the motion and doesn't stop the horizontal motion; conservation of energy would only be violated if energy was not conserved within the system, but in this case, kinetic energy is transformed into heat; lastly, no external force, other than friction in our discussion, is applied to stop the ball. Therefore, the correct answer is that the ball stops rolling because of external force acting upon it, specifically the force of friction.