Final answer:
The billiard ball slowing down on the pool table is due to friction, which acts as an external force opposing its motion, in line with Newton's First Law of Motion.
Explanation:
Newton's First Law of Motion, also known as the law of inertia, states that an object in motion tends to stay in motion unless acted upon by an external force. When a billiard ball is rolled across a pool table and slows to a stop, it is due to friction, which is the external force causing the deceleration. Friction between the ball and the table surface opposes its motion, eventually bringing it to a stop.
Learn more about Newton's First Law of Motion here:
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