Unless acted upon by an unbalanced external force, a physical body remains at rest or continues to travel at a constant velocity in a straight path. Which fundamental principle of mechanics is described by this statement?

The correct answer is Newton’s first law of motion.

Explanation:
Newton’s first law of motion, commonly known as the law of inertia, states that an object will remain in its state of rest or continue moving with a constant velocity along a straight path unless acted upon by an unbalanced external force.

Key aspects of this fundamental principle include:
1. Inertia: Inertia is the inherent property of matter by which it resists any change in its existing state of rest or uniform motion.
2. Unbalanced External Force: When the net force acting on a body is zero, its acceleration is zero, meaning its velocity remains constant. An unbalanced force is necessary to change either the speed or the direction of motion of the body.

Let me briefly review the remaining options:
• Principle of conservation of momentum: States that the total linear momentum of an isolated system remains constant provided no external force acts on it.
• Newton’s third law of motion: States that whenever one body exerts a force on a second body, the second body simultaneously exerts an equal and opposite force on the first body.
• Law of universal gravitation: States that every particle attracts every other particle in the universe with a force proportional to the product of their masses and inversely proportional to the square of the distance between their centers.