What is Newton's first law of motion?

• A. An object in motion must stay in motion unless acted upon by an external force
• B. For every action, there is an equal and opposite reaction
• C. The acceleration of an object is proportional to the net force acting on it
• D. An object at rest stays at rest, and an object in motion stays in motion unless acted upon by a net external force

Answer: (D)

Newton's first law of motion states that an object at rest will remain at rest, and an object in motion will continue in motion at a constant velocity in a straight line unless acted upon by a net external force. This principle emphasizes the concept of inertia, which is the tendency of objects to maintain their state of motion unless influenced by an external force.

The law encapsulates the idea that an unbalanced force is necessary to change the motion of an object. If no net force acts on the object, there will be no change in its motion, meaning it will not speed up, slow down, or change direction. This forms the foundational understanding of motion and force in classical physics.

The other options present concepts from Newton's laws or related principles, but they do not accurately define the first law. The second option refers to Newton's third law, which involves action-reaction pairs, while the third describes Newton's second law, relating force and acceleration. The first option does not fully capture the concept that addresses both objects at rest and in motion, making it less comprehensive than the correct definition.