To every action, there is an equal and opposite reaction.
Explanation
Newton's laws of motion are three physical laws that form the foundation for classical mechanics. They describe the relationship between a body and the forces acting upon it, and its motion in response to those forces.
Option Analysis
- An object remains at rest unless acted upon by an external force. is incorrect: This statement represents Newton's First Law of Motion, also known as the Law of Inertia. It states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force.
- The rate of change of momentum is proportional to the applied force. is incorrect: This statement represents Newton's Second Law of Motion. It quantifies force, stating that the force acting on an object is equal to the mass of that object times its acceleration ($F=ma$), or more precisely, the rate of change of momentum.
- To every action, there is an equal and opposite reaction. is correct: This statement represents Newton's Third Law of Motion. It posits that for every action (force) in nature, there is an equal and opposite reaction. These forces always act in pairs on two different objects.
- The total energy of an isolated system remains constant. is incorrect: This statement represents the Law of Conservation of Energy, a fundamental principle of physics stating that the total energy of an isolated system remains constant over time.
Key Takeaway: Newton's Third Law of Motion establishes that forces always occur in action-reaction pairs acting on different bodies; a single isolated force cannot exist.