What is an example of Newton's first law of motion?

An example of Newton's first law of motion is indeed that an object at rest stays at rest unless acted upon by a force. This is the fundamental principle of inertia, which states that an object will not change its state of motion unless an external force is applied to it. This law describes the tendency of objects to resist changes in their motion, whether they are at rest or moving at a constant velocity.

Understanding this concept helps illustrate why it's important to apply a force to initiate movement; for instance, a stationary ball will remain in place until someone kicks it or applies some other force to move it. This illustrates how the state of rest or motion is maintained in the absence of an external influence.

The other options relate to different aspects of motion. The second option describes the principle of action and reaction, which pertains to forces acting in pairs. The third option reflects the aspect of maintaining motion, which is also part of Newton’s first law but doesn’t convey the complete definition. The fourth option presents Newton's second law, which addresses the relationship between force, mass, and acceleration, rather than inertia.