When is Newton's second law applicable?

Newton's second law is applicable specifically when considering the net external force acting on an object. The law states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. This relationship is typically expressed with the formula F = ma, where F is the net force, m is mass, and a is acceleration.

The net external force is crucial because it accounts for all influences acting on an object, including friction, tension, gravity, and any other forces. In situations where forces are balanced, the net force is zero, and therefore the acceleration is zero, indicating that the object will not change its state of motion.

In contrast, the other options do not represent situations accurately fitting Newton's second law. The law does not universally apply regardless of forces, nor is it valid in cases where mass is changing, such as with rockets losing mass as fuel burns. Lastly, its application is not limited to situations involving relativistic speeds, where other physics principles come into play. Thus, understanding that only the net external force should be considered is critical for applying Newton's second law correctly.