In Newtonian mechanics, which of Newton laws of motion is also commonly known as the law of inertia?

Introduction / Context:
Newton formulated three fundamental laws of motion that form the basis of classical mechanics. The concept of inertia, which is the tendency of a body to resist changes in its state of rest or uniform motion, is central to the first of these laws. This question checks whether you know which numbered law is usually referred to as the law of inertia in physics textbooks.

Given Data / Assumptions:

• There are three Newton laws of motion: first, second and third.
• Inertia describes the tendency of a body to maintain its current state of motion unless a net external force acts.
• One of Newton laws explicitly formalises this idea.
• We must match the law of inertia with the correct numbered law.

Concept / Approach:
Newton first law states that a body remains at rest or moves with uniform velocity in a straight line unless acted upon by a net external force. This law directly expresses the concept of inertia and defines inertial frames of reference. Newton second law relates net force to acceleration and mass, and Newton third law deals with action reaction pairs. Therefore the first law is the one that is also called the law of inertia, because it formalises and quantifies the idea that bodies resist changes in motion.

Step-by-Step Solution:
Step 1: Recall the wording of Newton first law: a body continues in its state of rest or uniform straight line motion unless acted on by a net external force.Step 2: Recognise that this is a direct statement of the property of inertia.Step 3: Recall that Newton second law provides the equation F = m * a, relating force, mass and acceleration.Step 4: Recall that Newton third law states that every action has an equal and opposite reaction.Step 5: Identify that only the first law focuses explicitly on inertia and unforced motion.Step 6: Conclude that Newton first law is also called the law of inertia.

Verification / Alternative check:
Introductory physics books sometimes introduce inertia even before giving the formal statement of Newton first law, then explain that the law of inertia is an alternative name for the first law. Historical discussions of Galileo and Newton also emphasise that understanding inertia was a key step towards formulating this first law. You rarely see the phrase law of inertia applied to the second or third law, which confirms the correct association.

Why Other Options Are Wrong:
Newton second law describes how force causes acceleration and introduces the concept of momentum, but it does not itself define inertia. Newton third law deals with the mutual forces between interacting bodies, the action reaction pair, which again is distinct from inertia. Saying that all of the above are the law of inertia is misleading because each law addresses a different fundamental aspect of motion.

Common Pitfalls:
Some students mix up the numbering of Newton laws or think that inertia is part of all laws equally. Others may remember F = m * a from the second law and incorrectly associate inertia with it because mass appears in that equation. To avoid confusion, remember that the first law emphasises what happens when net force is zero, which is exactly the behaviour that defines inertia.

Final Answer:
The law of inertia is Newton first law of motion.