Difference Between Newton's First Law and Inertia | Compare the Difference Between Similar Terms

The key difference between Newton’s first law and inertia is that Newton’s first law describes the relationship between the motion of an object and the forces that are acting upon the object whereas the term inertia refers to the resistance of an object to any change in its velocity.

Generally, Newton’s first law of motion is also named the Law of inertia. This is because Newton’s first law explains the inertia of a physical object.

1. Overview and Key Difference
2. What is Newton’s First Law
3. What is Inertia
4. Side by Side Comparison – Newton’s First Law vs Inertia in Tabular Form
5. Summary

What is Newton’s First Law?

Newton’s first law states that a physical object that is at rest will stay at rest and an object that is moving will stay in motion until a net external force is applied to that object. In other words, if the net force acting on a particular object is zero, then the velocity of that object remains constant. We can say this law is a statement of inertia.

Moreover, if any change in the motion of the object involves an acceleration, then we need to know Newton’s second law in order to understand that motion of the object. Therefore, we can view Newton’s first law as a special case of Newton’s second law.

Difference Between Newton's First Law and Inertia

Figure 01: Sir Isaac Newton

More importantly, we need to know the reference frame in which the motion of the object occurs. In the case of Newton’s first law, we are considering a reference frame which is not itself accelerating. Usually, we call these frames “inertial frames”.

According to Newton’s first law of motion, any object that is at rest in one reference frame will appear to be moving in a straight line to the observer in another reference frame which is moving by the object.

What is Inertia?

Inertia is the resistance of a particular object to any change in its velocity. In this case, the term includes the changes in the object’s speed or direction of motion. This term describes the tendency of an object to keep moving in a straight line at a constant speed when there is no external force acting upon the object. In simple and common terms, inertia refers to the resistance to any change in motion.

On Earth’s surface, gravity, and the effects of friction and air resistance mask the inertia. Both these factors tend to decrease the speed of moving object. The concept of inertia is a fundamental principle in classical physics that’s still in use even today. It describes the motion of objects and the effect of forces applied on the object.

What is the Difference Between Newton’s First Law and Inertia?

The key difference between Newton’s first law and inertia is that Newton’s first law describes the relationship between the motion of an object and the forces that are acting upon the object whereas the term inertia refers to the resistance of an object to any change in its velocity. The Newton’s first law states that a physical object that is at rest will stay at rest and an object that is moving will stay in motion until a net external force is applied to that object.

Below is a summary of the difference between Newton’s first law and inertia in tabular form.

Summary – Newton’s First Law vs Inertia

Usually, Newton’s first law of motion is also called the Law of inertia. This is because Newton’s first law explains the inertia of a physical object. The key difference between Newton’s first law and inertia is that Newton’s first law describes the relationship between the motion of an object and the forces that are acting upon the object whereas the term inertia refers to the resistance of an object to any change in its velocity.