What is Galileo's Law of Inertia?

Inertia is a natural tendency of an object to stay at rest or in motion. That is, an object will resist anything against its current state of motion. It is a fundamental property that applies to every object we can think of, from a dust particle to the largest planet. It's because of inertia, we know in physics, especially classical mechanics, that a book stays firmly on the desk and how a meteor travels at thousands of kilometres per hour. 

Historical Context of Inertia

Year/Period	Contributor	Contribution
Ancient Times	Aristotle	He observed that a continuous force is required to maintain motion. This led to the worldview that rest is the natural state of objects.
11th Century	Ibn Sina (Avicenna)	He was the first to counter the Aristotelian law of motion. He suggested that in a vacuum, a projectile would continue moving indefinitely unless acted upon.
14th Century	Jean Buridan	Buridan developed the theory of impetus, which stated that objects retain motion imparted to them until opposed by another force.
16th Century	Giambattista Benedetti	Benedetti argued against the idea of circular impetus. He further suggested that motion naturally tends toward straight-line (rectilinear) paths.
Early 17th Century	Johannes Kepler	Introduced the term inertia, though he defined it as resistance to motion and still linked motion with force.
Early 17th Century	René Descartes	He incorporated inertia into his physics, and argued that motion continues in the absence of external forces due to the geometrical nature of matter.
Early 17th Century	Galileo Galilei	Refined the idea by demonstrating that objects remain in uniform motion in a straight line unless acted upon.

To understand the law of inertia, it's ideal to have a historical background. In the sections below, we look into how Galileo Galilei deduced it.  

Inertia is a property of any mass that opposes changes to its motion. It was Galileo Galilei who first recognised this principle in the early 17th century. He showed that, without friction, a rolling object would neither stop nor accelerate. This object will sustain speed and direction continuously because there is no external force yet to stop it. And this practically explains the concept of the equilibrium of a particle, which you will learn in this fourth chapter. 

This was later (over an 80-year period) refined and mathematically defined by Sir Isaac Newton with his First Law of Motion. 

This becomes the starting point for understanding the motion (or lack of it) of every object. And this principle of inertia underlines the basics of classical mechanics. 

In Chapter 4, Section 4.3 on Law of Inertia, NCERT mentions, “if the net external force is zero, a body at rest continues to remain at rest and a body in motion continues to move with a uniform velocity. This property of the body is called inertia. Inertia means ‘resistance to change’. A body does not change its state of rest or uniform motion, unless an external force compels it to change that state.”

This law of inertia will be much clearer when we look into the inclined plane experiments of Galileo.  

Galileo observed that a ball rolling down a ramp accelerates in the same way as an object in free fall. But this happens at a much slower and more measurable pace.

The Single Inclined Plane Experiment

If you imagine a ball on a ramp. 

What happens when you let it go?

When the Ball Rolls Downhill 

The ball speeds up. This is because a portion of gravity pulls it along the downward slope. That causes it to accelerate.

When the Ball Rolls Uphill

If you give the ball a push to go up the ramp, it slows down. Gravity is now working against its direction of motion. That creates deceleration. 

The Ball on a Flat Surface

Here, gravity isn't going to push the ball forward or pull backwards along the surface. 

Galileo reasoned that with no force to speed it up or slow it down, the ball should maintain a constant velocity. 

Galileo's Final Observation: An Object’s Natural State is Not Rest 

On a perfectly smooth, friction-free horizontal surface, an object in motion should continue to move at a constant speed forever. 

The Double Inclined Plane Experiment

Suppose there are two inclined planes, and they face each other. 

A ball is released from a certain height on the first plane.

The Observation 

The ball rolls down the first plane and up the second. Galileo noted that the ball almost reaches the same height from which it was released. 

A small amount of height is lost due to friction.

The Idealisation 

In a perfect, frictionless world, the ball would reach the exact same height on the second plane.

Key Insight of the Two Inclined Planes: Inertia Stays In Motion or Rest Unless There Is an External Force

He considered what would happen if the second plane were made less steep.

As the angle of the second plane decreases, the ball has to travel a longer distance to reach its original height.

If you make the second plane completely horizontal, the ball can never reach its original height. 

So, in an ideal, frictionless scenario, it would roll on forever.

This demonstrated the fundamental principle of inertia. An object in motion will stay in motion with a constant velocity unless acted upon by an external force.

The Law of Inertia explains various everyday phenomena.  

• We observe the inertia of a bus passenger's body when they lurch forward as the bus stops suddenly.
• The inertia principle of still motion is apparent when we see a book kept on a table. It does not move unless we push it. Based on the Law of Inertia, we can conclude that there is no net force acting on it.

 

If you need to revise some important concepts in Physics before exams, we have you covered. Check under these chapters.  

Units and Measurements Class 11 Notes	Mechanical Properties of Solids Class 11 Notes
Motion in a Straight Line Class 11 Notes	Mechanical Properties of Fluids Class 11 Notes
NCERT Class 11 Notes for Motion in a Plane	Thermal Properties of Matter Class 11 Notes
Laws of Motion Class 11 Notes	Thermodynamics Class 11 Notes
Work, Energy, and Power Class 11 Notes	Kinetic Theory of Gas Class 11 Notes
System of Particles and Rotational Motion Class 11 Notes	Oscillations Class 11 Notes
Gravitation Class 11 Notes	Waves Class 11 Notes

Also check these notes. 

NCERT Class 11 Notes for PCM

NCERT Class 11 Physics Notes

 

Find NCERT Solutions for Physics Class 11 while learning to solve exam-related questions quickly. Or gain more clarity with chapter-wise NCERT Solutions below.  

Units and Measurements Class 11 NCERT Solutions	Mechanical Properties of Solids Class 11 NCERT Solutions
Motion in a Straight Line Class 11 NCERT Solutions	Mechanical Properties of Fluids Class 11 NCERT Solutions
Motion in a Plane Class 11 NCERT Solutions	Thermal Properties of Matter Class 11 NCERT Solutions
Laws of Motion Class 11 NCERT Solutions	Thermodynamics Class 11 NCERT Solutions
Work, Energy, and Power Class 11 NCERT Solutions	Kinetic Theory Class 11 NCERT Solutions
System of Particles and Rotational Motion Class 11 NCERT Solutions	Oscillations Class 11 NCERT Solutions
Gravitation Class 11 NCERT Solutions	Waves Class 11 NCERT Solutions