Newton’s laws of motion do not hold good for objects

1. The Foundation: Newton's Three Laws of Motion (basic)

Welcome to our journey into basic mechanics! To understand how the physical world works, we must start with Sir Isaac Newton, whose work represents the climax of the scientific revolution Themes in world history, History Class XI (NCERT 2025 ed.), Changing Cultural Traditions, p.119. Newton formulated three fundamental laws that describe how objects move and interact. Before we dive in, remember that the SI unit of force is the newton (denoted by the symbol N) Science, Class VIII. NCERT (Revised ed 2025), Exploring Forces, p.65.

Newton's laws provide a framework for Classical Mechanics:

• First Law (Inertia): An object will remain at rest or move at a constant velocity unless acted upon by an external force. This resistance to change in motion is called inertia.
• Second Law (F = ma): The force acting on an object is equal to the mass of that object times its acceleration. This law defines how we measure force. It also helps us distinguish between mass (the amount of matter, which is constant) and weight (the force of gravity pulling on that mass) Science, Class VIII. NCERT (Revised ed 2025), Exploring Forces, p.77.
• Third Law (Action-Reaction): For every action, there is an equal and opposite reaction. If you push a wall, the wall pushes back on you with equal force.

Crucially, we must understand the domain of validity for these laws. While they perfectly describe "everyday" motion—like a ball rolling or a car braking—they begin to fail at extreme scales. When objects move at relativistic speeds (approaching the speed of light), Newtonian mechanics must be replaced by Einstein's Theory of Relativity, as space and time are no longer absolute at those speeds.

Concept	Mass	Weight
Definition	Quantity of matter in an object.	The force with which Earth pulls an object Science, Class VIII. NCERT (Revised ed 2025), Exploring Forces, p.72.
Variability	Remains unchanged everywhere.	Varies from place to place (e.g., Earth vs. Moon).
SI Unit	Kilogram (kg)	Newton (N)

Sources: Science, Class VIII. NCERT (Revised ed 2025), Exploring Forces, p.65; Science, Class VIII. NCERT (Revised ed 2025), Exploring Forces, p.72; Science, Class VIII. NCERT (Revised ed 2025), Exploring Forces, p.77; Themes in world history, History Class XI (NCERT 2025 ed.), Changing Cultural Traditions, p.119

2. Frames of Reference: Where Laws are Observed (intermediate)

In mechanics, before we can measure the speed of an object or the force acting upon it, we must establish a Frame of Reference. Think of a frame of reference as the "viewpoint" or the coordinate system from which an observer takes measurements. Motion is never absolute; it is always relative to the observer. For example, if you are sitting in a moving train, a book on your lap appears stationary. However, to someone standing on the platform, that same book is zooming past at 80 km/h. Both observers are correct within their respective frames.

Physics distinguishes between two primary types of frames, which determine whether Newton’s laws of motion can be applied directly:

Feature	Inertial Frame	Non-Inertial Frame
Acceleration	Zero (Rest or constant velocity)	The frame itself is accelerating or rotating
Newton's Laws	Laws hold true without modification	Newton's Laws seem to fail unless "pseudo-forces" are added
Example	A parked car or a plane flying at a steady speed	A car turning a corner or the rotating Earth

While we often treat the Earth as an inertial frame for daily calculations, it is technically non-inertial because it rotates. This rotation creates effects like the Coriolis force, which deflects winds and ocean currents (Physical Geography by PMF IAS, Pressure Systems and Wind System, p.308). Understanding the frame is vital because Newton's laws, which describe how forces change an object's motion (Science, Class VIII NCERT (Revised ed 2025), Exploring Forces, p.64), have a specific domain of validity. They work perfectly for everyday objects, but when we observe motion from a frame moving near the speed of light, classical mechanics breaks down and must be replaced by Einstein’s Theory of Relativity (Physical Geography by PMF IAS, Chapter 1, p.5).

Sources: Physical Geography by PMF IAS, Pressure Systems and Wind System, p.308; Science, Class VIII NCERT (Revised ed 2025), Exploring Forces, p.64; Physical Geography by PMF IAS, Chapter 1: The Universe, p.5

3. Gravitation and the Universe (basic)

At its most fundamental level, gravitation is the glue that holds the universe together. For centuries, our understanding was rooted in the classical view established by Isaac Newton. This scientific revolution reached its peak with Newton’s Law of Universal Gravitation, which suggests that every mass in the universe exerts an attractive force on every other mass. Whether it is an apple falling to the ground or the moon orbiting the Earth, Newton provided a single mathematical framework to explain both terrestrial and celestial motion Themes in world history, History Class XI (NCERT 2025 ed.), Changing Cultural Traditions, p.119.

However, modern physics underwent a paradigm shift with Albert Einstein’s General Theory of Relativity. Instead of viewing gravity as a mysterious "pulling force" through empty space, Einstein proposed that gravity is actually the curvature of spacetime. Think of spacetime as a flexible fabric; a massive object like a star or planet is like a bowling ball placed on a trampoline, causing the fabric to curve. Objects (like planets) aren't being "pulled" so much as they are following the natural curves in the fabric created by larger masses Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.4.

This modern understanding has led to the discovery of two fascinating phenomena that serve as proof for Einstein's theories:

• Gravitational Waves: These are 'ripples' in the fabric of spacetime caused by violent cosmic events, such as the collision of black holes. They travel at the speed of light, carrying information about their origin Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.4.
• Gravitational Lensing: Because gravity curves space, it also bends the path of light. When light from a distant star passes near a massive galaxy, the galaxy's gravity acts like a lens, distorting the light's path Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.5.

Closer to home, gravity is responsible for maintaining our atmosphere. It pulls air molecules toward the Earth's center, creating the homosphere—a region extending up to 80 km where the blend of gases is nearly uniform Environment and Ecology by Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.7. Because gravity is strongest near the surface, atmospheric pressure and density decrease rapidly as we gain altitude; for instance, at the top of Mt. Everest, the air pressure is roughly two-thirds less than at sea level Physical Geography by PMF IAS, Pressure Systems and Wind System, p.305.

Perspective	Newtonian Gravity	Einsteinian Gravity
Nature	An invisible force/pull between masses.	A geometric curvature of spacetime.
Medium	Acts instantly across empty space.	Propagates as waves through the fabric of space.

Sources: Themes in world history, History Class XI (NCERT 2025 ed.), Changing Cultural Traditions, p.119; Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.4-5; Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.7; Physical Geography by PMF IAS, Pressure Systems and Wind System, p.305

4. Conservation Principles: Momentum and Energy (intermediate)

In the study of mechanics, Conservation Principles are the bedrock of how we understand the universe's behavior. The word 'conservation' implies that while a system may undergo dramatic changes, certain physical quantities remain constant in their total value. The two most vital pillars are the Law of Conservation of Momentum and the Law of Conservation of Energy. These principles allow us to predict the outcome of events, from the simple motion of a train between stations to complex atomic collisions, without needing to know every tiny force acting in between Science-Class VII, NCERT (Revised ed 2025), Measurement of Time and Motion, p.116.

Linear Momentum is defined as the product of an object's mass and its velocity (p = mv). The principle of conservation states that in an isolated system—where no external forces like friction are acting—the total momentum before an event (like a collision) is exactly equal to the total momentum after. This is why, when two billiard balls collide, one might slow down while the other speeds up, but their combined 'quantity of motion' remains the same. This holds true regardless of whether the motion is uniform (constant speed) or non-uniform (changing speed) Science-Class VII, NCERT (Revised ed 2025), Measurement of Time and Motion, p.117.

Energy Conservation dictates that energy cannot be created or destroyed, only transformed from one form to another. For instance, in an electric circuit, electrical energy is dissipated as heat or light, and the rate at which this happens is known as Power Science, Class X (NCERT 2025 ed.), Electricity, p.191. In mechanical systems, we often see a trade-off between Kinetic Energy (energy of motion) and Potential Energy (stored energy, like a pendulum at its highest point). While the form changes, the sum of energy in a closed system stays fixed.

However, an exceptional coach must point out the limits of these rules. While Newtonian mechanics provides a perfect description for everyday speeds, it acts as an approximation. When objects approach relativistic speeds (near the speed of light), classical definitions of mass and momentum begin to fail. At these extremes, space and time are no longer absolute but interlinked, and we must use Einstein’s formulas to account for the increase in effective mass Physical Geography by PMF IAS, Chapter 1, p.5. For your UPSC preparation, remember that conservation laws are universal, but the mathematical formulas we use to describe them must change when we leave the 'slow' world of everyday life.

Sources: Science-Class VII, NCERT (Revised ed 2025), Measurement of Time and Motion, p.116-117; Science, Class X (NCERT 2025 ed.), Electricity, p.191; Physical Geography by PMF IAS, Chapter 1: The Universe, p.5

5. Fundamental Forces of Nature (intermediate)

To understand how the universe functions, we must look at the four fundamental forces of nature. These are the basic interactions that cannot be reduced to more simple mechanisms. While we often talk about 'endogenic' and 'exogenic' forces in geography to describe land-building or land-wearing processes Fundamentals of Physical Geography, Geomorphic Processes, p.37, those are actually complex macro-manifestations of these four primary physical forces. Every event, from a volcanic eruption to the melting of ice due to weakened interparticle attraction Science, Class VIII, Particulate Nature of Matter, p.103, is governed by these interactions.

The four forces vary drastically in their strength and range:

Force	Relative Strength	Range	Role in Nature
Strong Nuclear	1 (Strongest)	Very Short (Subatomic)	Holds the atomic nucleus together.
Electromagnetism	10⁻²	Infinite	Governs chemistry, friction, and interparticle forces.
Weak Nuclear	10⁻¹³	Very Short (Subatomic)
Gravity	10⁻³⁸ (Weakest)	Infinite	Governs planetary motion and large-scale structures.

Interestingly, Gravity is the weakest force, yet it dominates the cosmic scale because it is always attractive and has an infinite range. On the other hand, the Electromagnetic force is what allows plants to trap solar radiation and drive ecosystem energy flow Environment and Ecology, Basic Concepts, p.14. However, it is important to note that while Newton’s laws of motion provide a brilliant framework for these forces at everyday speeds, they begin to fail as objects approach the speed of light. In those extreme relativistic conditions, space and time warp, and we must transition from classical mechanics to Einstein’s Theory of Relativity.

Sources: Fundamentals of Physical Geography, Geomorphic Processes, p.37; Science, Class VIII, Particulate Nature of Matter, p.103; Environment and Ecology, Basic Concepts, p.14

6. The Relativistic Limit: Beyond Newton (exam-level)

For centuries, Newtonian mechanics reigned supreme, providing a perfect framework for understanding everything from falling apples to the orbits of planets. However, as our understanding of the universe deepened, we discovered that Newton’s laws have a domain of validity. They work exceptionally well for the "slow" world we live in, but they fail when we reach the Relativistic Limit—situations where objects move at speeds approaching the speed of light (c), which is approximately 300,000 km/s.

In 1905, Albert Einstein’s Theory of Special Relativity transformed our understanding by proving that space and time are not absolute or separate entities. Instead, they are interwoven into a four-dimensional fabric known as spacetime Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.5. In this regime, the classical formula for momentum (p = mv) no longer holds true because an object's effective mass increases as it nears the speed of light. This leads to fascinating phenomena like time dilation (time slowing down for the moving object) and length contraction (objects appearing shorter in the direction of motion).

Beyond just high speeds, Einstein’s General Theory of Relativity (1915) redefined gravity itself. While Newton viewed gravity as an invisible force pulling objects together, Einstein showed that massive objects actually distort the fabric of spacetime Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.5. Think of a heavy bowling ball placed on a trampoline; it creates a dip that causes marbles to roll toward it. When massive objects like black holes accelerate, they create gravitational waves—ripples in spacetime that travel at the speed of light Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.4.

Feature	Newtonian Mechanics	Relativistic Mechanics
Space & Time	Absolute and independent	Interwoven as "Spacetime"
Mass	Constant regardless of speed	Increases with velocity (as v → c)
Gravity	A force acting at a distance	Curvature of spacetime

Sources: Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.4; Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.5

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental principles of Classical Mechanics, this question serves as a crucial bridge to understanding the limitations of scientific theories. You’ve learned that Newton’s Laws govern the motion of macro-scale objects in our daily lives. However, the UPSC often tests your ability to identify the boundary conditions where these laws cease to be accurate. As discussed in Physical Geography by PMF IAS, Newtonian physics assumes that space and time are absolute, a premise that holds true only until we reach the extreme edges of the universe's speed limit.

To arrive at the correct answer, you must think like a physicist: ask yourself at what point the mass of an object is no longer constant. While Newton’s laws perfectly describe objects at rest or moving slowly (Options A and B), they begin to fail as an object approaches the velocity comparable to the velocity of light. In this regime, Einstein’s Theory of Special Relativity replaces classical formulas because, at such speeds, time dilates, lengths contract, and mass effectively increases. Therefore, Option (D) is the correct choice because the classical equations of motion do not account for these relativistic effects.

Be careful not to fall for the trap in Option (C). The term "high velocity" is a common UPSC distractor; in the context of physics, even a rocket traveling at thousands of miles per hour is considered "slow" compared to the speed of light. The breakdown of Newtonian mechanics only becomes significant when velocity is a substantial fraction of 'c' (the speed of light). Always look for that specific comparison to light speed to identify the transition from Classical to Relativistic physics.