A man is at rest in the middle of a horizontal plane of perfectly smooth surface of ice. He can move himself to the shore by making use of Newton’s :

1. Newton's First Law: The Concept of Inertia (basic)

Welcome to your first step in mastering mechanics! To understand how the world moves, we must first understand why things don't move or why they keep moving. Newton’s First Law of Motion, often called the Law of Inertia, states that an object will remain in its state of rest or continue to move with a constant velocity in a straight line unless acted upon by an external, unbalanced force. In simpler terms, objects are ‘lazy’— they want to keep doing exactly what they are already doing.

The root of this behavior is a property called Inertia. Inertia is the inherent tendency of an object to resist any change in its state of motion. It is important to realize that inertia is not a force itself; rather, it is a quality of matter. The amount of inertia an object possesses is directly proportional to its mass. For instance, it is much harder to push a stationary car into motion than it is to push a bicycle. The car has more mass, and therefore, more inertia or ‘resistance’ to change Science, Class VIII, Chapter 5, p.77.

In our daily lives, we rarely see objects moving forever in a straight line because of invisible forces like friction. Friction is a contact force that comes into play when an object moves or attempts to move over a surface, opposing its motion Science, Class VIII, Chapter 5, p.77. Imagine being on a perfectly smooth, frictionless sheet of ice. If you are at rest, you cannot simply ‘walk’ because walking requires your feet to push against the ground via friction. Without that external force to overcome your inertia, you would remain stuck in place regardless of how much you moved your legs Science, Class VIII, Chapter 5, p.68.

Sources: Science, Class VIII . NCERT(Revised ed 2025), Exploring Forces, p.77; Science, Class VIII . NCERT(Revised ed 2025), Exploring Forces, p.68

2. Newton's Second Law: Force and Momentum (basic)

To understand Newton’s Second Law, we must first grasp the concept of momentum. Think of momentum as the 'quantity of motion' an object possesses. It is the product of an object's mass (m) and its velocity (v), represented by the formula p = mv. A heavy truck moving slowly can have the same momentum as a light bullet moving very fast. Because velocity involves direction, momentum is a vector quantity. Changes in this 'quantity of motion' do not happen spontaneously; they require the application of a force, which is a push or pull resulting from an interaction Science, Class VIII NCERT, Exploring Forces, p.77.

Newton’s Second Law of Motion provides the mathematical backbone for mechanics. It states that the rate of change of momentum of an object is directly proportional to the applied unbalanced force and takes place in the direction of the force. In simpler terms, the harder you push an object, the faster its momentum changes. If the mass of the object remains constant, this law simplifies to the famous equation F = ma (Force = mass × acceleration). This tells us that force is required to change an object's speed or direction of motion Science, Class VIII NCERT, Exploring Forces, p.77.

The SI unit of force is the newton (N) Science, Class VIII NCERT, Exploring Forces, p.65. One Newton is defined as the amount of force required to give a 1 kg mass an acceleration of 1 m/s². This law explains why it is harder to stop a fast-moving cricket ball than a slow-moving one, or why a car requires more braking force to stop quickly than to slow down gradually. It bridges the gap between the 'cause' (Force) and the 'effect' (Acceleration/Change in Momentum).

Sources: Science, Class VIII NCERT, Exploring Forces, p.77; Science, Class VIII NCERT, Exploring Forces, p.65

3. Newton's Third Law: Action and Reaction Pairs (basic)

At the heart of how things move in our universe is Newton’s Third Law of Motion. It states a simple but profound truth: To every action, there is always an equal and opposite reaction. This means that forces never exist in isolation; they always occur in pairs. When you push against a wall, the wall is simultaneously pushing back against you with the exact same amount of force. In scientific terms, a force is a push or pull resulting from an object’s interaction with another object Science, Class VIII . NCERT(Revised ed 2025), Chapter 5, p. 77.

A crucial detail that students often miss is that the action and reaction forces act on two different bodies. For example, when you walk, your foot exerts a backward force on the ground (Action). In response, the ground exerts an equal forward force on your foot (Reaction), which is what actually moves you forward. This movement relies heavily on friction, which is the force that comes into play when one surface tries to move over another Science, Class VIII . NCERT(Revised ed 2025), Chapter 5, p. 77. Without friction, such as on a perfectly smooth sheet of ice, your feet would simply slip, and you wouldn’t be able to exert that initial "action" force against the ground to get a "reaction" back.

So, how do you move if you are stranded on frictionless ice? You must create an interaction with a different object. By throwing a heavy object (like a bag or a shoe) away from the shore, you exert an action force on that object. According to Newton’s Third Law, that object will exert an equal and opposite reaction force on you. This force will propel you toward the shore. This principle is deeply connected to the conservation of linear momentum: since the total momentum of you and the object was initially zero, the momentum gained by the object in one direction must be balanced by you gaining momentum in the opposite direction.

Aspect	Action Force	Reaction Force
Magnitude	Equal to Reaction	Equal to Action
Direction	Opposite to Reaction	Opposite to Action
Target	Object A acts on Object B	Object B acts on Object A

Sources: Science, Class VIII . NCERT(Revised ed 2025), Chapter 5: Exploring Forces, p.77; Science, Class VIII . NCERT(Revised ed 2025), Chapter 5: Exploring Forces, p.66

4. Understanding Friction as a Component of Motion (intermediate)

To understand motion, we must first recognize that friction is not merely a force that slows things down; it is often the very force that makes motion possible. At a microscopic level, every surface has tiny peaks and valleys called irregularities. When two surfaces come into contact, these irregularities 'interlock,' creating a resistance known as the force of friction Science, Class VIII . NCERT(Revised ed 2025), Chapter 5: Exploring Forces, p.68. When you walk, your foot pushes backward against the ground; the ground, through friction, provides an equal and opposite reaction force that propels you forward. Without this 'grip,' your feet would simply slide in place.

Now, consider a theoretical scenario: you are at rest in the middle of a perfectly smooth, frictionless ice pond. Because there is no friction, you cannot walk, crawl, or even push yourself off the surface. To move toward the shore, you must leverage Newton’s Third Law of Motion—the principle that every action has an equal and opposite reaction Science, Class VIII . NCERT(Revised ed 2025), Chapter 5: Exploring Forces, p.77. By throwing an object, such as a heavy coat or a stone, in the direction away from the shore, you exert an 'action' force on that object. In response, the object exerts a 'reaction' force on you, pushing you toward your destination.

This movement is governed by the Conservation of Linear Momentum. Since the total momentum of the system (you + the object) was initially zero, it must remain zero. When the object gains momentum in one direction, you must gain an equal amount of momentum in the opposite direction. This same logic applies to global scales; for instance, the friction created by the Earth's surface irregularities significantly slows down wind speeds and alters wind direction compared to the minimal friction found over open seas Physical Geography by PMF IAS, Pressure Systems and Wind System, p.307.

Feature	Movement with Friction (Walking)	Movement without Friction (Ice)
Primary Requirement	Contact and surface irregularities.	Internal mass to eject/throw.
Force Origin	External reaction force from the ground.	Internal reaction force via Newton's 3rd Law.
Example	Walking on a carpeted floor.	A rocket engine or throwing a bag on ice.

Sources: Science ,Class VIII . NCERT(Revised ed 2025), Chapter 5: Exploring Forces, p.68, 77; Physical Geography by PMF IAS, Pressure Systems and Wind System, p.307

5. Law of Conservation of Linear Momentum (intermediate)

The Law of Conservation of Linear Momentum is a fundamental principle stating that the total momentum of a closed system remains constant if no external force acts upon it. In physics, momentum (p) is defined as the product of an object's mass (m) and its velocity (v), represented as p = mv. When objects move along a straight line, we refer to this as linear motion Science, Class VII, Chapter 8, p. 116. If the velocity is constant, the motion is uniform; if it changes, it is non-uniform Science, Class VII, Chapter 8, p. 117. Regardless of the type of motion, the total momentum before an interaction must equal the total momentum after the interaction, provided the system is isolated.

A classic application of this law involves a person stranded on a perfectly frictionless surface, like smooth ice. Normally, walking requires friction to provide an external force that pushes us forward as we push the ground backward. On frictionless ice, walking is impossible. To reach the shore, the person must create an internal interaction within a system (man + object). By throwing an object, such as a bag or a shoe, away from the shore, the person exerts an "action" force on the object. According to Newton’s Third Law, the object exerts an equal and opposite "reaction" force on the person Science, Class VIII, Chapter 5, p. 77.

From the perspective of momentum conservation, the math is simple: if the initial momentum of the man and the object is zero (at rest), the final total momentum must also be zero. Therefore, if the object gains momentum in one direction, the man must gain an equal amount of momentum in the opposite direction. This is the same principle that causes a gun to recoil when a bullet is fired or a rocket to lift off by ejecting exhaust gases downward. Even in advanced physics, such as observing particles like protons in magnetic fields, momentum remains a defining property that helps scientists track motion and force Science, Class X, Chapter 12, p. 203.

Sources: Science, Class VII, Chapter 8: Measurement of Time and Motion, p.116-117; Science, Class VIII, Chapter 5: Exploring Forces, p.68, 77; Science, Class X, Chapter 12: Magnetic Effects of Electric Current, p.203

6. Real-world Applications: Rockets and Jet Propulsion (exam-level)

To understand how rockets soar into space, we must first look at a classic physics puzzle: a man standing on a perfectly frictionless surface of ice. Since there is no friction to push against, he cannot walk or crawl to the shore Science, Class VIII, Exploring Forces, p. 67. To move, he must apply Newton’s Third Law of Motion, which states that for every action, there is an equal and opposite reaction. By throwing his coat or a stone away from the shore (the action), the object exerts an equal force back on him (the reaction), propelling him toward safety Science, Class VIII, Exploring Forces, p. 77. This movement is also governed by the Law of Conservation of Linear Momentum: as the object gains momentum in one direction, the man must gain equal momentum in the opposite direction to keep the system's total momentum at zero.

Rockets and jet engines operate on this exact principle of recoil. A rocket does not 'push' against the air or the ground to move; instead, it ejects mass (exhaust gases) at incredibly high velocities. The downward action of the escaping gas creates an upward reaction force, known as thrust, on the rocket. This is why rockets can function in the vacuum of space where there is no atmosphere to push against. In India, this science was pioneered at the Thumba Equatorial Rocket Launching Station (TERLS), chosen because its location near the magnetic equator allows scientists to study unique atmospheric currents like the equatorial electrojet Physical Geography by PMF IAS, Earths Magnetic Field, p. 78.

While both rockets and jets use propulsion, they differ in how they obtain their 'working fluid' and oxygen for combustion:

Feature	Jet Engine	Rocket Engine
Oxygen Source	Sucks in oxygen from the surrounding atmosphere.	Carries its own oxidizer (can work in a vacuum).
Medium	Limited to the atmosphere.	Can operate in space and the atmosphere.

Sources: Science, Class VIII . NCERT(Revised ed 2025), Exploring Forces, p.67, 77; Physical Geography by PMF IAS, Earths Magnetic Field, p.78

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental building blocks of force and friction, this question tests your ability to apply those concepts in an idealized environment. In physics, a perfectly smooth surface implies that friction is zero. Normally, when we walk, we push backward against the ground, and friction provides the reaction force to move us forward. Without friction, you cannot "grip" the ice. To move, you must find a way to generate an external force from within the system, which brings the principle of action and reaction into play.

By applying the Third Law of Motion, you can create motion by throwing an object (like a bag or a piece of clothing) away from the shore. As you exert an action force on the object, the object exerts an equal and opposite reaction force on you. This reaction force provides the necessary propulsion to move you toward the shore, a concept rooted in the conservation of linear momentum as detailed in Science, Class VIII. NCERT (Revised ed 2025). Therefore, the correct answer is (C) Third law of motion.

UPSC often uses the other laws as distractors. While the First Law describes inertia, it doesn't explain how to initiate movement without an external agent. The Second Law defines the relationship between force and acceleration ($F=ma$), but it doesn't provide the mechanism for generating that force in a frictionless void. Option (D) is a common trap designed to catch students who assume all laws of physics are equally involved; however, the specific strategy for a man at rest to gain momentum relies specifically on the action-reaction pair.