Assertion (A) : A man standing on a completely frictionless surface can propel himself by whistling. Reason (R) : If no external force acts on a system, its momentum cannot change.

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

Welcome to the beginning of our journey into Physics! To understand how waves move or why sounds travel, we must first master Newton’s Laws of Motion. These laws describe how objects interact with forces. At its simplest, a force is a push or a pull, and its SI unit is the newton (N) Science, Class VIII, Exploring Forces, p.65.

Newton gave us three pillars of motion. The First Law (Inertia) tells us that an object at rest stays at rest, and an object in motion stays in motion unless an external force acts on it. The Second Law provides the formula F = ma, meaning the force required depends on the mass of the object and how fast you want it to accelerate. Finally, the Third Law states that for every action, there is an equal and opposite reaction. This third law is vital for understanding propulsion and momentum: if you push against a wall, the wall pushes back on you with the same intensity.

One common area of confusion in competitive exams is the difference between mass and weight. Mass is the amount of matter in an object and remains constant everywhere in the universe. Weight, however, is a force — specifically, the force with which Earth pulls an object toward itself Science, Class VIII, Exploring Forces, p.72. Because weight is a force, it is also measured in Newtons, and unlike mass, it can change depending on gravity Science, Class VIII, Exploring Forces, p.77.

Feature	Mass	Weight
Definition	Quantity of matter in an object.	Force of gravity acting on an object.
SI Unit	Kilogram (kg)	Newton (N)
Variability	Constant everywhere.	Changes with location (e.g., Moon vs. Earth).

Sources: Science, Class VIII, Exploring Forces, p.65; Science, Class VIII, Exploring Forces, p.72; Science, Class VIII, Exploring Forces, p.77

2. Newton’s Third Law and Action-Reaction Pairs (basic)

To understand how things move—from a rocket launching to a person walking—we must look at Newton’s Third Law of Motion. At its heart, this law tells us that forces never exist in isolation; they always come in pairs. Formally, it states: 'To every action, there is always an equal and opposite reaction.' This means if you push against a wall, the wall is simultaneously pushing back against you with the exact same amount of force. As noted in Science, Class VIII, Exploring Forces, p.64, a force is an interaction that can change the speed or direction of an object, and in the Third Law, this interaction is a two-way street.

A crucial point that often confuses students is why these forces don't just cancel each other out. The reason is simple: action and reaction forces act on different objects. For example, when you swim, your hands push the water backward (Action on the water). In response, the water pushes your body forward (Reaction on you). Because the forces are acting on two different bodies—the water and the swimmer—they do not result in a net zero force on a single object. Instead, they cause motion. This principle is also why we feel 'pressure' when we interact with surfaces; as defined in Science, Class VIII, Pressure, Winds, Storms, and Cyclones, p.82, pressure is force per unit area, and that force is often the reaction force we experience during an interaction.

In the context of momentum, this law implies that for every bit of 'push' given to a substance (like air or water) in one direction, an equal 'push' is felt by the source in the opposite direction. This is the foundation of recoil. When a jet of air is expelled forward, it carries momentum. To satisfy Newton's Third Law, the body expelling that air must experience a reaction force in the opposite direction. This is not a 'reflex' or an involuntary biological response like those discussed in Science, Class X, Control and Coordination, p.102, but a fundamental physical necessity of the universe.

Sources: Science, Class VIII, Exploring Forces, p.64; Science, Class VIII, Pressure, Winds, Storms, and Cyclones, p.82; Science, Class X, Control and Coordination, p.102

3. Friction: The Role of Surface Resistance (intermediate)

At its heart, friction is a contact force that arises whenever two surfaces attempt to move across one another. It is essentially a force of resistance. Even when a surface looks perfectly smooth to the naked eye, a microscopic view reveals a landscape of tiny peaks and valleys known as irregularities. When two surfaces are pressed together, these irregularities interlock, creating a mechanical bond that opposes motion Science, Class VIII, NCERT, Exploring Forces, p.68. To initiate or maintain movement, an external force must be applied to overcome this interlocking resistance. Friction is not limited to solid-on-solid contact; it is a universal phenomenon occurring in fluids (liquids and gases) as well. In fluid dynamics, this resistance is commonly called drag. For example, as an airplane moves through the atmosphere, the air molecules collide with its surface, creating a resistive force that engineers must counteract with streamlined designs Science, Class VIII, NCERT, Exploring Forces, p.68. Similarly, on a planetary scale, the earth's surface irregularities resist wind movement. This friction is so significant that it influences wind speed and direction up to an altitude of 1-3 km, whereas friction over the relatively smooth sea surface is minimal Physical Geography, PMF IAS, Pressure Systems and Wind System, p.307. Understanding the nature of surface resistance helps us understand energy dissipation. In any mechanical system—including the movement of sound waves through a medium—friction acts to convert kinetic energy into thermal energy (heat). This is why a rolling ball eventually stops and why sound gradually fades as it travels; the energy is literally "worn away" by the resistance of the medium it moves through.

Feature	Solid Friction	Fluid Friction (Drag)
Primary Cause	Interlocking of surface irregularities.	Collisions with molecules and viscosity.
Effect of Shape	Dependent on surface texture/roughness.	Highly dependent on the object's streamlining.
Medium	Occurs between two solid surfaces.	Occurs in liquids and gases.

Sources: Science, Class VIII, NCERT, Exploring Forces, p.68; Physical Geography, PMF IAS, Pressure Systems and Wind System, p.307

4. Law of Conservation of Linear Momentum (intermediate)

Concept: Law of Conservation of Linear Momentum

5. Applications: Recoil and Jet Propulsion (exam-level)

To understand recoil and jet propulsion, we must return to the bedrock of physics: Newton’s Third Law of Motion and the Law of Conservation of Momentum. At its simplest, a force is a push or a pull that can change an object's speed or direction Science, Class VIII . NCERT, Exploring Forces, p.64. When an object exerts a force on another, the second object exerts an equal and opposite force back Science, Class VIII . NCERT, Exploring Forces, p.69. In an isolated system, the total momentum (mass × velocity) remains constant. Therefore, if a system ejects mass in one direction, the remaining mass must move in the opposite direction to balance the momentum equation. This 'kickback' is what we define as recoil.

Jet propulsion is the practical application of continuous recoil. By accelerating and ejecting a stream of fluid—such as the hot gases in a rocket engine—a vehicle generates forward thrust. This principle allows rockets to function even in the vacuum of space because they do not need an atmosphere to 'push against'; they simply push against their own exhaust. This technology has a long history, from the early military rockets used during the Mysore War to the sophisticated sounding rockets launched from the Thumba Equatorial Rocket Launching Station Geography of India, Majid Husain, Transport, Communications and Trade, p.54 Physical Geography by PMF IAS, Earths Magnetic Field, p.78.

A nuanced distinction must be made between mass transport and wave propagation. In pure acoustics (sound waves), particles generally oscillate back and forth without a significant net movement of matter. However, actions like whistling involve more than just sound; they involve the physical ejection of a jet of air. Because this air jet carries momentum away from the person, the person technically experiences a recoil force. While the sound waves themselves carry energy, it is the mass of the air being expelled that generates the measurable reaction force.

Sources: Science, Class VIII . NCERT, Exploring Forces, p.64, 69; Geography of India, Majid Husain, Transport, Communications and Trade, p.54; Physical Geography by PMF IAS, Earths Magnetic Field, p.78

6. Defining the 'System': Internal vs. External Forces (exam-level)

In physics, the first step to solving any problem is defining the 'System'. A system is simply a portion of the universe that we choose to analyze, separated from the rest of the world (the environment) by a boundary. Whether a force is considered internal or external depends entirely on where you draw that boundary.

Internal forces are those exerted by objects within the system on each other. According to Newton’s Third Law, these forces always exist in equal and opposite pairs. Because they cancel each other out within the boundary, internal forces cannot change the total momentum or the motion of the system’s center of mass. For instance, if you are sitting inside a car and push on the dashboard, the car won't move because your push and the dashboard's reaction are internal to the "Car + Person" system.

External forces, however, originate from outside the system boundary. These are the only forces capable of changing the system's total momentum. As noted in Science, Class VIII, Exploring Forces, p.64, a force can cause a moving object to change its speed or direction. In a geographic context, air masses move because of external forces like the pressure gradient force and friction (FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Atmospheric Circulation and Weather Systems, p.78).

When we apply this to the Law of Conservation of Momentum, we state that if the net external force on a system is zero, the total momentum remains constant. Consider a person whistling:

• If the system is just the Person: The air being ejected is an external mass leaving the system. The force it exerts back on the person is an external force, causing recoil.
• If the system is Person + Air: The interaction is internal. The air moves forward with some momentum, and the person moves backward with equal momentum, keeping the total system momentum at zero.

Feature	Internal Forces	External Forces
Source	Objects inside the system boundary.	Environment outside the boundary.
Effect on Total Momentum	No change (they cancel out).	Can increase or decrease total momentum.
Example	Tension in a string connecting two blocks.	Gravity, Friction, or a person pushing a box (Science, Class VIII, Exploring Forces, p.67).

Sources: Science, Class VIII (NCERT 2025), Exploring Forces, p.64, 67; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Class XI (NCERT 2025), Atmospheric Circulation and Weather Systems, p.78

7. The Physics of Whistling: Mass Transport vs. Sound (exam-level)

To understand the physics of whistling, we must distinguish between two distinct physical processes occurring simultaneously: acoustic radiation (the sound you hear) and mass transport (the air you blow). When you whistle, your lungs create a high-pressure zone, forcing a stream of air through a narrow opening between your lips or teeth. According to the principles of fluid dynamics, this pressure difference generates a steady flow of air, or a 'jet.' As explored in Science, Class VIII, Pressure, Winds, Storms, and Cyclones, p.88, air moves from regions of high pressure to low pressure, and this movement of a physical substance is what we call mass transport.

The 'recoil' one might feel while whistling is primarily a result of Newton’s Third Law of Motion. As the person exhales, they accelerate a specific mass of air (m) to a certain velocity (v), giving that air momentum (p = mv). For the air to gain this momentum, the person must exert a forward force on it; consequently, the air exerts an equal and opposite reaction force on the person. This is a classic application of the Law of Conservation of Momentum. While Science, Class VIII, Exploring Forces, p.69 discusses how forces like magnetism can act from a distance, the force in whistling is a contact force between the internal tissues and the air being expelled.

Crucially, there is a difference between the jet of air and the sound wave it produces. A pure sound wave is a longitudinal disturbance where air particles simply oscillate back and forth about a mean position. In such a wave, there is negligible net mass transport; the energy moves forward, but the air itself stays roughly in the same place. However, because whistling involves 'blowing' (the physical ejection of air), it results in a net transfer of mass out of the body. In the context of Life Processes, as mentioned in Science-Class VII, Life Processes in Animals, p.136, the act of exhaling air is a fundamental biological function, but in physics, it serves as a propellant that produces a tiny, though theoretically calculable, recoil force.

Feature	Acoustic Wave (Sound)	Jet Flow (Whistling Act)
Primary Action	Particle oscillation (back & forth)	Net directional movement of air
Mass Transport	Negligible / Zero	Significant
Recoil Cause	Acoustic radiation pressure (tiny)	Momentum of ejected air (primary)

Sources: Science, Class VIII (Revised ed 2025), Pressure, Winds, Storms, and Cyclones, p.88; Science, Class VIII (Revised ed 2025), Exploring Forces, p.69; Science-Class VII (Revised ed 2025), Life Processes in Animals, p.136

8. Solving the Original PYQ (exam-level)

This question is a classic application of the Law of Conservation of Momentum and Newton’s Third Law. Having just mastered the building blocks of dynamics, you can see that whistling is more than just sound—it involves the acceleration and ejection of a jet of air from the lungs. On a completely frictionless surface, there is no external resistive force to prevent motion. By ejecting air (mass with velocity), the man creates a momentum change in the air, which necessitates an equal and opposite recoil for himself to keep the total momentum balanced. This confirms that Assertion (A) is true, as the ejected air provides the necessary reaction force for propulsion.

To evaluate the Reason, we look at the fundamental definition: if no external force acts on a system, its total momentum remains constant. This is a true statement (R), representing a universal law of physics. However, the correct answer is (B) because the Reason does not provide the specific causal link to the Assertion. While momentum is conserved for the entire system (man plus the ejected air), the Reason as stated describes a condition of no change, whereas the Assertion describes a process of movement. As noted by University of Sydney - Mechanics, the man moves because he is part of a system where internal forces redistribute momentum, a nuance that the general law in R does not explicitly detail as an explanation for the man's specific acceleration.

The common trap in UPSC Assertion-Reason questions is selecting Option (A) simply because both statements are scientifically factual. Students often assume that if the Reason is a famous law of physics, it must be the explanation. To avoid this, you must ask: "Does the Reason explain the specific mechanism of the Assertion?" Here, R is a universal principle, but it fails to mention the internal interaction—the ejection of mass—required to propel the man. Additionally, Stanford CCRMA - Acoustics highlights that sound waves alone involve back-and-forth motion without net mass transport; therefore, it is the breath (the jet of air), not the acoustic whistle, that does the work—a distinction the Reason fails to bridge.