Which one among the following is correct for resultant of balanced forces ?

1. Introduction to Force and Motion (basic)

At its simplest level, a force is a push or a pull acting upon an object resulting from the object's interaction with another object. In the study of mechanics, we quantify this interaction using the Newton (N), which is the standard SI unit of force Science, Class VIII. NCERT(Revised ed 2025), Exploring Forces, p.65. Forces do not exist in isolation; they always come in pairs and can change the state of motion, the direction, or even the shape of an object. For instance, in physical geography, we see massive forces like gravity and the Coriolis force initiating the movement of ocean waters FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Movements of Ocean Water, p.111.

To understand how an object will move, we must look at the Net Force (or resultant force), which is the vector sum of all individual forces acting on it. When multiple forces act on an object, they can either cancel each other out or combine to create motion. This leads us to the critical distinction between balanced and unbalanced forces:

Feature	Balanced Forces	Unbalanced Forces
Net Force	Zero (Sum = 0)	Non-zero (Sum ≠ 0)
Effect on Motion	No change in state (remains at rest or constant velocity)	Causes acceleration (change in speed or direction)
Example	A book resting on a table (Gravity vs. Normal Force)	Kicking a stationary ball

When forces are balanced, the object is said to be in equilibrium. This doesn't necessarily mean the object is still; it simply means there is no change in its current state of motion. If it was already moving at a constant speed in a straight line, it will continue to do so. Conversely, an unbalanced force is the only way to make an object speed up, slow down, or turn. Interestingly, the application of force over a specific surface area also defines pressure (Force/Area), measured in Pascals (Pa) or N/m² Science, Class VIII. NCERT(Revised ed 2025), Pressure, Winds, Storms, and Cyclones, p.82.

Sources: Science, Class VIII. NCERT(Revised ed 2025), Exploring Forces, p.65; Science, Class VIII. NCERT(Revised ed 2025), Pressure, Winds, Storms, and Cyclones, p.82; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Movements of Ocean Water, p.111

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

At its heart, Newton's First Law describes a fundamental property of all matter: Inertia. Think of inertia as a kind of physical "stubbornness" or laziness. It is the inherent tendency of an object to resist any change in its state of motion. If an object is sitting still, it wants to stay still; if it is moving, it wants to keep moving at the same speed and in the same direction forever. This revolution in scientific thinking, which began with thinkers like Galileo and reached its peak with Isaac Newton, changed how we understand the very fabric of the universe Themes in world history, History Class XI, Changing Cultural Traditions, p.119.

To understand this law, we must look at Balanced Forces. Imagine a book resting on your study table. Gravity pulls it downward, but the table pushes it upward with an equal and opposite force. Because these two forces are equal in magnitude but opposite in direction, their net force (or vector sum) is zero. In physics, we say the object is in a state of equilibrium. The SI unit we use to measure these forces is the newton, symbolized by N Science, Class VIII, Exploring Forces, p.65. When the net force is zero, the object's velocity remains constant—which means if it was at rest, it stays at rest; if it was moving, it continues to move in a straight line at a steady pace.

It was Galileo Galilei who first challenged the ancient idea that a force is needed to keep an object moving. Through his experiments—such as those involving pendulums and rolling balls—he realized that in the absence of friction, an object would simply never stop Science-Class VII, Measurement of Time and Motion, p.108. Newton formalized this into his First Law: an object will maintain its state of rest or uniform motion in a straight line unless acted upon by an unbalanced (non-zero) external force. If you see an object speeding up, slowing down, or turning, you can be certain that the forces acting on it are no longer balanced.

State of Object	Net Force (ΣF)	Resulting Motion
Balanced Forces	Zero	Rest OR Constant Velocity (Straight line)
Unbalanced Forces	Non-Zero	Acceleration (Change in speed or direction)

Sources: Themes in world history, History Class XI (NCERT 2025 ed.), Changing Cultural Traditions, p.119; Science, Class VIII. NCERT (Revised ed 2025), Exploring Forces, p.65; Science-Class VII. NCERT (Revised ed 2025), Measurement of Time and Motion, p.108

3. Understanding Vectors in Mechanics (intermediate)

To master mechanics, we must first distinguish between simple numbers and quantities that have a 'sense of direction.' In physics, we categorize physical quantities into two types: Scalars (which only have magnitude, like mass or temperature) and Vectors (which have both magnitude and direction). Force is a classic example of a vector quantity. As noted in Science, Class VIII NCERT, Exploring Forces, p.77, a force is a push or pull resulting from an interaction, and it is measured in Newtons (N). Because force is a vector, if you push an object to the right with 10 N and I push it to the left with 10 N, the 'net' effect isn't 20 N—it is zero.

When multiple forces act on a single object, we calculate the Resultant Force (or Net Force) by finding the vector sum of all individual forces. This is more complex than simple addition because we must account for the angles and directions. If the forces are Balanced, their vector sum equals zero. In this state, the object is in Equilibrium. This doesn't necessarily mean the object is stationary; it means the object's state of motion is unchanging. According to Science, Class VIII NCERT, Exploring Forces, p.77, forces can change an object's speed or direction; therefore, if the forces are balanced (Net Force = 0), the object will either remain at rest or continue moving at a constant velocity in a straight line.

Consider a book resting on your study table. It experiences the downward pull of gravity, but it doesn't fall through the table. This is because the table exerts an equal and opposite upward normal force. These two vectors cancel each other out perfectly. However, if you apply an unbalanced force—one where the resultant vector is non-zero—the object must undergo acceleration in the direction of that net force. Understanding this vector balance is the foundation for solving almost every problem in structural engineering and classical mechanics.

Feature	Scalar Quantity	Vector Quantity
Definition	Has only magnitude (size).	Has magnitude AND direction.
Examples	Mass, Time, Temperature, Energy.	Force, Velocity, Acceleration, Momentum.
Addition	Simple arithmetic (5kg + 5kg = 10kg).	Vector addition (takes direction into account).

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

4. Friction: The Opposing Force (intermediate)

Friction is a fundamental contact force that acts as an invisible resistance whenever two surfaces attempt to move or actually move relative to each other. It always acts in the direction opposite to the motion (or intended motion) of the object. For instance, if you roll a ball on the ground, friction is the force responsible for gradually reducing its speed until it stops Science, Class VIII. NCERT (Revised ed 2025), Exploring Forces, p.78. Without friction, an object in motion would continue to move indefinitely unless acted upon by another external force.

At a microscopic level, no surface is perfectly smooth. Even surfaces that appear polished have tiny "hills and valleys" known as irregularities. When two surfaces are placed in contact, these irregularities interlock with one another. To move one surface over the other, we must apply enough force to overcome this interlocking mechanism Science, Class VIII. NCERT (Revised ed 2025), Exploring Forces, p.68. This explains why rougher surfaces, which have larger and more frequent irregularities, offer greater resistance than smooth surfaces.

Friction is not limited to solid objects sliding against each other; it also plays a critical role in fluid dynamics and geography. For example, the Earth's surface exerts friction on the atmosphere, resisting the movement of wind. This resistance is most significant over land due to topographical obstacles (like mountains and forests) and is minimal over the relatively uniform surface of the sea Physical Geography by PMF IAS, Pressure Systems and Wind System, p.307. This atmospheric friction typically influences wind behavior up to an altitude of 1-3 km, affecting both wind speed and direction.

Sources: Science, Class VIII. NCERT (Revised ed 2025), Exploring Forces, p.68; Science, Class VIII. NCERT (Revised ed 2025), Exploring Forces, p.78; Physical Geography by PMF IAS, Pressure Systems and Wind System, p.307

5. Newton's Third Law: Action and Reaction (intermediate)

In our journey through mechanics, we have seen that a force is not just a 'thing' an object possesses, but an interaction between two objects Science, Class VIII, Exploring Forces, p.77. Newton’s Third Law takes this a step further by stating a fundamental truth: Forces always exist in pairs. Formally, whenever one object exerts a force (Action) on a second object, the second object exerts a force of equal magnitude but in the opposite direction (Reaction) on the first. If we label the objects as A and B, the relationship is expressed as Fᴀʙ = -Fʙᴀ. This means you cannot touch a wall without the wall touching you back with the exact same intensity.

A common point of confusion for students is why these forces don't simply 'cancel each other out' if they are equal and opposite. The key lies in the point of application: the action and reaction forces act on different objects. For example, when you walk, your foot pushes the ground backward (Action on the ground); simultaneously, the ground pushes your foot forward (Reaction on you). Because the reaction force acts on you, it causes your motion. This law applies to both contact forces, like friction, and non-contact forces, like gravity or magnetism Science, Class VIII, Exploring Forces, p.77.

It is also vital to understand that there is no time lag between action and reaction. They happen simultaneously the moment the interaction begins. Whether a car is accelerating or a train is moving at a constant linear motion Science, Class VII, Measurement of Time and Motion, p.116, these interaction pairs are always present. Even if the two objects have vastly different masses—like a bat hitting a ball—the force the bat exerts on the ball is exactly equal to the force the ball exerts on the bat. The difference in their resulting motion (acceleration) is due to their different masses, not a difference in the force applied.

Sources: Science, Class VIII, Exploring Forces, p.77; Science, Class VII, Measurement of Time and Motion, p.116; Science, Class VIII, Exploring Forces, p.64

6. Resultant Force and Equilibrium (exam-level)

In our study of mechanics, we must understand that objects are rarely acted upon by a single force in isolation. Whether it is the muscular force used to push a cart or the frictional force resisting that motion, forces usually work in groups Science Class VIII NCERT, Exploring Forces, p.66, 77. The Resultant Force (also called the Net Force) is the single force that represents the combined effect of all individual forces acting on an object. Because force is a vector quantity, calculating the resultant requires us to account for both magnitude and direction. If two people pull a rope in opposite directions with equal strength, the resultant force is zero, even though the tension in the rope is high.

The concept of Equilibrium describes a specific state where the resultant of all forces acting on a body is exactly zero. When an object is in equilibrium, it experiences no acceleration. This leads to two possible scenarios: either the object remains perfectly still (static equilibrium) or it continues to move at a constant velocity in a straight line. If the forces are not balanced—meaning the resultant force is non-zero—the object’s state of motion will change, resulting in a change in speed or direction Science Class VIII NCERT, Exploring Forces, p.77.

To help distinguish between these states, consider the following comparison:

Feature	Balanced Forces (Equilibrium)	Unbalanced Forces
Resultant Force	Exactly zero (ΣF = 0)	Non-zero (ΣF ≠ 0)
Effect on Motion	Constant velocity or stays at rest	Acceleration (change in speed/direction)
Example	A book resting on a table	A ball falling through the air

Sources: Science Class VIII NCERT, Exploring Forces, p.66, 77

7. Solving the Original PYQ (exam-level)

Now that you have mastered the building blocks of Newtonian mechanics and the nature of vector addition, this question serves as a perfect application of those principles. In your previous modules, you learned that forces do not act in isolation; they interact to determine an object's state of motion. The term balanced forces refers to a specific scenario where all individual force vectors acting upon a body cancel each other out perfectly. As we discussed in the concept of equilibrium, for a body to remain at rest or move with constant velocity, the net influence—or what we call the resultant—must satisfy the condition of being zero. This is exactly what SATHEE IITK and NASA GRC highlight when defining the physical state of a system where no acceleration occurs.

To arrive at the correct answer, think like a physicist: if a force is a push or pull, then multiple forces can either reinforce each other or oppose each other. When they are balanced, it implies a perfect tug-of-war where neither side wins. Since the resultant force is simply the vector sum of all these interactions, its value must be (A) It is zero for the forces to be considered balanced. If the forces were non-zero, the object would inevitably undergo acceleration according to Newton’s Second Law, which contradicts the very definition of the forces being in a state of balance.

UPSC often uses options like "it varies continuously" (Option C) to distract students who might confuse dynamic equilibrium with varying forces. Remember, even if individual forces change, for the system to remain balanced at any given moment, their sum must remain zero. A non-zero resultant (Option B) is the hallmark of unbalanced forces, which is the primary cause of a change in an object's state of motion. By mastering this distinction, you avoid the common trap of overcomplicating a foundational definition. Always look for that state of net zero effect when you encounter the word "balanced" in your exam.