Which one of the following remains constant while throwing a ball upward?

1. Basics of Motion: Scalar vs. Vector Quantities (basic)

In our journey to master mechanics, we must first understand how to describe the physical world. Every physical quantity we measure falls into one of two categories based on how much information it carries: Scalar or Vector. This distinction is the bedrock of physics because it determines how we add, subtract, and analyze the movement of objects.

Scalar quantities are the simpler of the two. They are fully described by their magnitude (size or numerical value) alone. Think of mass, time, or temperature. If you are told the temperature is 30°C, you have all the information you need. In geography, for instance, we note that the distance between two latitudes remains constant everywhere—this distance is a scalar value INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT 2025 ed.), India — Location, p.2. It tells us "how much" space is between two lines, but not the direction of travel.

Vector quantities, however, require both magnitude and direction to be fully understood. Imagine someone tells you a treasure is 5 kilometers away. Without a direction (North, South, up, or down), that information is incomplete. Force is a primary example of a vector. When we measure force in newtons (N), we must also know which way it is being applied Science, Class VIII. NCERT (Revised ed 2025), Exploring Forces, p.65. Whether you are pushing a bag upward or dragging it across a rough surface where friction opposes your movement, the direction is vital to calculating the outcome Science, Class VIII. NCERT (Revised ed 2025), Exploring Forces, p.68.

To help you distinguish them in mechanics, compare Distance (Scalar) with Displacement (Vector). If you walk 5 meters forward and 5 meters back, your total distance covered is 10 meters, but your displacement—your change in position from the start—is zero. Vectors account for the path and direction, while scalars only account for the total amount.

Feature	Scalar Quantities	Vector Quantities
Definition	Magnitude only	Magnitude + Direction
Examples	Mass, Time, Distance, Speed	Force, Velocity, Acceleration, Displacement
Change	Changes with magnitude change	Changes with magnitude OR direction change

Sources: INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT 2025 ed.), India — Location, p.2; Science, Class VIII. NCERT (Revised ed 2025), Exploring Forces, p.65; Science, Class VIII. NCERT (Revised ed 2025), Exploring Forces, p.68

2. Newton’s Laws of Motion (basic)

At its most fundamental level, force is simply a push or a pull on an object resulting from its interaction with another object. While we often think of force as something we apply with our muscles (a contact force), it can also act over a distance without any physical touch (a non-contact force), such as gravity or magnetism Science, Class VIII, Chapter 5, p. 77. The strength or magnitude of this force is measured in the SI unit called the newton (N) Science, Class VIII, Chapter 5, p. 65.

Newton’s laws describe how these forces change the motion of objects. A force can change an object's speed, its direction of motion, or even its physical shape Science, Class VIII, Chapter 5, p. 77. For instance, if the speed of an object moving in a straight line keeps changing, it is in non-uniform linear motion, meaning a force is actively working on it Science, Class VII, Chapter 9, p. 118. A crucial application of this is weight. Many people confuse weight with mass, but weight is actually the force with which the Earth pulls an object toward itself Science, Class VIII, Chapter 5, p. 72. Because weight is a force, it is also measured in newtons (N).

Force Type	Definition	Examples
Contact Force	Requires physical interaction between surfaces.	Friction, Muscular force
Non-contact Force	Acts through a field without physical touch.	Gravity, Magnetic force, Electrostatic force

When we apply these laws to a specific scenario, like throwing a ball vertically upward, we see Newton's principles in their purest form. Once the ball leaves your hand, the only significant force acting on it is gravity (a non-contact force). Since this force (the ball's weight) is constant, the acceleration it produces remains constant in both magnitude and direction throughout the entire flight. However, because the force is pulling against the upward movement, the ball’s velocity decreases until it stops at the peak and then increases as it falls. Thus, while speed and direction change, the underlying gravitational pull—and therefore the acceleration—remains steady.

Sources: Science, Class VIII, Chapter 5: Exploring Forces, p.65; Science, Class VIII, Chapter 5: Exploring Forces, p.72; Science, Class VIII, Chapter 5: Exploring Forces, p.77; Science, Class VII, Chapter 9: Measurement of Time and Motion, p.118

3. Understanding Acceleration (intermediate)

To understand acceleration, we must first distinguish it from velocity. While velocity tells us how fast an object is moving in a specific direction, acceleration describes the rate of change of that velocity. In simpler terms, if an object is speeding up, slowing down, or even just changing its direction, it is accelerating. If an object moves at a perfectly steady speed in a straight line, its motion is 'uniform,' and its acceleration is zero Science-Class VII . NCERT(Revised ed 2025), Measurement of Time and Motion, p.117. However, in our daily lives, most motion is 'non-uniform' because forces—like a push, a pull, or even friction—are constantly acting upon objects to change their speed Science ,Class VIII . NCERT(Revised ed 2025), Exploring Forces, p.67.

A fascinating way to observe acceleration is by looking at vertical motion. Imagine throwing a ball straight up into the air. As it rises, its velocity gradually decreases until it reaches the very top, where it momentarily stops (velocity = 0). It then begins its descent, gaining speed as it falls. Throughout this entire journey—as it goes up, pauses, and comes down—the ball is under the constant influence of gravity. This gravitational pull provides a constant downward acceleration (approximately 9.8 m/s²). It is a common misconception that acceleration vanishes at the peak because the ball stops moving for a split second; in reality, the force of gravity never 'takes a break.' The acceleration remains steady in both magnitude and direction, even as the ball's velocity and kinetic energy fluctuate.

Parameter	During Upward Flight	At the Peak	During Downward Flight
Velocity	Decreasing	Zero	Increasing
Acceleration (g)	Constant (Downward)	Constant (Downward)	Constant (Downward)

Sources: Science-Class VII . NCERT(Revised ed 2025), Measurement of Time and Motion, p.117; Science ,Class VIII . NCERT(Revised ed 2025), Exploring Forces, p.67

4. Mechanical Energy: Kinetic and Potential (intermediate)

Mechanical Energy is the capacity of an object to do work due to its motion or its position. It is the sum of two distinct forms: Kinetic Energy (KE) and Potential Energy (PE). At its most fundamental level, energy is a measure of a system's ability to undergo change or perform work Environment and Ecology, Majid Hussain, p.14. When we look at a physical object, like a ball being thrown into the air, we are observing a continuous conversion between these two types of energy.

Kinetic Energy is the energy an object possesses because it is moving. It is calculated using the formula KE = ½mv² (where m is mass and v is velocity). This means that any change in speed significantly affects the kinetic energy Environment and Ecology, Majid Hussain, p.8. Conversely, Potential Energy is "stored" energy based on an object's position or configuration. In basic mechanics, we focus on Gravitational Potential Energy, calculated as PE = mgh (where m is mass, g is acceleration due to gravity, and h is height). As an object rises, its potential energy increases because its position relative to the Earth changes.

Consider the journey of a ball thrown vertically upward. As it leaves your hand, it has maximum velocity and thus maximum Kinetic Energy. As it rises, gravity acts against its motion with a constant downward acceleration of approximately 9.8 m/s² Science, Class VIII NCERT, p.72. This causes the ball to slow down, meaning its KE decreases. However, that energy isn't lost; it is converted into PE as the ball gains height. At the peak of its flight, the velocity momentarily becomes zero, meaning KE is at its minimum (zero), while PE reaches its maximum.

Stage of Motion	Kinetic Energy (KE)	Potential Energy (PE)	Acceleration (g)
Launch	Maximum	Minimum (Zero)	Constant (9.8 m/s²)
Ascending	Decreasing	Increasing	Constant (9.8 m/s²)
At the Peak	Minimum (Zero)	Maximum	Constant (9.8 m/s²)

Sources: Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.14; Environment and Ecology, Majid Hussain, BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY, p.8; Science, Class VIII NCERT, Exploring Forces, p.72

5. Gravity and the Universal Law of Gravitation (intermediate)

At its core, Gravity is not just a force that pulls things to the ground; it is a fundamental property of matter that ensures every object in the universe with mass attracts every other object. This concept reached its scientific climax with Isaac Newton’s theory of gravitation, which revolutionized our understanding of the physical world Themes in world history, History Class XI (NCERT 2025 ed.), Changing Cultural Traditions, p.119. According to Newton's Universal Law of Gravitation, the force of attraction between two bodies is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers (F = G · m₁ · m₂ / r²). Here, G is the Universal Gravitational Constant, a value that remains the same everywhere in the cosmos.

In a UPSC context, it is vital to distinguish between Mass and Weight, as these terms are often confused in daily speech. Mass is the actual quantity of matter present in an object and remains constant regardless of where the object is located. Weight, however, is the specific gravitational force with which a planet pulls an object toward itself Science, Class VIII. NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.142. Because the strength of gravity varies slightly across the Earth's surface and significantly between different planets, your weight would change if you traveled to the Moon, but your mass would remain identical Science, Class VIII. NCERT (Revised ed 2025), Exploring Forces, p.75.

Feature	Mass	Weight
Definition	Quantity of matter in an object.	Force of gravity acting on an object.
Nature	Intrinsic property; constant everywhere.	Variable; depends on local gravity.
Unit	Kilogram (kg)	Newton (N)

When an object is in free fall or moving vertically, it experiences a constant downward acceleration known as acceleration due to gravity (g). On Earth, this is approximately 9.8 m/s². While the velocity of a ball thrown upward changes every second (slowing down as it rises and speeding up as it falls), the acceleration itself remains constant in both magnitude and direction throughout the flight. Beyond Newton, modern physics describes gravity through Einstein's General Theory of Relativity, which views gravity as a curvature or "ripple" in the fabric of spacetime caused by massive objects Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.4.

Sources: Themes in world history, History Class XI (NCERT 2025 ed.), Changing Cultural Traditions, p.119; Science, Class VIII. NCERT (Revised ed 2025), The Amazing World of Solutes, Solvents, and Solutions, p.142; Science, Class VIII. NCERT (Revised ed 2025), Exploring Forces, p.75; Physical Geography by PMF IAS, The Universe, The Big Bang Theory, Galaxies & Stellar Evolution, p.4

6. Vertical Motion Under Gravity (Free Fall) (exam-level)

When we observe an object moving vertically—whether it is a ball thrown upward or a stone dropped from a balcony—we are witnessing **Vertical Motion Under Gravity**. According to the principles of mechanics, when an object is thrown upward, it undergoes a predictable cycle: it moves up straight, slows down, stops momentarily at the peak, and then accelerates downward Science, Class VIII, NCERT (2025), Chapter 5, p.72. Throughout this entire journey, the object's **velocity** and **displacement** are constantly changing. However, there is one critical constant: the **acceleration due to gravity (g)**. Even at the very top of the flight where the object's speed is zero, gravity is still pulling it downward at approximately 9.8 m/s². While we treat 'g' as a constant in basic physics problems, a deeper look reveals that gravity is not uniform across the entire planet. The Earth is not a perfect sphere; it is wider at the equator. Consequently, gravity is **greater near the poles** (closer to the Earth's center) and **less at the equator** Fundamentals of Physical Geography, Geography Class XI (NCERT 2025 ed.), Chapter 2, p.19. Furthermore, geophysicists identify **gravity anomalies**, which are variations in the expected gravity value caused by the uneven distribution of mass within the Earth's crust Physical Geography by PMF IAS, Earths Interior, p.58. These anomalies are particularly notable in oceanic trenches where subduction occurs, indicating a local loss of mass Physical Geography by PMF IAS, Tectonics, p.108. To master this concept for the exam, you must distinguish between the kinematics of the flight (velocity and position) and the force causing it (gravity). During the upward phase, the kinetic energy decreases as it converts into potential energy, reaching its minimum at the highest point. Yet, through all these changes, the direction and magnitude of the gravitational acceleration remain steadfastly downward toward the Earth's center.

Sources: Science, Class VIII, NCERT (2025), Chapter 5: Exploring Forces, p.72; Fundamentals of Physical Geography, Geography Class XI (NCERT 2025 ed.), Chapter 2: The Origin and Evolution of the Earth, p.19; Physical Geography by PMF IAS, Earths Interior, p.58; Physical Geography by PMF IAS, Tectonics, p.108

7. Solving the Original PYQ (exam-level)

Now that you have mastered the building blocks of kinetics and Newton’s Laws of Motion, this question serves as a perfect test of your conceptual clarity. In your previous lessons, you learned that an object's motion is governed by the forces acting upon it. When a ball is thrown upward, it enters a state of free fall (if we neglect air resistance), meaning the only force at play is the Earth's gravitational pull. Because the mass of the ball and the force of gravity remain essentially unchanged near the surface, the resulting acceleration must remain constant throughout the entire journey—on the way up, at the very peak, and on the way down.

To arrive at the correct answer, (C) Acceleration, think like a physicist: force equals mass times acceleration ($F=ma$). Since the force of gravity is a steady downward pull, the acceleration ($g$) stays fixed at approximately 9.8 m/s². Conversely, velocity must change; it decreases as the ball rises and momentarily hits zero at the peak. Because kinetic energy depends directly on the square of that velocity ($KE = ½mv²$), it also fluctuates, reaching its minimum at the highest point. Finally, displacement is a measure of the ball's position relative to your hand; since the ball is moving, its position is constantly shifting, making it impossible for displacement to remain constant.

UPSC often uses these options to trip up students who confuse motion with the rate of change of motion. Always remember: just because an object slows down or stops does not mean the force (and thus the acceleration) driving that change has disappeared. This fundamental principle is detailed in Science, Class VIII. NCERT (Revised ed 2025) > Chapter 5: Exploring Forces, where it emphasizes that gravity acts on all objects regardless of their state of motion.