The weight of any object is felt due to gravity of Earth. When any object goes inside the Earth or above the Earth, weight decreases. It will weigh minimum when an object is placed at :

1. Mass vs. Weight: The Fundamental Distinction (basic)

In our daily lives, we often use the terms mass and weight interchangeably, but in the world of physics, they represent two very different concepts. Mass is the measure of the total quantity of matter contained within an object Science, Class VIII. NCERT (Revised ed 2025), Chapter 9, p.142. Think of it as the 'stuff' an object is made of; because that amount of 'stuff' doesn't change based on where the object is, mass remains constant throughout the universe. Its standard units are the gram (g) and kilogram (kg).

Weight, however, is not a measure of matter, but a measure of force — specifically, the gravitational force with which a celestial body like the Earth pulls an object toward itself Science, Class VIII. NCERT (Revised ed 2025), Chapter 5, p.75. Since weight is a force, it is measured in Newtons (N) Science, Class VIII. NCERT (Revised ed 2025), Chapter 5, p.77. Because gravity varies depending on where you are, your weight can change while your mass stays exactly the same. For instance, on the Moon, where gravity is much weaker, you would weigh only about one-sixth of what you do on Earth, yet your body would still contain the same amount of matter.

Feature	Mass	Weight
Definition	The quantity of matter in an object.	The gravitational force acting on an object.
SI Unit	Kilogram (kg)	Newton (N)
Constancy	Remains constant everywhere.	Varies depending on local gravity.
Measurement	Measured using a two-pan balance.	Measured using a spring balance.

There are extreme cases where weight can even drop to zero. At the center of the Earth, the gravitational pull from all directions cancels out, making an object effectively weightless. Similarly, astronauts in an orbiting satellite experience weightlessness because they are in a state of continuous free fall, meaning there is no supporting surface (like a floor) to push back against them Science, Class VIII. NCERT (Revised ed 2025), Chapter 5, p.75. Even on the Earth's surface, minor variations exist; you actually weigh about 1% less at the Equator than at the Poles due to the Earth's bulge and centrifugal force.

Sources: Science, Class VIII. NCERT (Revised ed 2025), Chapter 5: Exploring Forces, p.75, 77; Science, Class VIII. NCERT (Revised ed 2025), Chapter 9: The Amazing World of Solutes, Solvents, and Solutions, p.142

2. Newton’s Universal Law of Gravitation (basic)

Imagine you throw a ball into the air; it always comes back down. This isn't just a quirk of nature; it is the result of a fundamental force called gravity. At its simplest level, gravity is an attractive force that acts between any two objects with mass. Unlike magnetic or electrostatic forces, which can both pull things together or push them apart, gravity only pulls Science, Class VIII (NCERT 2025), Exploring Forces, p.72. It is also a non-contact force, meaning the Earth can pull on a falling mango even before it touches the ground.

While we often think of gravity as just "Earth's pull," Isaac Newton revolutionized science by realizing it is universal Themes in world history, History Class XI (NCERT 2025), Changing Cultural Traditions, p.119. Every object in the universe—from the tiny pebble in your hand to the massive Sun—exerts a gravitational pull on everything else. However, the strength of this pull varies. For instance, the Sun is so massive that its surface gravity is about 28 times stronger than Earth's, while the Moon, being much smaller, has only about one-sixth of Earth's gravity Physical Geography by PMF IAS, The Solar System, p.23. This is why you would feel much lighter on the Moon, even though your body hasn't changed!

Celestial Body	Surface Gravity (m/s²)	Comparison to Earth
Sun	274	~28.0x Earth
Earth	9.8	1.0x Earth
Moon	1.62	~0.16x Earth (1/6th)

In the world of Geography, gravity is the "engine" behind almost everything we see on the landscape. Without gravity and the gradients (slopes) it acts upon, water wouldn't flow in rivers, and there would be no erosion or deposition of soil Fundamentals of Physical Geography, Geography Class XI (NCERT 2025), Geomorphic Processes, p.38. It is the force that keeps us firmly on the ground and governs the movement of all surface materials on Earth.

Sources: Science, Class VIII (NCERT 2025), Exploring Forces, p.72; Themes in world history, History Class XI (NCERT 2025), Changing Cultural Traditions, p.119; Physical Geography by PMF IAS, The Solar System, p.23; Fundamentals of Physical Geography, Geography Class XI (NCERT 2025), Geomorphic Processes, p.38

3. Variation of Gravity with Altitude and Depth (intermediate)

To understand how gravity changes, we must first look at the Earth's shape and mass distribution. Earth is not a perfect sphere; it is an oblate spheroid, meaning it bulges at the equator and is flattened at the poles. Because the distance from the Earth's center to the surface is greater at the equator than at the poles, the gravitational pull is slightly weaker at the equator and stronger at the poles FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), The Origin and Evolution of the Earth, p.19. Additionally, the uneven distribution of mass within the crust leads to slight variations known as gravity anomalies, which geologists use to map the materials beneath our feet Physical Geography by PMF IAS, Earths Interior, p.58.

When we move away from the surface into the atmosphere (increasing altitude), the value of gravity decreases. This is because gravity follows an inverse-square law: as the distance from the center of the Earth increases, the pull weakens. Conversely, when we go below the surface (increasing depth), gravity also decreases, but for a different reason. Even though you are closer to the center, the mass of the Earth 'above' you pulls you upward, cancelling out some of the pull from below. At the very center of the Earth, the gravitational pull from all directions cancels out completely, resulting in a net force of zero—making an object technically weightless Science, Class VIII NCERT (Revised ed 2025), Chapter 5: Exploring Forces, p.75.

Finally, we must distinguish between actual gravity and apparent weightlessness. In an orbiting satellite, gravity is still very much present (it’s what keeps the satellite in orbit!), but because the satellite and everything inside it are in a state of continuous free fall toward Earth, there is no supporting 'normal force' (like a floor) to push back against you. This creates the sensation of zero gravity. On other celestial bodies, the values change drastically; for instance, the Moon’s smaller mass means you would weigh only about one-sixth of your Earth weight Science, Class VIII NCERT (Revised ed 2025), Chapter 5: Exploring Forces, p.75.

Location	Gravitational Trend	Primary Reason
Poles	Maximum	Minimum distance to Earth's center.
Equator	Minimum (on surface)	Maximum distance from center + Centrifugal force.
High Altitude	Decreasing	Increased distance from the planet's mass.
Earth's Center	Zero	Mass pull from all directions cancels out.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), The Origin and Evolution of the Earth, p.19; Physical Geography by PMF IAS, Earths Interior, p.58; Science, Class VIII NCERT (Revised ed 2025), Chapter 5: Exploring Forces, p.75

4. Influence of Earth's Shape and Rotation (Equator vs. Poles) (intermediate)

To understand why your weight changes as you travel across the globe, we first need to look at the Earth's true shape. While we often imagine it as a perfect marble, it is actually a Geoid (or oblate spheroid). Because of the Earth's rotation over billions of years, it has developed a distinct equatorial bulge and a slight flattening at the poles Physical Geography by PMF IAS, Latitudes and Longitudes, p.241. This means if you are standing at the Equator, you are actually further away from the Earth's center of mass than if you were standing at the North or South Pole. Since the force of gravity weakens as distance from the center increases, the gravitational pull is naturally lower at the Equator.

Beyond the shape, the rotation of the Earth introduces another critical factor: centrifugal force. As the Earth spins, it generates an outward-acting force that is strongest at the Equator (where the rotational speed is highest) and effectively zero at the poles. This centrifugal force acts in direct opposition to gravity, 'lifting' you slightly and further reducing your net weight at the lower latitudes. Consequently, the acceleration due to gravity (g) is at its maximum at the poles and its minimum at the Equator Physical Geography by PMF IAS, Latitudes and Longitudes, p.241.

Feature	At the Equator	At the Poles
Distance from Center	Greater (Bulge)	Lesser (Flattened)
Centrifugal Force	Maximum (acts against gravity)	Zero
Weight of an Object	Minimum (~1% less)	Maximum

Interestingly, weight is not just about latitude. If you were to travel to the center of the Earth, you would experience complete weightlessness because the mass of the Earth would be pulling you equally in every direction, resulting in a net gravitational force of zero Science Class VIII NCERT (2025), Exploring Forces, p.75. A similar sensation occurs in an orbiting satellite; even though gravity is still pulling on the craft to keep it in orbit, the state of continuous 'free fall' means there is no floor pushing back against you, creating apparent weightlessness Science Class VIII NCERT (2025), Exploring Forces, p.78.

Sources: Physical Geography by PMF IAS, Latitudes and Longitudes, p.241; Science Class VIII NCERT (2025), Exploring Forces, p.75; Science Class VIII NCERT (2025), Exploring Forces, p.78

5. Orbital Mechanics and Kepler’s Laws (intermediate)

To understand how celestial bodies and satellites move, we look to the three fundamental laws proposed by Johannes Kepler. The first law, the Law of Orbits, clarifies that planets do not move in perfect circles; instead, they follow an elliptical path with the Sun located at one of the two 'foci' of the ellipse Physical Geography by PMF IAS, The Solar System, p.21. This means the distance between a planet and the Sun is constantly changing as it completes its revolution. Kepler’s second law, the Law of Areas, explains the variation in orbital speed. It states that a line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time. To maintain this equal area, a planet must travel faster when it is closer to the Sun (at perihelion) and slower when it is further away (at aphelion) Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.257. This has real-world consequences: because Earth is furthest from the Sun during the Northern Hemisphere's summer, its orbital velocity is at its lowest, making summer (approx. 92 days) slightly longer than winter (approx. 89 days) Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.256. Finally, we must distinguish between gravity and the sensation of weight. In an orbiting satellite, objects experience apparent weightlessness. This isn't because gravity is absent—gravity is the very force keeping the satellite in orbit—but because the satellite and its occupants are in a state of continuous free fall toward Earth. Without a supporting surface (like a floor) to push back against you, the 'weight' is not felt Science, Class VIII. NCERT, Exploring Forces, p.75. A similar state of zero net gravitational force occurs at the center of the Earth, where the gravitational pull from all directions cancels out.

Term	Position	Orbital Speed
Perihelion / Perigee	Closest to Sun / Earth	Maximum Speed
Aphelion / Apogee	Farthest from Sun / Earth	Minimum Speed

Sources: Physical Geography by PMF IAS, The Solar System, p.21; Physical Geography by PMF IAS, The Motions of The Earth and Their Effects, p.256-257; Science, Class VIII. NCERT, Exploring Forces, p.75

6. Escape Velocity and Gravitational Potential (exam-level)

To understand why rockets need massive engines or why the Moon lacks an atmosphere, we must look at Gravitational Potential and Escape Velocity. Think of a planet’s gravity as a 'well' in space. To get out of this well, an object needs energy. Gravitational Potential is the measure of potential energy per unit mass at a point in the gravitational field. Because gravity is an attractive force, we consider the potential to be zero at an infinite distance; consequently, the potential near a planet is always negative. The 'deeper' the potential well (determined by the planet's mass), the harder it is to climb out.

Escape Velocity is the minimum speed an object must reach to break free from this 'well' forever, without further propulsion. It is calculated using the formula vₑ = √(2GM/R), where G is the gravitational constant, M is the mass of the body, and R is its radius. Crucially, escape velocity does not depend on the mass of the escaping object—a tiny molecule and a massive spaceship both need the same speed to escape Earth's pull, which is approximately 11.2 km/s.

This concept explains the composition of planetary atmospheres. In the upper layers of our atmosphere (the exosphere), light gases like Hydrogen and Helium can gain enough thermal energy to reach escape velocity and 'leak' into space Physical Geography by PMF IAS, Earths Atmosphere, p.280. Since the Moon has a much smaller mass, its escape velocity is very low (about 2.4 km/s), making it impossible for it to retain any significant atmosphere—gas molecules simply fly away Physical Geography by PMF IAS, The Solar System, p.28.

Feature	Earth	Moon
Mass/Gravity	Higher (1g)	Lower (~1/6th of Earth)
Escape Velocity	~11.2 km/s	~2.4 km/s
Atmosphere	Rich (O₂, N₂)	Virtually none

Sources: Physical Geography by PMF IAS, Earths Atmosphere, p.280; Physical Geography by PMF IAS, The Solar System, p.28

7. The Concept of Apparent Weightlessness (exam-level)

To understand weightlessness, we must first distinguish between mass (the amount of matter in an object) and weight (the force of gravity acting on that mass). In our daily lives, what we perceive as 'weight' is actually the normal force—the push-back from the floor or a chair that prevents us from falling toward the center of the Earth. When this supporting force is absent, we experience the sensation of weightlessness, even if gravity is still present Science, Class VIII. NCERT (Revised ed 2025), Chapter 5: Exploring Forces, p. 75. There are two primary scenarios where an object experiences a net weight of zero. First, at the center of the Earth, the gravitational pull from the surrounding mass of the planet acts equally in every direction. These forces cancel each other out, resulting in a net gravitational force of zero. Second, in an orbiting satellite, objects experience what we call apparent weightlessness. The satellite and everything inside it are in a state of continuous free fall toward Earth. Because they are falling at the exact same rate, the floor of the satellite does not push back against the astronaut, removing the sensation of weight Science, Class VIII. NCERT (Revised ed 2025), Chapter 5: Exploring Forces, p. 78. It is a common misconception that gravity does not exist in space. In reality, gravity is what keeps a satellite in orbit; without it, the satellite would fly off in a straight line. Satellites are often placed in the exosphere, where the air is extremely thin, to minimize atmospheric drag which would otherwise slow them down and cause them to crash Physical Geography by PMF IAS, Earths Atmosphere, p. 280. On the Earth's surface, weight also varies slightly: it is greatest at the poles and least at the equator because the Earth is not a perfect sphere; the equator is farther from the center and experiences higher centrifugal forces FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), The Origin and Evolution of the Earth, p. 19.

Sources: Science, Class VIII. NCERT (Revised ed 2025), Chapter 5: Exploring Forces, p.75, 78; Physical Geography by PMF IAS, Earths Atmosphere, p.280; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), The Origin and Evolution of the Earth, p.19

8. Solving the Original PYQ (exam-level)

This question bridges your fundamental understanding of gravitational force and weight. You have recently learned that while mass remains constant, weight is a force ($W = mg$) that depends on the gravitational acceleration of your specific location. This problem tests your ability to identify the extreme limits of these variations—specifically, identifying the unique environments where the net gravitational pull or the normal reaction force effectively disappears.

To arrive at the correct answer, you must look for the condition of weightlessness. At the Centre of Earth, the mass of the planet surrounds you uniformly, causing the gravitational pull from all directions to cancel out, resulting in a net force of zero. In an orbiting satellite, although gravity is still present to maintain the orbit, you are in a state of perpetual free fall. Because there is no supporting surface to provide a normal force against your body, your weight is not "felt." As highlighted in Science, Class VIII. NCERT (Revised ed 2025), this state of zero net force makes (D) Centre of Earth and in an orbiting satellite the only option representing the absolute minimum weight.

UPSC often includes comparative reductions as traps to distract you from the absolute minimum. For instance, in Option (A), the Moon only reduces weight to 1/6th, and the Equator reduces it by a mere 1%—neither is zero. Option (B) is a "reversal trap"; the North Pole is actually where weight is at its maximum because you are closest to the Earth's center. Similarly, Option (C) mentions being "1 km beneath the Earth," which causes a negligible decrease compared to the total weightlessness found at the center. Always distinguish between a slight decrease and the theoretical minimum of zero.