What is the difference between mass and weight?

Mass is an intrinsic property of an object, representing the amount of matter it contains and its inertia. It is a scalar quantity, measured in kilograms (kg), and remains constant regardless of location. Weight, on the other hand, is the force exerted on an object due to gravity. It is a vector quantity, measured in Newtons (N), and varies depending on the strength of the gravitational field. For instance, an astronaut's mass remains the same on Earth and the Moon, but their weight on the Moon would be about one-sixth of their Earth weight due to the Moon's weaker gravity.

Why is the gravitational force considered the weakest fundamental force, even though it governs planetary motion?

Gravitational force is indeed the weakest among the four fundamental forces (gravitational, electromagnetic, strong nuclear, and weak nuclear). Its weakness is evident when comparing it to the electromagnetic force; a small magnet can lift a paperclip against the entire gravitational pull of the Earth. However, gravity becomes dominant on astronomical scales because it is always attractive, has an infinite range, and acts on all matter. Unlike electromagnetic forces, which can be screened or canceled out due to positive and negative charges, gravity's effects accumulate over vast masses, making it the primary force shaping planets, stars, and galaxies.

Does the value of the universal gravitational constant 'G' change with altitude or depth?

No, the universal gravitational constant 'G' is a fundamental constant of nature and its value remains constant throughout the universe, irrespective of location, medium, or the nature of the interacting bodies. It is distinct from 'g', the acceleration due to gravity, which does vary with altitude, depth, latitude, and the rotation of the Earth. 'G' is a fixed proportionality constant in Newton's Law of Universal Gravitation, reflecting the inherent strength of the gravitational interaction itself.

What is the significance of Kepler's third law, $T^2 propto a^3$?

Kepler's third law, also known as the Law of Periods, establishes a mathematical relationship between the orbital period ($T$) of a planet and the semi-major axis ($a$) of its elliptical orbit. It implies that planets farther from the Sun have longer orbital periods and move slower on average. This law was crucial because it provided a quantitative framework for planetary motion, which Newton later used to derive his Law of Universal Gravitation. It also allows scientists to calculate the orbital parameters of celestial bodies if one of the values is known, and is fundamental for understanding satellite orbits.

Can an object ever truly escape Earth's gravity?

Technically, no. Gravitational force has an infinite range, meaning it never truly becomes zero, no matter how far you go. However, the force diminishes rapidly with distance (inverse square law). When we talk about 'escaping Earth's gravity' (escape velocity), we mean achieving a velocity such that the object's kinetic energy is sufficient to overcome the gravitational potential energy, allowing it to move infinitely far away from Earth without ever falling back. At this point, its velocity relative to Earth would approach zero as its distance from Earth approaches infinity, effectively 'escaping' its dominant gravitational influence.

Why do astronauts experience weightlessness in space, even though Earth's gravity is still acting on them?

Astronauts in orbit are not in a region of 'zero gravity'. At the altitude of the International Space Station (ISS), Earth's gravity is still about 90% as strong as it is on the surface. The sensation of weightlessness arises because the astronauts and their spacecraft are continuously in a state of freefall around the Earth. They are constantly falling towards Earth, but their high tangential velocity keeps them moving horizontally enough to miss the Earth, thus maintaining an orbit. Since there's no surface pushing back against them (like the ground or a chair), they experience no normal force, leading to the sensation of weightlessness. It's an apparent weightlessness, not an actual absence of gravity.

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Version 1Updated 22 Mar 2026

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Gravitation is a fundamental natural phenomenon by which all things with mass or energy – including planets, stars, galaxies, and even light – are attracted to (or gravitate toward) one another. It is one of the four fundamental interactions of nature, alongside the strong nuclear force, the weak nuclear force, and electromagnetism. In classical physics, it is described by Newton's Law of Universa…

Quick Summary

Gravitation is the universal attractive force between any two objects with mass. Newton's Law of Universal Gravitation states that this force is directly proportional to the product of their masses ( $m_1 m_2$ ) and inversely proportional to the square of the distance ( $r^2$ ) between their centers, given by $F_g = G \frac{m_1 m_2}{r^2}$ .

Here, $G$ is the universal gravitational constant ( $6.674 \times 10^{-11},\text{N m}^2/\text{kg}^2$ ). The acceleration due to gravity ( $g$ ) on Earth's surface is approximately $9.8,\text{m/s}^2$ , and it varies with altitude, depth, latitude, and Earth's rotation.

Gravitational potential ( $V_g = -GM/r$ ) is the potential energy per unit mass, and gravitational potential energy ( $U_g = -GMm/r$ ) is the energy stored in a system of two masses. Escape velocity ( $v_e = sqrt{2GM/R}$ ) is the minimum speed needed to escape a planet's gravity, while orbital velocity ( $v_o = sqrt{GM/r}$ ) is the speed required to maintain a stable orbit.

Kepler's laws describe planetary motion: elliptical orbits, equal areas swept in equal times, and $T^2 propto a^3$ . These principles explain everything from falling objects to satellite motion and the structure of galaxies.

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Key Concepts

Variation of 'g' with Altitude

As an object moves to a height $h$ above the Earth's surface, the distance from the center of the Earth…

Gravitational Potential Energy

Gravitational potential energy ( $U$ ) represents the work done to bring a mass from infinity to a specific…

Relationship between Escape Velocity and Orbital Velocity

For an object orbiting very close to the surface of a planet (where orbital radius $r approx R$ ), the orbital…

Newton's Law: —

F = G \frac{m_1 m_2}{r^2}

Universal Gravitational Constant: —

G = 6.67 \times 10^{-11},\text{N m}^2/\text{kg}^2

Acceleration due to gravity (surface): —

g = \frac{GM_E}{R_E^2} approx 9.8,\text{m/s}^2

Variation of g (altitude): —

g_h = g left(1 - \frac{2h}{R_E}\right)

(for

h ll R_E

)

Variation of g (depth): —

g_d = g left(1 - \frac{d}{R_E}\right)

Gravitational Potential Energy: —

U = -\frac{GMm}{r}

Gravitational Potential: —

V = -\frac{GM}{r}

Escape Velocity: —

v_e = sqrt{\frac{2GM}{R}} = sqrt{2gR}

Orbital Velocity: —

v_o = sqrt{\frac{GM}{r}} = sqrt{\frac{gR^2}{r}}

Relationship: —

v_e = sqrt{2} v_o

(near surface)

Kepler's 3rd Law: —

T^2 propto a^3

To remember the variations of 'g': All Deep Layers Rotate Slowly.

Altitude: 'g' decreases.

Depth: 'g' decreases.

Latitude: 'g' minimum at equator, maximum at poles.

Rotation: Reduces 'g' at equator.

Shape (oblate spheroid): 'g' less at equator due to larger radius.

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