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Revision: Gravitation >> Gravitation Physics Science (English Medium) Class 11 CBSE

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Definitions [9]

Definition: Universal Law of Gravitation

"Every particle of matter attracts every other particle of matter with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them."

Answer the following question in detail.

What is a critical velocity?

The exact horizontal velocity of projection that must be given to a satellite at a certain height so that it can revolve in a circular orbit round the Earth is called the critical velocity or orbital velocity (vc).

Definition: Satellite

The objects that revolve around the Earth are called Earth satellites.

Definition: Artificial Satellite

A man-made object that orbits a planet or other celestial body is called an artificial satellite.

Definition: Geosynchronous Satellite

A satellite that orbits the Earth at a height of approximately 36,000 km above the equator with a period of revolution of 24 hours is called a geosynchronous satellite.

Definition: Polar Satellite

A satellite that travels over Earth's poles, passing close to the Earth's surface, with a period of revolution of nearly 85 minutes is called a polar satellite.

Answer the following question.

Define the binding energy of a satellite.

The minimum energy required by a satellite to escape from Earth’s gravitational influence is the binding energy of the satellite.

Definition: Binding Energy of Satellite

"The minimum energy required by a satellite to escape from Earth’s gravitational influence is the binding energy of the satellite."

Definition: Escape velocity

"The minimum velocity with which a body should be thrown vertically upwards from the surface of the Earth so that it escapes the Earth’s gravitational field, is called the escape velocity (ve) of the body."

Formulae [6]

Formula: Kepler's Law

Kepler’s Third Law relates the time period T of a planet’s revolution to the semi-major axis a of its elliptical orbit:
T2 ∝ a3
where,

  • T = time period of revolution of the planet,
  • a = semi-major axis of the elliptical orbit.
Formula: Universal Law of Gravitation

The gravitational force of attraction (F) between two bodies of mass m1 and m2 separated by a distance r is:

\[\mathbf{F} = \mathbf{G}\frac{m_1 m_2}{r^2}\]
  • F: Gravitational Force of attraction (in Newtons, N).

  • \[m_1, m_2\]: Masses of the two objects (in kilograms, kg).

  • r (or d in the first part): Distance between the two objects (in meters, m).

  • G: The constant of proportionality, called the Universal gravitational constant.

    • Value in SI units: \[G=6.67\times10^{-11}\mathrm{N}\cdot\mathrm{m}^2/\mathrm{kg}^2\]

    • Dimensions: \[[G]=[\mathrm{L}^3\mathrm{M}^{-1}\mathrm{T}^{-2}]\]

Formula: Gravity with Altitude

The formulas for acceleration due to gravity (g) are provided below:

On the Earth's Surface:

\[g = \frac{G M}{R^2}\]

At height $h$ above the Earth's Surface:

\[g_h = g \frac{R^2}{(R+h)^2} \quad \text{or} \quad g_h = g \left(I + \frac{h}{R}\right)^{-2}\]

Simplified Formula for Small Altitudes ($h \ll R$):

\[g_h = g \left(1 - \frac{2h}{R}\right)\]

Definition of Terms:

  • g: Acceleration due to gravity on the Earth's surface.
  • gh: Acceleration due to gravity at height h above the Earth's surface.
  • G: Universal Gravitational Constant.
  • M: Mass of the Earth.
  • R: Radius of the Earth.
  • h: Altitude or height above the Earth's surface.
Formula: Gravitational Potential Energy

U(r) = -\[\frac {GMm}{r}\]

Where:

  • U(r) = Gravitational potential energy at distance r from Earth's center
  • G = Universal gravitational constant (6.67 × 10⁻¹¹ N·m²/kg²)
  • M = Mass of Earth (kg)
  • m = Mass of the object (kg)
  • r = Distance between the centers of mass of Earth and object (m)
  • Negative sign = Shows that potential energy is negative (zero at infinity)
Formula: Binding Energy

Where:

  • G = Universal Gravitational Constant
  • M = Mass of the Earth
  • m = Mass of the satellite
  • r = Radius of the orbit (Distance from the center of the Earth)
Formula: Escape velocity

\[v_e=\sqrt{\frac{2GM}{R}}\]

  • ve = Escape velocity (minimum speed needed to escape Earth’s gravity)
  • G = Universal gravitational constant (6.674 × 10−11 Nm2/kg2)
  • M = Mass of the Earth (or celestial body)
  • R = Radius of the Earth (or distance from the centre of the mass to the object)

Theorems and Laws [1]

Law: Universal Law of Gravitation

Statement:

The law which states that every particle of matter attracts every other particle in the universe with a force whose magnitude is directly proportional to the product of masses and inversely proportional to the square of distance between them is called Newton's Law of Gravitation.

Derivation:

Newton's Universal Law of Gravitation states that every particle of matter attracts every other particle of matter with a force which is:

  • Directly proportional to the product of their masses: F ∝ m1 ⋅ m2
  • Inversely proportional to the square of the distance between them: F ∝ \[\frac {1}{r^2}\]

Combining both, the gravitational force is expressed as:

F = G\[\frac{m_1m_2}{r^2}\]

where G is the Universal Gravitational Constant, measured by Henry Cavendish using the Cavendish balance, with the value:

G = 6.67 × 10−11Nm2/kg2

Key Points

Key Points: Newton's Universal Law of Gravitation
  • Every object attracts every other with a gravitational force.
  • Force increases with mass — more mass means a stronger pull.
  • Force decreases with distance — doubling the distance halves the force.
  • A force acts along the line joining the centres (or centres of mass) of the two bodies.
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