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Revision: Class 11 >> Gravitation NEET (UG) Gravitation

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Definitions [5]

Definition: Gravitation

"Every particle of matter in the universe attracts every other particle with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them. The direction of the force is along the line joining the particles."

or

The force by which the Earth attracts objects towards its centre is called gravitational force.

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."

Definition: Potential Energy

"Potential energy is the work done against conservative force (or forces) in achieving a certain position or configuration of a given system."

OR

The energy stored in an object because of its position or state is called potential energy.

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."

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."

Formulae [8]

Formula: Gravitation

Newton’s Universal Law of Gravitation:
F = \[G\frac{m_1m_2}{r^2}\]

where:

  • F = Gravitational force between two objects
  • m1,m2 = Masses of the two objects
  • r = Distance between the centers of the two masses
  • G = Universal gravitational constant = 6.67×10−11 Nm2/kg2
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: Kepler's Second Law

The area swept by the planet of mass m in a given interval Δt is:

\[\Delta\vec{A}=\frac{1}{2}(\vec{r}\times\vec{v}\Delta t)\]

  • \[\vec r\]: Position vector of the planet (distance from Sun).
  • \[\vec v\]: Velocity vector of the planet.
  • Δt: Time interval.
  • \[\vec p\]: Linear momentum (\[\vec p\] = m\[\vec v\])
  • \[\vec L\]: Angular momentum (\[\vec L\] = \[\vec r\] × \[\vec p\])
Formula: Potential Energy

Based on the relationship between work and energy, the change in potential energy is given by:

\[\vec F\] · d\[\vec x\] = dU

  • \[\vec{F}\]: The force acting on the object (external force applied against the conservative force).
  • \[d\vec{x}\]: The small displacement of the object.
  • dU: The change (increase) in the potential energy of the system.
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)
Formula: Time Period of Satellite

T = \[2\pi\sqrt{\frac{(R+h)^3}{GM}}\]

Where:

  • T = Time period of the satellite (in seconds)
  • R = Radius of the Earth
  • h = Height of the satellite above Earth's surface
  • G = Universal gravitational constant
  • M = Mass of the Earth
  • (R + h) = r = Radius of the satellite's orbit
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)

Theorems and Laws [1]

Law: Kepler's Second Law

Kepler's Second Law (Law of Equal Areas)

  • A line joining the planet and the Sun sweeps out equal areas in equal time intervals.
  • When the planet is nearer the Sun (perihelion), it moves faster.
  • When the planet is farther from the Sun (aphelion), it moves more slowly.
  • This law reflects conservation of angular momentum.

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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