Gravitation

Force of attraction between any two bodies in the universe is known as Gravitation. The attractive force of earth, or other celestial body, on an object is called Gravity Force. Because of gravity, the earth and other planets remain in their orbits around the sun; the moon remains in its orbit around the earth; tides are formed; for heating the interiors of new stars and planets to extremely high temperatures; for natural convection, which is the process by which fluid flows under the influence of gravity and a density gradient; and for a number of other earthly phenomena. Gravitational field is the space around a massive body in which gravitational force of attraction is felt. Planets move slower along their orbits when they are farthest from the sun (at apogee) and they move faster along their orbits when they are nearest to the sun (at perigee).

Acceleration due to gravity (g)

The earth attracts every body towards its centre. The acceleration with which the freely falling bodies are attracted towards the earth is called the acceleration due to gravity (g). Normally the value of g is taken as 9.8 m/s2. The value of ‘g’ at the centre of earth is zero. The value of ‘g’ is maximum at polar region and minimum at equatorial region. The value of ‘g’ decreases due to rotation of earth. The value of ‘g’ decreases with height or depth from earth’s surface. The acceleration due to gravity is maximum on Jupiter and minimum on planet Mercury.

As the mass of the substances increases the gravitational force between them also increases. The weight of a body on earth is maximum in the polar region and minimum in the equatorial region. In the absence of an effective force of gravity, bodies become weightless in artificial satellites. The weight of a body on the moon will be less than its weight on the earth. This is so because the mass of the moon is 1/81 of the mass of the earth, its radius is 1/3.66 of the radius of the earth. Therefore, the acceleration due to gravity experienced on the moon will be less than the acceleration due to gravity experienced on the earth. This will be about 1/6 of that on earth.

Orbital Velocity

Orbit is the path of a natural or artificial object that moves under the influence of a central force. Orbital velocity is the velocity at which a body revolves around the other body. The orbital velocity of a satellite does not depend on the mass of the body. It depends upon the radius of the orbit. Greater the radius of orbit, lesser will be the orbital velocity. The orbital velocity of a satellite revolving near the surface of earth is 7.9 km/sec.

Period of Revolution

Time taken by a satellite to complete one revolution in its orbit is called its period of revolution. Period of Revolution of a satellite depends upon the height of satellite from the surface of earth. Greater the height, more will be the period of revolution. Period of Revolution of a satellite is independent of its mass.

Geo-Stationary Satellite

A satellite with an orbital period same as the Earth’s rotation period is called geo-stationary satellite. Its period of rotation is 24 hours. Geo-stationary satellite revolves around the earth at a height of 36000 km. The orbit of geo-stationary satellite is called parking orbit.

Escape Velocity

Escape Velocity is the minimum speed which a body must have to escape from the earth’s gravitational force. Escape Velocity of moon is 2.4 km/s and that of earth is 11.2 km/s (7 miles/sec). The escape velocity of the planet Jupiter is 60 km/s and of Mercury it is 4.2 km/s.

Kepler's laws of planetary motion

Three laws discovered by the German astronomer Johannes Kepler regarding the motion of planets in the solar system.

1. First Law (Law of Orbits)

All planets move in elliptical orbits around the Sun, which is located at one of its foci. This law is a major change from the Copernicus model, which prescribed only circular paths for planetary motion.

2. Second Law (Law of Areas)

A straight line connecting a planet and the Sun moves at equal intervals of time. Planets move more slowly when they are farther from the Sun and faster when they are closer.

3. Third Law (Law of Periods)

The square of the period of a planet's orbit is proportional to the cube of the semi-major axis of its elliptical orbit.

Newton’s Law of Universal Gravitation

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

F = G.Mm/r2

Where ‘G’ is gravitational constant and value of ‘G’ is 6.67 x 10-11 Nm2/kg2