Introduction
Astronomy has been a fascinating subject from ancient times. The geocentric system (around the 2nd century A.D.) considered the Earth to be at the centre with the sun and other planets revolving around it.
Newton's Universal Law of Gravitation
Every particle of matter in the universe attracts every other particle with a force which varies directly as the product of their masses and inversely as the square of the distance between them.
Acceleration due to Gravity 'g'
Bodies allowed to fall freely were found to fall at the same rate irrespective of their masses (air resistance being negligible). The velocity of a freely falling body increased at a steady rate i.e., the body had acceleration. This acceleration is called acceleration due to gravity - 'g'.
Gravitational Field
The space around a body within which its gravitational force of attraction is experienced, is called its gravitational field.
Gravitational Potential
The work done in moving a unit mass from infinity to a point in a gravitational field is called the gravitational potential at that point.
Inertial Mass and Gravitational Mass
The mass of a body can be determined by measuring the acceleration 'a' produced in it by a known force 'F'.
Satellites
The moon is a natural satellite of the Earth. We now have a number of artificial satellites (man-made) revolving around the Earth.
Kepler's Laws of Planetary Motion
The path of each planet around the sun is an ellipse with the sun at one focus.
Weightlessness
The difference between the gravitational force and the centripetal force acting on our body is equal to our weight.
Vector Form of Newton's Law of Gravitation
Let m1 and m2 be the masses of two bodies separated by a distance d.
Torque due to Gravity
Every particle of matter in a body is attracted to the Earth and the result of all these attractive forces is the weight of the body.
Cavendish's Experiment to Determine G
The Cavendish balance consists of a light, rigid T-shaped member supported by a fine vertical fibre. Two small spheres of masses m are mounted at the ends of the horizontal portion of the T. A mirror M on the vertical reflects the beam of light on to a scale.
Conclusion
The discovery of gravitation by Sir Isaac Newton, due to his observation of the falling apple, explained the movements of planets and satellites not only in our solar system but also in other systems as well. It explained why the Earth and other planets where spherical or nearly spherical in shape.
