Chapter at a glance (Gravitation)

• Force of gravitation : The force of attraction which exists between all objects, big or small, not only on the surface of the earth, but anywhere and everywhere in the universe, irrespective of the distances separating them, is called force of gravitation.

• Newton’s law of gravitation : Every body in this universe attracts every other body with a force, which is directly proportional to the product of their masses and inversely proportional to the square of distance between their centres.

• Gravitational constant : It is equal to the force of attraction between two unit masses, when they are separated by a unit distance, as measured from their centres.

• SI unit of gravitational constant : The SI unit of gravitational constant is Nm2/kg2 or Nm2kg–2. Its value is 6.67 × 10–11 Nm2/kg2.

• Force of gravity : It is a special case of force of gravitation, when one amongst the two bodies has an infinitely large mass as compared to the other body.

• Force of gravity of the earth : The force of gravitation acting on the bodies close to the surface of the earth is called force of gravity of the earth.

• Acceleration due to gravity of the Earth : The acceleration with which the bodies fall towards the earth is called acceleration due to gravity. Its average value is 9.81 ms–2.

• Variation of acceleration due to gravity.

(i) Acceleration due to gravity changes with the change in distance from the centre of the earth.

(ii) Acceleration due to gravity is maximum at the poles of the earth. Its value decreases as we move towards the equator, such that its magnitude is least at the equator.

(iii) Acceleration due to gravity decreases as we move inside the earth, such as deep mines.

(iv) Acceleration due to gravity decreases as we move away from the surface of the earth, such as on the mountains, in aeroplanes, in spaceships, etc.

• Mass : The amount of matter contained in a body is called mass. It is a scalar quantity and is measured by a physical balance. It is always a constant quantity and its unit in SI system is kilogram.

• Weight : The force with which a body is attracted towards the centre of the Earth is called weight.