Newton ‘s law of gravitation states that the force of attraction between particles is directly proportional to their mass and inversely proportional to the square of distance apart.

Where G is the universal gravitational constant,

**7.2 GRAVITATIONAL FIELD STRENGTH**

•

**A gravitational field is a region where gravitational force acts on massive bodies. Eg . gravitaional field of the Earth.**•

**The gravitational field strength tells us how strong a gravitational field is. The gravitational field strength of the Earth near its surface is 9.81***m*/*s*^{2}.•

**The gravitational field strength , E at a point is the force of gravity per unit mass exerted on a mass placed.**The variation of the acceleration due to gravity g’ with distance r from the centre of the Earth is illustrated by the graph above.

**7.3 Gravitational Potential**

• The strength of the gravitational force at a point in a gravitational field is described by the gravitational field strength

*E*or*g*is a**vector quantity.**• Another quantity associated with the point in the gravitational field is the gravitational potential. It is a

**scalar quantity**.• The gravitational potential

*V*at a point P in a gravitational field is defined as the work done per unit mass to bring a body*from infinity to P*. The unit for gravitational potential is Jkgˉ¹.• The gravitational potential energy

*U*of a body at a point P in a gravitational field is defined as the work done to bring the body*from infinity to P*. The unit for gravitational potential energy is J.• Hence the gravitational potential energy

*U*of a body of mass m at a point where the gravitational potential,*V*is given by U = mV

• On the surface on the Earth, r = R

– Gravitational potential, V = -

– Gravitational potential energy, U = -

– The graph illustrates the variation of the gravitational potential V with distance r from the centre of the Earth.

**7.4**

**RELATIONSHIP BETWEEN g AND G**

-G is the Universal Gravitational Constant.

- It is a scalar quantity with dimension

- It is a vector quantity with dimension

**R = constant radius of earth**

**G = universal gravitational constant**

**M = mass of Earth**

**7.5 Satellite Motion in Circular Orbits**

Satellite is a body that revolves round a planet. Satellites can be categorized as natural satellites or man-made satellites. The moon, the planets and comets are examples of natural satellites. Examples of man-made satellites are Sputnik I , Measat I ,II and III which are communication satellites. In order to launch satellite into orbit , rockets are used. When rocket that carries the satellite reaches the required height , the satellite is launched into circular orbit with a certain velocity v that is tangential to intended orbit.

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