Thursday, 4 April 2013


Newton's Law of Universal Gravitation
The gravitational force of attraction between any two particles of mass m1 and m2 separated by a distance r has the magnitude

It is worth noting about the gravitational force that
  • It is applicable for point masses (size of body is neglected in comparison to their distance of separation).
  • It is always attractive.
  • It is independent of presence of medium between particles.
  • Universal gravitational constant G = 6.672 x 10-11 Nm2 kg-2.
  • [ G ] = [ M-1  L3  T-2 ]
  • Exchange particle is gravitation.
  • It is conservative force.

    Line integral of gravitational force over a closed path is always zero.

    OR    Work done by gravitational force is displacing a body is independent of path followed.
  • Force between two bodies is independent of presence of surrounding bodies.
Gravitational Intensity or Gravitational Field

Qualitative Definition:
It is the region in space in which effect of presence of a mass is felt.

Mean to say

It is the region in which a massive object experiences a force.

Earth is in the gravitational field of Sun.

Moon is in the gravitational field of Sun and Earth.

Test Mass:
It is massive particle which experience a force in the gravitational field but it does not have its own field or it can never influence the field of other.

e.g., Chalk, duster, pen, book etc. are test masses in the Earth gravitational field. Motion of the falling chalk and duster are influenced by earth gravitational field but the duster can not influence the motion of the duster.

Qualitative Definition of Gravitational Field Intensity:
Gravitational field intensity (l) at a point in a region is defined as the gravitational force experience per unit test mass present at that point.

If a test mass mo experiences a gravitational force  at a point, then gravitational field intensity at that point is

It is also said to be acceleration due to gravity at that point.

Unit: N kg-1  or  ms-2.

Gravitational Field Lines:
These are imaginary lines in the gravitational field such that tangent drawn to it at a point represents the direction of gravitational intensity at that point.

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