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Motion In Straight Line

Question
CBSEENPH11017643

Three point masses each of mass m are placed at the vertices of an equilateral triangle of side 1. What is the gravitational field due to three masses at the centriod of triangle?

Solution
Let three masses each of mass m be placed at the vertices of an equilateral triangle ABC of side 1 unit.

Let O be the centroid of the triangle.
The distance of the centroid from each vertex is, 
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The gravitational field at O due to mass m at A is, 
                straight E with rightwards arrow on top subscript straight A space equals space Gm over straight r squared 
                     equals fraction numerator 3 Gm over denominator ell squared end fraction comma space space space space space space along OA with rightwards arrow on top 
Similarly,
                  straight E with rightwards arrow on top subscript straight B equals fraction numerator 3 Gm over denominator ell end fraction comma space space space space space space space space space space along space OB with rightwards arrow on top 
and            straight E with rightwards arrow on top subscript straight C equals fraction numerator 3 Gm over denominator ell squared end fraction comma space space space space space space space space space along OC with rightwards arrow on top 
Here, the three coplanar vectors straight E with rightwards arrow on top subscript straight A. space straight E with rightwards arrow on top subscript straight B and space straight E with rightwards arrow on top subscript straight C are equal in magnitude and make an angle of 120 degree with the preceding vector.
Therefore, the resultant field is zero.