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Three identical charged balls, each carrying charge 2 C, hang from a common point P by silk threads of length 2 m each. They settle into an equilateral triangle of side 1 m. Find the ratio of the net Coulomb force on any one ball to the force between any two of the balls.

1. sqrt(3): 1
2. 1: 1
3. 1: 4
4. sqrt(3): 2

Correct answer: sqrt(3): 1

Solution

The two pairwise forces on a corner ball are equal in magnitude F and the angle between them (the interior angle of the equilateral triangle) is 60 degrees. Their resultant is sqrt(F² + F² + 2F² cos60) = F*sqrt(3). So net: pairwise = sqrt(3): 1.

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