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Two identical balls each having a charge of 2×10-7 and a mass of 100g are suspended from a common point by two insulating strings each 50 cms long. The balls are held at a separation 5 cm apart and then released . find the
a)the electric force on one of the charged balls.
b)the components of the resultant force on it along and perpendicular to the string.
c)the tension in the string
d)the acceleration of one of the balls. Answers are to be obtained only for the instant just after the release

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b) Lets suppose that the angle made by the string and the vertical line is x.

Then sin x = 5/2x50 = 1/20

so cos x = 0.9987

Since the tension in the string will balance the force along the string, The net force along the string will be zero.

The force perpendicular to the string will be 

F= mgsinx - Fecosx = 0.1x9.8x0.05 - 0.9987x3.6x10-4 = 0.0486 N

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