A particle having a charge of `2.0xx10^(-4) C` is placed directly below and at a separtion of 10 cm from the bob of a simple pendulum at rest. The mass of the bob is 100g. What charge should the bob be given so that the string becomes loose?

Explanation

` Mass of the bob=100g=0.1g `
So, tension in the string
` =0.1xx9.8=0.98N `
for the tension to be 0, the charge below should repel the 1st bob
` and F=mg `
` rArr (9xx10^9xxqxx2xx10^-4)/10^-2=10^-1xx10 `
` rArr q=5.4xx10^-9C`