Question

A charge of +1.7 micro-coulombs lies on a horizontal surface. It has a mass of 3 kg and the coefficient of static friction between it and the surface is 1.57. Another charge of -3.1 micro-coulombs lies on the horizontal surface 9 cm to the right of the charge. A third charge which is negative is located 19 cm above the positive charge on the surface. What is the smallest magnitude this charge needs to be in order for the positive charge to overcome static friction such that the negative charge on the surface begins to move it? Answer in micro-coulombs.

Answer 1

The situation can be piictured as follows

Now, drawing the FBD of q1

In order to balance the friction force on q1 so that it can be overcome and the charge can move towards q2, we balance the forces along x axis.

The friction on q1 can be given by = friction coefficient x normal force on q1 ( which is the resultant of force due to gravity as well as electric force due to q3)

Electric force between two charges is given by

EF= kq1q2/(rxr)