Consider three charges `q_1,q_2,q_3` each equal to q at the vertices of an equilateral triangle of side l. what is the force on a charges Q ( with the same sign as q) placed at the centroid of the triangle,as shown in figure?

Explanation

The forces `F_1 ,F_2 and F_3` are equal ( in magnitude) and act along the medians with inclination `120^(@) ` each. They form a closed vector triangles and hence the resultant is zero.