Someone has challenged you in a friendly bet that, given a set of six charged spheres with three carrying positive charge and three carrying negative charge, you can't place any three of them in a line (a board with a groove in it serves the purpose) and have them be in translational equilibrium. You determine which of the three spheres are the positively charged ones and call them 1 , 2 , and 3. By comparing the electric force exerted by sphere 1 on sphere 3 with the electric force exerted by sphere 2 on sphere 3 , you rank 1 as smaller than 2, placing them in the groove several hundred millimeters apart, temporarily blocking them from moving. You can release them only when the third sphere is in place, but there are so many things to consider!