I think if they were free to move the forces between them would continuously change (because the force is proportional to distance squared). So, them being stationary makes sense, but I recall my teacher stating that in each part of the problem a different sphere was free to move.
If we assume...
Homework Statement
Three separate spheres next to each other. The one on the left is positive (charge of +25.5μ C). The one on the right is negative(-25.5μ C). The two are separated by 0.25m. The third sphere has a charge of +2.5μ C and is placed exactly halfway between the two. Find the force...