at the initial time, we can get the equation for A: k*QA*QB/L^2=m*a, where QA, QB are the electric charges of A and B, respectively.
some time later, suppose the distance between A and B is X, then we can get the same equation for B: k*QA*QB/X^2=2m*a
with these two equations, we can get: X=L/sqrt(2)
done!
答案是（根号2）/2L
解：设A，B电荷量为qA，qB，则有：
（ kqAqB）/L^2=ma
因为mB=2m
所以aB=a/2
当aB=a时：A，B间电场力增大为原来的2倍，即L减小为根号2/2倍
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