Answer
$40 N$
Work Step by Step
In the last section, we saw that the maximum force is $80 N$, as this is the maximum acceleration ($8*10^3 m/s^2$) divided by the mass, $0.01 kg$. In this situation, we have to find the force at half the maximum displacement. However, since this is SHM, force is proportional to displacement, so the force is simply half of the maximum force, or half of $80$ $N$, or $40$ $N$.
