Past exam of the physics 2 paper of the University of Oxford / 2013 resit / 7 / iii

Here $q_1=3.8q_3$ and $q_2=-1.9q_3$. The part of the electrostatic potential energy that changes isAt the two endpoints, $U(-0.9)=37k_e{q}_3^2/L$ and $U(0.9)=-17k_e{q}_3^2/L$. The fixed-source interaction energy is unchanged, and conservation of energy givesThe force points toward increasing $f$ throughout this interval: the left charge repels $q_3$ and the right charge attracts it, so release from rest reaches the stated endpoint without a turning point.