n the formula, v is proportional not equal to squared root of k over u$.$ The proportional change in v will equal to the proportional change of squared roof of k over u$.$ Therefore, you should use ratio.In part A$,$ it said assuming the force constant $($k$)$ is the same, so when you do ratio, it will cancel out $($no need to find k$).$ The lecture said when u decreases, v increases.