24P1Q12

12 A
Time interval between collisions, $\displaystyle \Delta t=2x\div v\text{ }$
Change in momentum per collision, $\displaystyle \Delta p=2mv$
Average force, $\displaystyle \left\langle F \right\rangle =\frac{\Delta p}{\Delta t}=\frac{2mv}{2x\div v}=\frac{m{{v}^{2}}}{x}$