The cart impacts the safety barrier with speed $v_{0}=3.25 \mathrm{m} / \mathrm{s}$ and is brought to a stop by the nest of nonlinear springs which provide a deceleration $a=$ $-k_{1} x-k_{2} x^{3},$ where $x$ is the amount of spring deflection from the undeformed position and $k_{1}$ and $k_{2}$ are positive constants. If the maximum spring deflection is $475 \mathrm{mm}$ and the velocity at half-maximum deflection is $2.85 \mathrm{m} / \mathrm{s}$, determine the values and corresponding units for the constants $k_{1}$ and $k_{2}$