A car with a total mass of \(1400 \mathrm{~kg}\) (including passengers) is driving down a washboard road with bumps spaced \(5.0 \mathrm{~m}\) apart. The ride is roughest-that is, the car bounces up and down with the maximum amplitude-when the car is traveling at \(6.0 \mathrm{~m} / \mathrm{s}\). What is the spring constant of the car's springs?