The cart impacts the safety barrier with speed v0 = 3.25 m /s and is brought to a stop by the nest of nonlinear springs which provide a deceleration a = −k1x − k2x3 , where x is the amount of spring defl ec- tion from the undeformed position and k1 and k2 are positive constants. if the maximum spring defl ec- tion is 475 mm and the velocity at half-maximum defl ection is 2.85 m /s, determine the values and cor- responding units for the constants k1 and k2.