Determine the amplitude of vertical vibration of the spring-mounted trailer as it travel...

Determine the amplitude of vertical vibration of the spring-mounted trailer as it travels at a velocity of over the corduroy road whose contour may be expressed by a sine or cosine term. The mass of the trailer is 500 kg and that of the wheels alone may be neglected. During the loading, each 75 kg added to the load caused the trailer to sag 3 mm on its springs. Assume that the wheels are in contact with the road at all times and neglect damping. At what critical speed is the vibration of the trailer greatest?