A steam engine drop-valve is closed by a spring after the operation of a trip gear. The stiffness of the spring is such that a force of 4 N is required per mm of compression. The valve is lifted against the spring, and when fully open the compression is 75 mm. When closed the compression is 30 mm. The mass of the valve is 5 kg and the resistance may be taken as constant and equal to 70 N. Find the time taken to close the valve after the operation of the trip.