1. The problem statement, all variables and given/known data A 4.50 ✕ 10^5 kg subway train is brought to a stop from a speed of 0.500 m/s in 0.900 m by a large spring bumper at the end of its track. What is the force constant k of the spring? M- 4.50E5 kg IV- .5m/s FV- 0m/s 2. Relevant equations k=mg/x F=-kx PE = (.5)kx^2 3. The attempt at a solution k=(450000kg)(9.81m/s^2)/ .9 = 4900000, which isn't the right answer. Can someone help me out?