1. The problem statement, all variables and given/known data Superman must stop a 120-km/hr train in 150 m to keep it from hitting a stalled car on the tracks. If the trains mass is 3.6 x 10^5 kg, how much force must he exert? Vi = 33 m/s (120 km/h) Vf = 0 m/s Displacement (Xf - Xi) = 150 m M = 3.6 x 10^5 kg 2. Relevant equations Vf^2 = Vi^2 + a(Xf - Xi) - used to calculate the acceleration without a time value. F = Ma - used to calculate the force required to stop the train with the calculated acceleration. 3. The attempt at a solution First convert Km/h to m/s. (120*1000)/60^2 ~ 33 m/s Then find the acceleration. A = (Vf^2 - Vi^2)/(Xf - Xi) A = -(33 m/s)^2 / 150 m ~ -7.3 m/s^s Now calculate the force required to achieve that acceleration. F = Ma F = 3.6 x 10^5 kg* -7.3 m/s^2 F ~ -2.6 x 10^6 N The answer at the back of the book is half that value. (1.3 x 10^6 N) I don't understand what I'm doing wrong.