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 v = x / t a = v / t F = m a 3. The attempt at a solution v = x / t 33 = 150 / t t = 4.55 secs a = v / t a = 33 / 4.55 a = 7.26 m/s^2 F = 3.6 * 10^5 * 7.26 F = 2613600N Ignoring the sign convention for just a second, the answer is half the value of what I calculated. I would like to know why I have to use the formula below, and why my method is incorrect.