1. The problem statement, all variables and given/known data Two trains, one traveling 60.0mph east and the other traveling 90.0mph west, are on the same track. When they are 10000 ft apart both train's brakes are applied. If they both have the same deceleration, what is the minimum deceleration to prevent a collision? 2. Relevant equations x = xo + vot + 0.5at2 3. The attempt at a solution I converted velocities to ft/s, 60mph = 88ft/s , 90mph = 132ft/s I chose the location of the eastbound train to be the origin of the x-axis, then wrote out the positions of both trains as: x1 = 88t - 0.5at2 x2 = 10000 - 132t + 0.5at2 I supposed that the minimum acceleration to prevent a collision implies that x1 approaches x2. So x1 = x2 88t - 0.5at2 = 10000 - 132t + 0.5at2 Solving for a gives: a = (88+132)/t - 10000/t2 I am currently stuck here, and really puzzled as to why a is dependent on t. Acceleration must be a constant, so I think I must have some error somewhere that I cannot find. Any help is greatly appreciated.