1. The problem statement, all variables and given/known data A car starts from rest. With constant acceleration, the car increases its speed from 10m/s to 50m/s over a distance of 60m. How long does this take? Purported answer: 4.0s 2. Relevant equations Constant acceleration, so the following equations apply: Vf^2 = Vi^2 + 2ad Vf = Vi + at 3. The attempt at a solution Vf^2 = Vi^2 + 2ad 2500 = 100 + 2a(60) 2400 = 120a 240/12 = a a = 20 m/s^2. Vf = Vi + at 50 = 10 + 20(t) 40/20 = 2s. Why does the key say 4s? I'm guessing it's because the key is wrong!