1. The problem statement, all variables and given/known data An airplane touches down at a speed of 100m/s. It travels 1000 metres along the runway while deceleration at a constant rate before coming to rest. How long did it take the airplane to come to rest on the runway? Xi=0m/s Xf=1000m Vix=100m/s 2. Relevant equations Xf=Xi+Vixt 3. The attempt at a solution 1000=0+100t t=10s Why is the answer 20 s? and why does this formula fail?