1. The problem statement, all variables and given/known data This is a question from the June 2009 Regents. The question asks what is the average acceleration of a roller coaster cart as it reaches the bottom of a hill. You know that Vi = 0 m/s (The cart starts from rest from the top of a hill) Vf = 46.5 m/s (velocity at bottom of hill) d = 110 m (This is the height of the hill) t = 5.3 s (Time it takes for it to start from rest and reach the bottom of the hill) 2. Relevant equations The Kinematics equations on the reference table: a = Δv / t d = (Vi)t + 1/2at2 Vf2 = Vi2 + 2ad Where Vi is initial velocity, and Vf is final velocity. 3. The attempt at a solution According to the answer key: a = Δv / t a = (46.5m/s) / 5.3 s and a = 8.8 m/s2 However, I did Vf2 = Vi2 + 2ad I plugged in all the correct numbers and got 9.8m/s2 Why isn't my answer valid/ Why do the two formulas yield different answers? Thank you.