Answer to Question #53222 in Mechanics | Relativity for Rebecca

Question

Hello,

This question relates to finding a constant acceleration using average velocity. In the acceleration questions posted at the beginning of the Khan Academy's MCAT Prep Acceleration section, there is one that asks for the constant acceleration to be solved for when a dragster travels 400 m in 4 seconds. The correct answer is displayed as 50 m/s/s, and the hints suggest solving with the d = 1/2at^2 formula.

Here is what I don't get: when I use the d = 1/2at^2 formula to solve for acceleration, I get the correct answer (50m/s/s). However, when I try to use the average velocity formula, a = 1/2(vi+vf) / t ---- I don't get the correct answer (12.5 m/s/s).

Could someone explain this for me, please? I would love to understand why I can't use acceleration = average velocity/time to solve this problem correctly, or what I have done wrong in trying to do so.

Thanks!!