"A car accelerates from rest over 400 metres in 19 seconds. The driver then brakes and the car stops in 5.1 seconds with constant deceleration." Part one of the question is "calculate the acceleration for the first 400m" which went just fine: x=ut+(1/2)at^2 400=180.5a a=2.22 m/s Now the second part of the question asks for the average speed over the entire journey, both accelerating and braking sections. So this is what I did: Final velocity after 400m: x=(1/2)(u+v)t 400=(1/2)(0+v)19 v=42 It's not exactly 42 but I did use the exact answer for the rest of this. Anyway, I then did: Braking distance: x=(1/2)(42+0)5.1 x=107.1 So it should be 507.1/24.1=21 m/s But when you do an acceleration/time graph and solve the area under the line you get the apparently correct answer of 26 m/s. I cannot for the life of me understand why or how my method doesn't work.