Here is the question, it is very simple. A toy car accelerates by means of a rocket-type engine for 20 m. If the acceleration during the burn is [tex]5 m/s^2[/tex] and the burn lasts for 3 s, determine the velocity of the car at the end of the burn. It seems like the obvious one, and we use. [tex]v=v_i + at[/tex] ...and we get 15. This works for uniform acceleration, which it says it is. They are using... [tex]s = v_f t - 1/2 a t^2[/tex] They got that by differentiating from [tex]v_i[/tex], instead of the usual [tex]v_f[/tex] In the end, they get 14.2 m/s.