Question

Two cars adjacent to each other on a highway. The first car accelerates uniformly at 3m/s2 the moment the light turns green. The second car is moving at 18m/s and is beside the first car at the instant the light changes and continues driving with a constant velocity. When are the cars side by side again?

Answer 1

Write equations for position from the light (X) vs time (t) and set them equal to each other.
Solve for t.

X1 = (a/2) t^2 = 1.5 t^2
X2 = 18 t

18t = 1.5 t^2

1.5 t (t - 12) = 0

One answer is t=0. What is the other?