Question

if a car rolls gently off a cliff how long does it take to reach 85km/

Answer 1

Is the car in free fall? And did you mean 85 km/h?

If yes to both, use the following equation:

\[a=\frac{v-v_0}{t}\]
Where a is the acceleration, v is the final velocity, v(0) is the initial velocity, and t is the time. To make things a bit easier, convert 85 km/hr to x m/s and solve for t from there!