Kinematics and Gravity?

Question

If a car rolls gently (v0 = 0) off a vertical cliff, how long does it take it to reach 80 km/h?
Answer in seconds

Answer 1

2.13 seconds.

Answer 2

Vf = Vo + a*t
where a = g = 9.8 m/s^2

You have Vf in km/hr but you need velocity in units of m/s. To convert to m/s, multiply by one, as follows.
(km/hr)*(1000 m/1 km)*(1 hr/3600 s)

How's that work?
1 km = 1000 m, so when you multiply by (1000 m/1 km) you are just multiplying by one. (If you divide a quantity by something that is equal to it, the result is one.) But you accomplish changing units to something that's equivalent.