A projectile is launched off a 71m high cliff with and lands 71m away. What was the initial velocity?

Initial velocity = 18.68  m/s

Explanation:

If the projectile is launched horizontally, then its vertical motion is that of free fall (no initial velocity in the vertical direction) and we can estimate the time of its flight via the free fall equation:

y = - 1/2  g  t^2

- 71 = - 4.9  t^2

t^2 = 71/4.9

t = 3.8 s

Now, knowing the time of flight, we can estimate the horizontal launching velocity:

x = v  t

71 = v (3.8)

v = 71/ 3.8

v = 18.68  m/s