Question

A ball is thrown directly downward with an initial speed of 8.00 m/s, from a height of 30.0 m. After what time interval does it strike the ground?

Answer 1

We can use the equation of motion:
s = ut + 1/2 x at²
Both the velocity and acceleration are downwards so we'll consider the downward direction positive
30 = 8t + 4.91t²
4.91t² + 8t - 30 = 0
Solving the quadratic:
t = 1.79 s; t = -3.42 s
Negative time is not possible so second value discarded. Time taken = 1.79 seconds