Question

A rock is thrown straight down at 7.2 m/s off a bridge that is 45 m above the ground. Find the
time it takes to reach the ground.

Answer 1

The rock's height y at time t is given by

y = 45 m + (7.2 m/s) t - 1/2 g t ²

where g = 9.80 m/s² is the magnitude of the acceleration due to gravity. Set y = 0 and solve for t :

0 = 45 m + (7.2 m/s) t - 1/2 g t ²   →   t ≈ 3.9 s