Understanding the Physics

* Free Fall: This means the only force acting on the rock is gravity.

* Acceleration due to Gravity: The acceleration due to gravity (g) is approximately 9.8 m/s². This means the rock's velocity increases by 9.8 m/s every second it falls.

* Initial Velocity: The rock is dropped, so its initial velocity (v₀) is 0 m/s.

Formula

We can use the following kinematic equation to find the final velocity (v):

v = v₀ + at

where:

* v = final velocity

* v₀ = initial velocity

* a = acceleration due to gravity (g)

* t = time

Calculations

1. Plug in the values:

v = 0 m/s + (9.8 m/s²) * (4.0 s)

2. Calculate:

v = 39.2 m/s

Answer: The velocity of the rock as it reaches the ground is 39.2 m/s.