Final answer:
The correct option is B. 39m/s.
Using the equation of motion for free fall, the ball's velocity at the time of impact after falling for 4.0 seconds is 39.2 m/s. This value is rounded to 39 m/s, corresponding to option B.
Step-by-step explanation:
To determine the ball's velocity at the time of impact when it hits the ground after being dropped from an observation tower, we can use the equation of motion under the influence of gravity. The equation that relates the velocity (v), acceleration due to gravity (g), and time (t) is v = gt, assuming the initial velocity is zero since the ball is simply dropped.
Given that the acceleration due to gravity is g = 9.8 m/s², and the time taken to hit the ground is t = 4.0 s, we can calculate the final velocity (v) as follows:
v = gt = (9.8 m/s²)(4.0 s) = 39.2 m/s
Therefore, the ball's velocity at the time of impact would be approximately 39 m/s, which most closely corresponds to option B.