Final answer:
The stone takes approximately 2 seconds to reach the river and has a velocity of approximately 19.6 m/s upon impact.
Step-by-step explanation:
Using energy considerations and ignoring air resistance, we can determine the time it takes for the stone to reach the river and its velocity upon impact. Since the stone is thrown horizontally, it does not have any initial vertical velocity. The only force acting on the stone is gravity, which causes it to accelerate downwards at a rate of 9.8 m/s². We can calculate the time it takes for the stone to reach the river using the equation:
h = (1/2)gt²
Where h is the vertical displacement, g is the acceleration due to gravity, and t is the time. In this case, the initial vertical displacement is 20 meters, so the equation becomes:
20 = (1/2)(9.8)t²
Simplifying, we find that the time it takes for the stone to reach the river is approximately 2 seconds.
To find the velocity upon impact, we can use the equation:
v = gt
Where v is the velocity and g is the acceleration due to gravity. Substituting the values, we get:
v = (9.8)(2) = 19.6 m/s
Therefore, the stone has a velocity of approximately 19.6 m/s upon impact.