Final answer:
The speed of the stone when it reaches a height of 20.7 m is 0 m/s.
Step-by-step explanation:
To find the speed of the stone when it reaches a height of 20.7 m, we can use the principle of conservation of energy. At the initial height, the stone has a potential energy (PE) given by PE = mgh, where m is the mass of the stone, g is the acceleration due to gravity, and h is the initial height. When the stone reaches a height of 20.7 m, it has a potential energy given by PE = mg(20.7). Since energy is conserved, we can set these two expressions for potential energy equal to each other:
mgh = mg(20.7)
Cancelling the mass and g, we get:
h = 20.7 m
Therefore, the height the rock reaches is 20.7 m. Since the stone is thrown vertically upward, its final velocity when it reaches this height will be zero. Hence, the speed of the stone when it reaches a height of 20.7 m is 0 m/s.