Understanding the Concepts

* Free Fall: The stone is moving under the influence of gravity alone.

* Acceleration due to Gravity (g): This is a constant value of approximately 9.8 m/s² downward.

* Kinematic Equations: We can use these equations to relate displacement, velocity, acceleration, and time.

The Solution

1. Identify the knowns:

* Final velocity (v) = 0 m/s (at the highest point, the stone momentarily stops)

* Displacement (Δy) = 20 m

* Acceleration (a) = -9.8 m/s² (negative since gravity acts downward)

2. Choose the appropriate kinematic equation: We'll use the following equation:

v² = u² + 2aΔy

Where:

* u = initial velocity (what we want to find)

* v = final velocity

* a = acceleration

* Δy = displacement

3. Plug in the values and solve for 'u':

0² = u² + 2(-9.8 m/s²)(20 m)

0 = u² - 392

u² = 392

u = √392 ≈ 19.8 m/s

Answer: The stone was thrown upward with a speed of approximately 19.8 m/s.