When a ball is thrown straight upward, its speed gradually decreases due to the opposing force of gravity until it reaches its highest point. At that point, the ball momentarily comes to a stop before descending back down.
At the highest point of its trajectory, the ball's speed is zero. This occurs because the ball has overcome the initial upward velocity imparted to it and gravity has gradually decelerated it until it comes to a momentary halt. Therefore, the speed of the ball at its highest point is 0 m/s.
Regarding the potential energy, it is a measure of the energy an object possesses due to its position. As the ball is thrown upward, it gains potential energy. At its highest point, all of the initial kinetic energy from the throw has been converted into potential energy, and the ball has no kinetic energy. The potential energy gained by the ball is equal to the amount of kinetic energy it possessed initially.
In the absence of additional information about the mass or velocity of the ball when it was thrown, it is not possible to determine the exact amount of potential energy lost in the motion. However, it can be said that the potential energy gained by the ball at its highest point is equal to the initial kinetic energy it had when it was thrown.