Emeka and Yumiko are standing on opposite sides of a tall fence that they cannot see through. A ball is thrown straight up into the air along the exact middle of the fence's height. Emeka, who threw the ball, watches it go up and then come back down. Without communication, what does Yumiko know about the ball's flight as soon as she sees it appear over the fence?
Provide a very brief response.
As soon as Yumiko sees the ball appear over the fence, she knows that the ball is on its way down. The reasoning being that if the ball was thrown straight up, it must first ascend to its highest point, which is when it momentarily stops, and then it starts to descend due to gravity. Since the fence obstructs her view and she cannot see the ball during its ascent, the first point at which she can see it is when it has reached a height greater than the fence, which can only occur after it has reached the peak of its trajectory and started falling down. Therefore, Yumiko knows the ball is descending when she first catches sight of it.