83. When the falling ball bounced off the rising board, why did the ball go upward very quickly? Because of your frame of reference?
The frame of reference from which you observe the situation doesn't cause the rebounding ball to move quickly, but it does help you to understand why the ball rebounds so quickly. Instead of describing the ball bounce from the rising board, let's look at the ball bouncing from a horizontally moving bat. That way, we won't have to worry about gravity—we can pretend it doesn't even exist for a moment. Let's begin from the fan's inertial frame of reference as a pitched ball heads toward a bat at home plate. As the ball approaches the bat, the bat approaches the ball. Both objects are moving, which makes things complicated. So we'll now shift to the bat's frame of reference for a while. In this frame of reference, the bat is stationary and the ball is approaching at high speed. (This rapid approach speed reflects the fact that the two objects are each moving toward the one another in the fan's reference frame.) The ball now bounces from the bat. Because it approached the bat at such a high speed, the ball rebounded at a high speed, too—it heads away from the bat at high speed. Now we'll shift back to the fan's reference frame. The ball is still going away from the bat at high speed, but now we must notice that the bat itself is heading toward the outfield at a high speed, too. So the ball must really be heading toward the outfield fast—it's outrunning the bat toward the outfield. And that is the case. The ball heads toward the outfield at a much higher speed than it had when it was heading toward the bat originally. In the fan's frame of reference, there is a large transfer of energy from the bat to the ball