Form when muscle cells convert lactic acid to lactate.

For the answer to the question above, "hydrogen ions" form when muscle cells convert lactic acid to lactate.
this is the reason why we feel fatigue when exercising it is that we accumulate a compound called lactic acid. I hope this helps

When the ball is rising up, it gains kinetic energy. when the ball reaches the very top, it will have all potential energy, until it falls back down and gains it kinetic energy. finally when you catch the ball or it falls back down, it would have gained its potential energy back.