He cells of the act as the heart's pacemaker, which establishes the pace for cardiac activity.

He cells of the sinoatrial (SA) node act as the heart's pacemaker, which establishes the pace for cardiac activity.
The SA node is one of the major elements in the cardiac conduction system. It consists of a cluster of cells that are situated in the upper part of the wall of the right atrium (the right upper chamber of the heart) and is responsible for generation of electrical impulses.

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.