As the first ball is released, its potential energy is converted to kinetic energy, and when it is abruptly halted during the impact with the second ball, most of that energy is transferred into the second ball by compressing it. The same thing happens to the third and fourth balls, but when the energy transfers into the fifth and final ball in the chain, the energy causes it to move. In an ideal Newton’s cradle, the fifth ball would move with the exact same energy as the first ball, but in real life, friction between the balls prevents this from happening.