The Newton’s Cradle is a terrific way of demonstrating some key ideas about energy and motion
The Law of Conservation of Energy says that energy cannot be created or destroyed. It can however change forms.
In Jiwi’s Newton’s Cradle image (above) you can see this as the first ball is lifted and held. It gains gravitational potential energy. This means it has an amount of stored energy. How much energy it has relates to the object’s mass and how high up it is. When the ball is dropped, the energy is changed into a moving, kinetic energy. As the moving first ball hits the stationary second ball, all of its energy is transferred into ball 2 and ball 1 is now stationary. This transfer of energy continues directionally from ball 2 to 3, ball 3 to 4, ball 4 to 5. When the last ball gains the kinetic energy, it is able to move and swings to its highest point. It is losing kinetic energy as it goes but this is being transferred into potential energy until it has no more moving energy but lots of potential energy. For a split moment it has zero kinetic energy and all potential energy before it swings in again, gaining kinetic energy and losing potential energy. You should be able to see the ball slowing as it reaches its highest point before it falls again and picks up speed before bumping into the next ball.