Newton's cradle stops because the in the swinging projectiles is transformed into quantities other than kinetic energy and gravitational potential energy

Newton's cradle stops because the in the swinging projectiles is transformed into quantities other than kinetic energy and gravitational potential energy.

The other forms of energy are sound and heat. When the balls collide (provided you are not in a vacuum) sound energy moves from the collision all around the room. This costs the cradle a certain amount of energy and is deducted from the initial energy the ball started with.

The balls also heat up over the series of collisions. This energy is also deducted from the initial energy. After enough time has passed, all of the kinetic energy is gone and you are left with a slightly higher temperature Newton's cradle.