Final answer:
The motion in Newton's Cradle stops due to friction and air resistance, which convert the system's kinetic and potential energy into thermal energy, eventually causing the balls to stop swinging.
Step-by-step explanation:
The motion of Newton's Cradle ultimately stops because of the effects of friction and air resistance, which are forms of dissipative forces. These forces act to gradually convert the kinetic energy and potential energy of the swinging balls into thermal energy, which is dissipated into the environment. While kinetic energy is transferred from one ball to the next in the cradle, a small amount of energy is lost with each collision and swing due to these forces, ultimately causing the balls to come to a rest.
In the idealized physics examples, such as a perfectly elastic collision where no kinetic energy is lost, the motion would continue indefinitely. However, in the real world, due to non-conservative forces like friction and air resistance, kinetic energy is not perfectly conserved in collisions and is instead transformed into other forms of energy, such as heat. This gradual loss of energy to the surroundings is why the height the balls reach in Newton's Cradle decreases over time, and they eventually stop moving.