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Elastic collision and friction of newtons cradle balance balls
An elastic collision occurs when two metal balls collide in newtons cradle balance balls. The kinetic energy remains unchanged before and after the collision. Under ideal conditions, ie the ball is only affected by momentum, energy and gravity, all collisions are perfectly elastic collisions and the structure of newtons cradle balance balls is also perfect, the metal ball will move forever. But there can be no perfect newtons cradle balance balls, because they are always subject to friction and energy loss. Part of the friction comes from air resistance, and most of it comes from the ball itself.
So the collision in newtons cradle balance balls is not a real elastic collision but an inelastic collision, because the kinetic energy after the collision is lost (due to friction) than before the collision. But according to the law of conservation of energy, the total energy remains the same. Due to the deformation of the ball, the molecules that make up the ball convert kinetic energy into thermal energy. The ball vibrates with the crisp crashing sound that is characteristic of newtons cradle balance balls.