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Newton's cradle consists of five iron balls, each hanging on two threads to prevent the ball from spinning.
Originally Newton's cradle was created to demonstrate Newton's third law. If you collide a ball from one side, the exact impact returns through the other.
The process of movement
If you drop one marble, the fallen ball hits other balls and stops completely. One ball on the other side immediately moves up at the same speed as the ball from which it fell, and the upward ball moves to the same height that the ball was initially pulled.
This indicates that the bounced ball has inherited most of the energy and momentum of the falling ball. The balls in the middle remain still and only transmit waves created by compression. During the propagation of the wave, some of the energy is lost as heat.
Assuming no energy loss, the number of balls that bounce always equals the number of dropped balls. If you drop two balls, two balls bounce on the other side, and if you drop three, three balls bounce on the other side.
When dropping a ball
Newton's cradle is described by the law of conservation of momentum (= mv) and kinetic energy (= 1/2 mv2) of an elastic body.
The collision can be explained simply if the two balls have the same mass. The impacted object completely takes over the momentum and kinetic energy of the impacted object. In the case of an utterly elastic body, no loss occurs due to heat and sound energy. A hard iron ball does not compress well but is elastic, so it does not cause energy loss and transfers energy efficiently.
When two or more balls are dropped
Let's think about lifting two balls and then dropping them toward the third. With a subtle difference, the second ball hits the third ball first. According to the law of collision of an elastic body, the third ball takes over momentum and kinetic energy and moves, and the second ball that has fallen stops. The first ball hits the second ball that has stopped. This process is repeated so that if you lift and drop two balls, the two opposite balls will bounce at the same speed.
Phenomena caused by friction
In Newton's cradle, friction reduces momentum and kinetic energy gradually. A ball falling from Newton's cradle is pushed back slightly by conflict immediately after impact. When these effects accumulate, after time, all the balls turn into shaking little by little at the same time, eventually stopping the whole movement as well.