Physics of Newton's Cradle 

great , i don't know if you have done more i'm just about to look but if you havn't then you really should of lots more bite size topics,really enjoyed it

How does Newton's cradle count balls? Maybe it is because when we have two balls for example the pulse travels in two directions. Since the collision produces an equal but opposite reaction, the pulse wave travels back through both balls that were used as the carriers of energy although no backwards motion is registered. Since the pulse travels through two balls the pulse length of two balls is rebounded back through the system so that when the pulse length reaches the end it will act to maintain conservation along the entire length of the pulse, whatever that length is, if it didn't it would actually be disobeying the law. Some added thoughts: I've read through many explanations on the internet concerning Newton's cradle but where I think they fail is that they try to describe why the center ball(s) absorb and release energy the way they do exhibiting barely any motion, by describing it through Newtonian laws of motion and the conservation of energy. What they should be doing is describing the phenomenon through the physics of resonance. Why? Because resonance is about the science of energy absorption, especially when resonant frequency is maintained through a system. What do I mean when I say resonant frequency is maintained through a system? It means the object that brings the energy into the system (the carrier), which in the case of Newton's cradle is the raised ball that will introduce the energy via the first collision) vibrates at a frequency which matches the resonant frequency of the other hanging balls. Since all the balls are made of the same materials and dimensions it means they all share or vibrate at the same resonant frequency. Yes quantum physics tells us no object is ever truly at rest but is in a constant state of fluctuation or vibration that is particular to the dimensions and materials of that object. Everything has a particular resonant frequency. And if the energy that is placed upon that carrier vibrates at the same resonant frequency as the object that will receive the energy, then that energy will tend to build on that object until the object is either destroyed or really rattled. The opera singer hits a certain high note, and the sound from her voice (the carrier) introduces a vibration on the crystal glass that happens to match the internal frequency the glass is resonating at long enough for the energy to build until the glass shatters. When we say energy builds it means the glass is absorbing ever greater levels of energy. However, when you think about it the phrase "the energy builds" may not be a good way of describing what is happening, because the glass is made to vibrate, and if it is made to move, work is being done and therefore energy is constantly being released, and if something is constantly being released how can that energy build up? Well if the energy is not building up per se what are we witnessing? Maybe what we are really seeing is that the energy being applied is finding more efficient ways of moving across the glass and less energy is being wasted in extraneous movement so that the final work of creating vibration is ever more potent. When you have increasingly less energy being wasted producing extraneous, or chaotic motion, that would mean you have more energy to bring to the task of producing the final bit of work, which is the vibration or movement on the glass. The potency builds until the glass shatters if the note is held long enough Of course there are other ways of thinking about it. If, for example, you increase efficiency you can say you decrease the resistance inhibiting the flow of energy across a system or object. If this is so then just the fact that you have objects in a system that match resonant frequency it has an implication on the way energy moves through a system, namely it opens up a pathway of least resistance which allows the flow or movement of energy through that system to be so efficient that it doesn't waste itself producing chaotic or extraneous motion. Which in Newton's cradle means the energy rebounding back to the first ball(s) used in the initial collision (Newton's notion of an equal and opposite reaction means the pulse wave from the initial collision travels in two directions, one being back towards the ball(s) used in the first collision) as well as the balls in the center load do not move until we get to the last ball, where almost like magic energy departs from a stationary, or non moving ball. And this is essentially what we are seeing with Newton's cradle. All the objects/balls in that system resonate at, the same frequency The flow of energy through that system is so efficient that it moves without wasting energy on outward motion until it gets to the end. In this way energy is kind of like most of us. It will obey the laws, but if you give it an option to do the LEAST amount of obeying and work until the very end, it will naturally chose that option. Energy is like, "Dude I'll do the damn work, but if you give me a path of least resistance so I don't have to waste energy chaotically moving things around, all things considered I'd rather take that path and do the least amount of work moving things around until the end and then I'll do the push off in whatever proportion that satisfies the laws governing conservation." Like us energy would rather not do unnecessary work if you give it an option, and when a system matches resonant frequency you will give energy the path of least resistance and it will take it and produce very little motion until it reaches the end, or the last object at resonance.

science doesn't "prove" things. Science is a method of evaluating evidence and there can always be more evidence. Proof would exclude new evidence. "Proof" is subjective outside math and logic. Other than that - brilliant video.