We often think that the humans are the culmination of the Creation. However, kittens are ahead of us.
Dr Panda, Dr Pikachu, and Dr Pavel (the principal investigators above) decided to do some research of Newton's cradle. Their research strategy was wise and the tactics were clever.
And the performance was so cute and their creativity was so mesmerizing that it is no wonder they received a $300 million grant from NSF.
For those readers who are not that far, Newton's cradle demonstrates the conservation of energy and the conservation of momentum.
Both quantities are conserved during a collision – if we neglect some losses that are small if the balls are hard enough. Such constraints are so strong that they effectively imply something very simply about the final velocities of the balls.
For example, if \(L\) balls with the same velocity \(V\) hit a collection of \(N\) balls at rest, the total momentum is \(LMV\) and the total kinetic energy is \(LMV^2/2\). The only way to guarantee the same final value of these quantities is to separate \(L\) balls from the total group of \(L+N\) balls on the right side and make them move to the right with the same velocity \(V\).
We get the same result even if we separate the collision of "groups of balls" to a sequence of several collisions of pairs of balls (1 on 1), assuming that they are separated by a tiny but nonzero distance. The animation above shows one of the neat but counterintuitive processes in the case \(L=2\), \(N=3\).
At the beginning, two balls on the left side are moving and the right three balls are at rest. At the end, two balls on the right side are moving and the left three balls are at rest. The process looks counterintuitive because in the approximation of vanishing distances between the balls, the middle (third) ball stays at rest all the time. Nevertheless, it is able to transmit the energy and momentum from the left two balls to the right two balls! You may make this conclusion look less counterintuitive if you admit that there are tiny gaps between the balls most of the time. In that case, the middle ball is moving by the velocity \(V\) to the right side for a short moment of time, too.
In the NSF research project, the three kittens also studied the condensed matter physics of the balls and the ropes, knot theory, and the invisibility cloak.