Summary
Highlights
Newton's Cradle consists of five identical metal spheres suspended as pendulums. While the motion seems obvious, explaining what's happening reveals an odd behavior: why only the end sphere is displaced when one is raised.
Raising a sphere gives it gravitational potential energy, which converts to kinetic energy upon release. This kinetic energy is efficiently transferred through the spheres in an elastic collision, displacing the end sphere. If perfectly efficient, the end sphere would rise to the same height, conserving energy. Friction eventually brings the system to a halt, converting energy to heat.
The video questions why only one sphere is displaced when one is released, and two when two are released. This phenomenon cannot be fully explained by the conservation of energy alone; the conservation of momentum is also crucial. Momentum is the tendency of a moving object to continue moving, and it is also conserved during collisions. Momentum is calculated as mass times velocity.
The video explains that both momentum (P = mv) and kinetic energy (KE = 1/2 mv²) must be conserved. A thought experiment is conducted where one sphere strikes the series, and two spheres are assumed to be displaced. Calculations show that while momentum could be conserved with two spheres moving at half the initial velocity, kinetic energy would not be conserved. Therefore, for both laws to hold true, one sphere entering the system must result in only one sphere exiting.
The only action that respects both the laws of conservation of energy and momentum is when the number of spheres entering the system equals the number of spheres exiting, with the same velocity. This equally applies when two spheres are released, resulting in two spheres being displaced from the other end. This serves as an impressive demonstration of these fundamental physics principles.