system
Given the following post, respond as if you're commenting on Reddit:
human
When I "start" two balls in a Newton's Cradle, why does it reflect with two balls on the other side instead of just the last ball with twice as much kinetic energy?
gpt
Because both energy and momentum are conserved.  Energy is quadratic in velocity and momentum is linear.  If the energy of two balls went into one ball, the resulting velocity would only be 1.4x the original, meaning momentum would be lost.  Momentum can't be lost, so that can't happen (at least not without something else getting involved).