And on we go, talking about more physics in Newton's Cradle.
We already talked about conservation of energy, so now let's talk about conservation of momentum. Momentum is a vector quantity, meaning the direction it's in matters. When the first ball is dropped into the second ball, the second ball must keep moving in the same direction, and the first ball doesn't just bounce off the 2nd in the opposite direction; this would be a change in momentum, which cannot happen without the application of an outside force.
So in order to conserve momentum, the energy must move in the same direction through all five spheres.
Of course, Newton's Cradle isn't a completely closed system so as to warrant conservation of momentum, as the force of gravity and friction will always act as outside forces. But for the sake of this explanation, we'll assume we live in a perfect world without gravity or friction and everything is a closed system. Then, the collisions in Newtons Cradle would be perfectly elastic, where kinetic energy is conserved as well as momentum.
A bit drab, but hopefully you stayed awake till the end. I'll try to make the next one more engaging, but it's hard to find motivation in a dark hotel room while you're leaning over your phone listening to your parents snore and typing about physics instead of partyin it up in Pittsburgh.