A marble rolling on a curved track appears to violate conservation of momentum. Please help me understand why this is illusion and/or what mechanism is acting such that momentum is cancelled and recreated in a new direction.
Consider a marble on a flat track on Earth's surface. You push the marble to create momentum. All forward motion is from that initial push. The track is curved such that the marble returns to its starting point. An observer standing next to the track would say the marble's entire journey was apparently funded by its initial forward momentum, but this seems a violation of the vector velocity part of conservation.
Contrast that with a space ship traveling away from Earth, coasting on the momentum of an initial thruster blast. In an attempt to return to Earth, you apply side rocket thrust (like the track did to the marble). But while you can spin the ship and/or alter its trajectory to one side, you are still traveling away from Earth at the same steady speed. Only by using your rockets to directly cancel momentum away from Earth (which takes exactly as much energy as it did to create the momentum), and then adding more energy to push the ship back towards earth, can you get home.
So while the space ship can never return using only its own existing momentum, a marble on Earth can?
The marble track is somehow translating the marble's forward momentum into some other form of energy, then giving it back to the marble in the form of reduced momentum with a new vector.
Or I'm completely misunderstanding how momentum should be described in the case of the marble and thus creating the illusion by my mischaracterization.
I have an intuition that gravity is a significant part of the issue, so perhaps the marble's momentum (and all planetary surface objects) should be considered as a case of angular momentum?
Or maybe there's no illusion, I'm confusing various motions and energies being displayed by the marble, with its much smaller forward momentum, and in fact the cancellation and recreation of momentum can be fully explained by the appropriation of energy from these other factors that are inherent to and traveling with the ball?