ELi5: Why is it that if you are in a moving train and jump, you stay in the same spot, but if you stand on top of a moving train and jump, you do not?
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Internally jumping in the car has no acceleration(assuming a perfect vertical jump and no other forces). You are moving at 10m/s relative to the ground, the train is also moving at 10m/s relative to the ground being propelled by an engine with constant force, the air inside the train is moving at 10m/s relative to the ground. You jump, there are very little forces impacting you in the air inside a train car. You land in the same spot.
Outside of the train, the train is moving relative to the ground and being propelled by an engine at a constant force, you are moving relative to the ground, but the air is stationary.
Standing on the top of the train, your shoes impart a force(assuming you can even stand up there depending how fast the train is going) which keeps your velocity “linked”.
By jumping you unlink yourself from the train and now become an object that is flying through the air. There is no propulsion pushing you forward(as there is with the train engine), so effectively you are being acted on by all drag pushing back from the relatively stationary air(wouldn’t be truly stationary and would be disturbed from the passing train but that’s getting into the weeds of fluid dynamics).
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