Eli5: why when I’m standing on a moving train and I jump, the train doesn’t move around me?
In: Physics
Essentially because objects will always move in a straight line at a constant speed. Unless you do something to change it:
In you scenario, you are moving at the same speed as the train. So unless you do something about it, you will keep moving at the same speed as the train.
You push upwards when jumping, so you do not change your horizontal speed when you jump. It remains the same as the speed of train. If, however, the train accelerated or slowed down exactly at the moment you jump, then it will move around you, forwards or backwards.
Because you were already moving in the same direction by standing in the train. If the train crashed with a sudden stop, what would kill you is the fact that you were moving just as fast but now the train isn’t.
You’re already moving the same speed as the train.
If the train were to suddenly stop while you were mid-jump, you would find yourself moving forward.
Case in point: Some years ago, a US Navy submarine hit an uncharted underwater mountain while cruising.
A couple sailors died, many more injured, because they were moving the same speed as the sub when it hit the mountain. Once it impacted, you have a bunch of unsuspecting humans being thrown around in a cramped environment made of steel.
Now if you really want to start messing with your brain [check out what happens to a balloon](https://www.youtube.com/watch?v=y8mzDvpKzfY) in a car when you start to accelerate.
Imagine driving your 100 miles an hour and it immediately (like in a second) stops.
You’d fly out the window with the momentum.
It is called inertia.
Things in motion (or not in motion) stay in their current state unless there is an outside force acting upon them.
When you jump you keep moving forward. The train will accelerate slightly underneath you if it is currently increasing your speed and the air will probably slow you down a bit.
So when you jump up high you will usually land a little further back on the train.
It is the same reason why you fall over when the train abruptly brakes. Your body keeps moving forward but the train stops moving. Your feet stick to the train floor because of friction, so your upper body just moves forward without your feet and you fall over.
TL;DR: The train is at a uniform speed, you are at a uniform speed, and therefore both you and the train continue forward at the same rate when you jump.
There are two basic ideas of motion that you need to understand here. The first is velocity, without a change in velocity(acceleration), and assuming a uniform speed or an environment without friction, a body that has a velocity will maintain its velocity. A body in motion wants to stay in motion. When the velocity changes(acceleration), you feel force, and if you jump during a change in acceleration, the train will move forwards or comparatively backwards around you.
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