Eli5: why when I’m standing on a moving train and I jump, the train doesn’t move around me?
In: Physics
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.
[This](https://www.youtube.com/watch?v=pyBNImQkRuk&t=295s) old educational film from the 60’s wonderfully shows the concept. The part about the relative motion of a ball falling on a moving cart starts about 5 minutes into the clip.
This video explains it pretty well and tries the theory of what happens if you counter the momentum and jump.
Objects in motion tend to stay in motion. You’ve moving in the train, you jump up. Absent any force on you while you are in the air, your velocity will remain the same and you’ll land on the same spot. Now try this on the top of the train with the wind pushing against you, and you will quickly decelerate while the train moves forward under you.
So don’t jump on an elevator, though. Some people I know did this and the elevator broke and they were stuck for hours and might have gotten sued because they were on camera jumping simultaneously, but they were trial lawyers and the building managers decided to let bygones be bygones.
Latest Answers