Eli5: why when I’m standing on a moving train and I jump, the train doesn’t move around me?

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Eli5: why when I’m standing on a moving train and I jump, the train doesn’t move around me?

In: Physics

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.

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.

Because you are moving as fast as the train. If you were standing on the top of the train and jumped your forward motion would stop because you’d be hitting a wall of air that isn’t moving. When you’re inside the train, the air, you and the train are all moving at the same speed.

The train gives you its forward motion after you climb aboard and it starts moving. Everything in the train, you, other people, the luggage, the air, everything is moving forward at the same speed as the train.

Things in motion tend to stay in motion unless there’s some force acting upon them. Friction, or actually being pushed, etc.

So if you were to jump straight up, and at that moment the train slammed the brakes for some reason, the train would slow down abruptly, but you would still have the same motion as before, because while you’re airborne (from the jump) only the force of gravity affects you, and gravity pulls straight down, not sideways.

So you would see the train suddenly stop, and a camera bolted to the train would record you suddenly moving forward.

This is why they have seatbelts in cars. You’re going at the same speed as the car, but if the car hits something, IT would suddenly stop, but you’d be still [going forward at the same speed](https://youtu.be/YLCWGcNpY94?t=76), and basically hit the front of the cabin and/or with possibly enough force to be thrown through the window.

Velocity isn’t an absolute thing, you’re always moving relative to something else. Right now you assume you’re standing still, but in reality you’re only standing still relative to the earth. You’re actually moving at thousands of kilometers per hour relative to our solar system.

When you’re in a train it’s the same story. You’re standing still relative to the train, but you’re moving relative to the ground. A jump won’t affect your movement in the horizontal direction, according to the train you’re still standing still (albeit 20cm off the ground) and according to the ground you’re still moving. In order to start moving (relative to the train) you need to add some force in that direction, for example by walking or running.

Some confusion might come into play because of air resistance. If you’re watching a movie where some guy is standing on top of the train, jumps and flies off like the train is “moving away” below him, he’s actually being pushed off by the force of air slamming into him. If you’re inside a train this doesn’t apply.

When you stand on the train, you are connected to the train.

Power is used to push the train, and because you are attached to it, you move as well (you sometimes feel a jolt as your body matches the speed of the train at the beginning).

Because you’re attached to the train, when it gets faster, you get faster. When the train hits it’s too speed, and stops going faster, you are travelling at the same speed as the train.

When you jump upwards, you’re still travelling at the same speed. Nothing is slowing you down. Nothing else is pushing on you.

If you were to jump when the train was still getting faster, there would be a little bit of movement under you in the time that you were jumping, and you may feel a little jolt when you land again, like when you first started.

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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.

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.

This video explains it pretty well and tries the theory of what happens if you counter the momentum and jump.
https://youtu.be/8Q4fam5UOPU

[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.

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.

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.

Imagine driving your 100 miles an hour and it immediately (like in a second) stops.

You’d fly out the window with the momentum.

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.

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.

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.