# i simply can not understand how. if you were to stand on a train and jump, you would end up in the same place you were when you jumped. similarly, how do flies fly on a train??

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i simply can not understand how. if you were to stand on a train and jump, you would end up in the same place you were when you jumped. similarly, how do flies fly on a train??

In: Physics

it is because you (and the fly) are moving as fast as the train.

so if you are standing on a train and jump, you’re not just moving up, you’re also moving forward, at the same speed the train is

Earth is rotating and moving in outer space. If you jump, how do you not end up in outer space?

Imagine a pool on the train. You could swim in that pool normally (aside from any bumpiness in the tracks) just like in a stationary pool. Why? Because the pool and the water inside it are all moving along with the train at the same speed.

Same goes for the air in the train that the fly is “swimming” in.

When you’re in a car and it suddenly stops, you end up lurching forward because you’re still moving.

TLDR: momentum

Newton’s law of inertia states *”An object at rest will remain at rest, and an object in motion will continue moving with a constant velocity, unless acted upon by an external force.”*

In other words once an object is accelerated to a certain speed, it will remain at that speed until an outside force slows it down. Like applying the breaks on a car, or friction from the air slowing it down.

Humans don’t perceive the speed we are moving at, we only feel acceleration and deceleration. If you are in a car you can feel the car going to to 60km/h but once at speed you don’t feel it anymore.

Once that car or train is done accelerating you need to think of everything in it from the train itself, to the air, to the passengers and freight as being 1 unit all moving at the same speed. This potential speed is it’s inertia.

So when you jump straight up on a train, how fast are you moving?

If the train is moving at 60km/h when you jump up you are moving up and also 60km/h in the direction of the train. That’s why you land in the same spot.

Similarly if you were to shoot a gun from a moving train, and the bullet travels at 1000 km/h, how fast is the bullet moving?

The answer is 1060km/h, the speed of the bullet plus the speed of the train.

(If you are wondering if there’s an upper limit to this effect, there is. Once you get near the speed of light things start getting really wacky, but that’s a different ELI5.)

If you want to really bake your noodle, you sitting still are moving at 1600km/h because the Earth is spinning.

Since the Earth spins West > East, when you are walking East at 5km/h your speed is 1605km/h, but walking West you’re moving slower than the rotation so 1595km/h.

This is why all measures of speed are relative. When measuring speed you always have to measure compared to something else. Are you measuring relative to an object standing still on the ground? or compared to the spin of Earth? or the speed at which we are orbiting around the sun? context matters.

The train is sling-shooting you forward as well as your jump up. It’s the same as a football player running with a ball and throwing it up in the air and catching it. The ball is moving forward too.

You are still moving at the same speed as you were standing when you jumped. That speed is the speed of the train.