ELI5, why if you jump inside a moving train you will land on the same spot, but if you jump on the roof of a moving train, you land on a different spot?

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seen it on twitter and I can’t get my head around it
EDIT: thanks guys I get it now 😅

In: Physics

In most cases you’ll see in high school physics, both should give the same result. But there is a big factor that high school physics (and depending on your field, uni physics) igbores: air. When you jump inside a train, the air inside it is moving along with the train as well. But the air outside of the train is still with respect to the ground, or moving with the speed of the train, in the opposite direction, with respsect to the train. The air then pushes against you, and when you jump, it blows you back. Of course this depends on how fast the train is going, and how strong the wind blows (and in what direction), but in general, that’s what happens.

Inside the train the air is not moving relative to you. Outside the train it is so it will blow you back. Like when you stick your hand outside a car window.

When you stand on the roof of a train, the train is pushing you forward (through your feet), and the wind is pushing you back (air resistance). As soon as you jump, you no longer have the train pushing you forward, but the wind is still pushing you back, so you move backwards.

When you’re in the train, you don’t have the wind pushing you backwards, so when you jump, you keep moving forward at the same speed as the train.

If you were on the roof of a train in a vacuum, and you jumped, you would land on the same spot, because again, there would be no wind pushing you back.

It’s basically Newton’s First Law of motion. If you’re moving, you keep moving until an external force stops you.

2 things here to consider: air resistance and velocity of the air relative to the train.

In the train, the air moves with the same velocity as the train, so no air resistance. This means, if you jump, you land on the same spot.

On the roof, the air does not move with the train, so there is a relative velocity in the opposite direction of the moving train. so if you jump, air resistance will cause you to be moved, so you land more to the back of the train.

Everything inside the train is moving with the train, *including the air inside*. When you’re on the roof, you’re moving with the train but the outside air is still stationary. When you jump, you keep your forward momentum, as you would if you were on the train, but this time your body is trying to move forward through stationary air, which slows it down.