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?

Question

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

In: Physics

Answer 1

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.