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

I would say speed and distance.
Typically a moving train that you’re jumping into, is still going very slowly (otherwise I wish you good luck…..). And also the distance you’re jumping is very small. You’re jumping only a few feet horizontally, after you matched your speed to the slowly moving train. So yeah, as long as you match the speed you will land kinda where you were aiming. Kinda, because typically the train will be accelerating (slowly) and you can’t accelerate mid air. 😋

Whereas people jumping on to trains in movies (don’t try it at home) typically jump onto a train moving at moderate to fast speed. So they are unable to match the forward momentum of the train. AND they are jumping maybe 10 to 20 feet vertically from a bridge. More like “aimed falling” than jumping. 😅 And during falling, simple gravity acceleration rules will apply. During that second or two that falling 10 to 20 feet will take, the train will be moving under you.

From km/h to m/s, you need to divide by 3.6.
So let’s say the train is moving at a modest speed of 36km/h to keep it easy. That’s 10 meters that the train will have moved forward if let’s say you are falling for 1 second. Sure, you too will have moved forward some if you took a running start before jumping off the bridge. But surely not 10 meters.