why is it that if your in a train and jump you move with the train but if your on top of the train and jump you stay in

Question: why is it that if your in a train and jump you move with the train but if your on top of the train and jump you stay in the same place and the train carries on moving?
- Keywords:
  jump,
  movement,
  physics,
  train,
  travel
Asked by saadialouise to Philip, Paddy, Michael, Eva, Chris on 13 Jun 2011.
- Chris Jordan answered on 10 Jun 2011:
  
  If you’re on a train your body is moving forward at the same speed as the train, doesn’t make any difference if you jump up and down, you can’t lose that forward speed. The same would happen if you were on top of the train and there was no air … think of the wind that you would be fighting on the roof of a high speed train – it would knock you backwards as soon as your feet left the roof. You maybe shouldn’t go and test this though – take my word for it.
- Michael Wharmby answered on 12 Jun 2011:
  
  Great answer from Chris. Just to add to it, you move forward with the train because there’s friction between your shoes and the train carpet. If the train floor were perfectly smooth (frictionless), you wouldn’t move with the train – it’d be like with a plastic box full of breakfast cereal. Move the box fast and the breakfast cereal stays where it is (until the back edge of the box catches up with it!) because there’s very little friction between the cereal and the plastic.
  Also why is there wind on top of the train to blow you off? Well the train is moving through the air. The air wants to stay where it is, so the train and anything on the outside of the train (moving with the train) is having to push through the air. This pushes you back if you were to jump on the top of a train. And if the train were travelling in an airless tube there would be nothing to push against you so you would jump up and because of the forward movement you had from the train when you jumped up you would keep travelling forward without loosing any speed (no air to push you back).
- Philip Denniff answered on 13 Jun 2011:
  
  I cant add anything else to these excellent answers
- Paddy Brock answered on 13 Jun 2011:
  
  Ditto, well covered guys.

Question: why is it that if your in a train and jump you move with the train but if your on top of the train and jump you stay in the same place and the train carries on moving?

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